Ecology Reproductive Allocation in Animals
James Gilbert
  • LAST REVIEWED: 06 May 2016
  • LAST MODIFIED: 23 May 2012
  • DOI: 10.1093/obo/9780199830060-0064


Reproductive allocation is a term used in ecology and evolutionary biology that refers to the proportion of an organism’s energy budget allocated to reproduction at any given time. Reproduction must be balanced (or traded off) against opposing expenditures such as growth, survival, maintenance, and future reproduction. The term also covers division of resources among offspring size and number. Studying reproductive allocation trade-offs is fundamental to the fields of behavioral ecology and physiological ecology, which use evolutionary theory to explain and predict animal behavior and physiology, respectively. More specifically, these trade-offs are central to the field of life history, which studies how growth and reproduction is distributed across animals’ lifetimes. Animals show a vast degree of variation in reproductive allocation. Kiwis, for example, famously lay a single egg that is up to 20 percent of body weight. As if this were nothing, caecilians (amphibians) can bear live litters of offspring that are up to 65 percent of the mother’s body weight. Egg numbers vary enormously and can reach spectacular numbers: tsetse flies bear as few as six live offspring in a lifetime, whereas ghost moths can lay more than 50,000 eggs. Social insects have truly mind-boggling fecundity: driver ants can lay several million eggs per month, and can live for decades; ocean sunfish release about 300 million eggs at a time, more than any other vertebrate. At the other extreme, very many organisms have one offspring at a time. Usually this goes hand in hand with repeated breeding, but perhaps the most puzzling of allocation decisions is found in dung beetles, which have only one ovary; some species lay as few as five to ten eggs. Careful parental care ensures that more than 90 percent of offspring survive, explaining why these species have not become extinct. Clearly, variation of this order of magnitude requires evolutionary explanation. Research into reproductive allocation has progressed from a simple household-economics outlook based on the division of a fixed energy budget toward more sophisticated approaches based on quantitative and mechanistic genetics. Of particular use have been model systems such as Drosophila and Daphnia, where traits of reproductive allocation (body size, egg size, egg number, etc.) have become model traits for modern genetic analyses. Modern approaches to reproductive allocation typically involve elucidating genetic bases for trade-offs expressed across a range of environments. Nevertheless, the classical life history approaches remain relevant, especially in systems in which controlled quantitative genetics are not possible.

General Overviews

Reproductive allocation lies across several fields and is covered by relevant textbooks from each. Krebs and Davies 1993 and Krebs and Davies 1997 are excellent, readable introductions to behavioral ecology, and they are standard undergraduate textbooks. Sibly and Calow 1986 and Schmidt-Nielsen 1997 provide reviews of literature on physiological ecology. Stearns 1992 and Roff 1992 are the classic texts on life history. Peters 1983 and Schmidt-Nielsen 1984 discuss reproductive allocation in the context of body size, while Reiss 1989 offers a more mathematical treatment of the same issue.

  • Krebs, John R., and Nicholas B. Davies. 1993. An introduction to behavioural ecology. 3d ed. Oxford: Blackwell Science.

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    Excellent and thoroughly readable introduction to all key concepts in behavioral ecology, including all concepts discussed in this article. Suitable for undergraduates.

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    • Krebs, John R., and Nicholas B. Davies, eds. 1997. Behavioural ecology: An evolutionary approach. 4th ed. Oxford: Blackwell Science.

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      A slightly more in-depth edited volume of contributions to a spectrum of topics in behavioral ecology. Suitable for final-year undergraduates and more advanced researchers.

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      • Peters, Robert H. 1983. The ecological implications of body size. Cambridge, UK: Cambridge Univ. Press.

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        A survey of the literature and comprehensive review of body size as a concept and its role in ecology, evolution, and life history. Fairly mathematical. Includes many painstakingly compiled tables of literature data. Schmidt-Nielsen 1984 is perhaps more readable but contains fewer relevant data and references.

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        • Reiss, Michael J. 1989. The allometry of growth and reproduction. Cambridge, UK: Cambridge Univ. Press.

          DOI: 10.1017/CBO9780511608483Save Citation »Export Citation »E-mail Citation »

          Mathematical and equation-based textbook on the scaling of reproductive parameters with body size. Short and surprisingly accessible, this text begins at first principles and rewards the effort.

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          • Roff, Derek A. 1992. The evolution of life histories: Theory and analysis. London: Chapman and Hall.

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            Derek Roff provides a readable and comprehensive overview of the theory of life history evolution; complements Stearns 1992 and should be the starting point for any interested reader. Unlike Stearns, Roff does not attempt to reduce or distill the mathematics—readers who are interested in the mathematical history of the subject would do better to start here.

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            • Schmidt-Nielsen, Knut. 1984. Scaling: Why is animal size so important? Cambridge, UK: Cambridge Univ. Press.

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              Textbook on the physiology of animal scaling, with sections on reproductive scaling. Highly readable and suitable for undergraduates.

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              • Schmidt-Nielsen, Knut. 1997. Animal physiology: Adaptation and environment. 5th ed. Cambridge, UK: Cambridge Univ. Press.

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                Classic, very well-written guide to animal physiology from an ecological perspective, more physiologically oriented than Sibly and Calow 1986—organized around the major physiological challenges facing animals. Suitable for undergraduates.

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                • Sibly, Richard M., and Peter Calow. 1986. Physiological ecology of animals: An evolutionary approach. Oxford: Blackwell.

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                  In-depth and comprehensive treatment of physiological ecology organized around the mathematics of ecological models.

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                  • Stearns, Stephen C. 1992. The evolution of life histories. Oxford: Oxford Univ. Press.

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                    Excellent, and fluently written, this textbook contains minimal mathematics and is still a classic. Stearns manages to draw an enormous body of work together into a coherent volume while still maintaining his preferred emphasis in stressing that the field of life history is not yet at the stage where predictions are possible. Should be the first port of call when studying trade-offs.

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                    No specific journal is dedicated to publishing papers on reproductive allocation or life history. However, most journals in the fields of evolution, ecology, and physiological and behavioral ecology publish works dealing with these topics. Many of the classic advances in life history have been published in the journal American Naturalist, which is dedicated to publishing detailed and thorough studies. Short reviews that are typically pithy and readable can be found in Trends in Ecology & Evolution, while longer and more detailed reviews appear in journals such as Annual Review of Ecology, Evolution and Systematics. Typically, standard tests of life history hypotheses, when not in American Naturalist, appear in journals such as Evolution, Proceedings of the Royal Society B: Biological Sciences, Journal of Evolutionary Biology, and Functional Ecology. Many other relevant articles appear in journals specific to the biology and behavior of one relevant group of animals. Purely theoretical developments can be found in Journal of Theoretical Biology, but they also often appear in American Naturalist and Evolution.

                    • American Naturalist.

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                      Published monthly since 1867 and with all issues now online, American Naturalist is the oldest scientific journal in the world dedicated to ecology, evolution, and behavior. Publishes extended and detailed articles (eight to fifteen pages) typically covering both theory and empirical tests of theory, and aimed at researchers looking for thorough treatment of a topic. Many (if not most) of the major developments in life history, both theory and tests, have appeared in this journal.

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                      • Annual Review of Ecology, Evolution and Systematics.

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                        Part of the Annual Reviews series of journals. Formerly simply Annual Review of Ecology and Systematics. Published annually since 1970; features long and involved essay-review articles (twenty to forty pages) aimed at the serious and committed researcher and strives for detailed and comprehensive coverage of a topic with typically hundreds of references.

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                        • Evolution.

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                          The journal of the Society for the Study of Evolution. Stated aim is to publish high-impact articles that represent “either important additions to evolutionary theory or methodology, or careful empirical studies that bear on significant questions in evolutionary biology.” Articles are typically medium- to extended length (eight to fifteen pages) and contain detailed theory and data.

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                          • Functional Ecology.

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                            The journal of the British Ecological Society. Publishes high-impact and thorough papers on all aspects of organismal ecology, including life histories, with a stated focus on experimental papers. Given this rigorous focus on experiments, papers appearing in this journal typically have impeccable methodological integrity.

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                            • Journal of Evolutionary Biology.

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                              The journal of the European Society for Evolutionary Biology. Bimonthly; publishes standard-length articles with a stated aim to “integrate perspectives across molecular and microbial evolution, behavior, genetics, ecology, life histories, development, palaeontology, systematics, and morphology.” Committed to open access, with all articles free to access two years after they are published.

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                              • Journal of Theoretical Biology.

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                                Forum for purely theoretical and mathematical advances relating to biological processes. Many advances in life history theory have appeared in this journal.

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                                • Proceedings of the Royal Society B: Biological Sciences.

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                                  Flagship journal of the Royal Society in the United Kingdom. Biweekly; publishes short- to medium-length, high-impact articles of a diverse scope on topics of major importance to evolutionary biology and with a stated focus on organismal biology (as opposed to, for example, molecular biology).

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                                  • Trends in Ecology & Evolution.

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                                    Short but comprehensive review articles of approximately two to five pages on diverse ecological and evolutionary topics that are excellent and highly readable; generally suitable for undergraduates. TREE is a recommended starting point for any student or researcher interested in mining the literature on a chosen topic.

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                                    Historical Background

                                    The principle of trade-offs in nature has been recognized for centuries: for example, Aristotle’s “physiological limitation” and Goethe and Geoffroy Saint-Hilaire’s “law of compensation” or “balancement of growth.” These formed the basis for the “principle of allocation” in Cody 1966, upon which classical life history theory rests. We can trace the formal study of reproductive allocation to Cole 1954, a seminal paper in which it is recognized that an organism’s life history is made up of a set of traits that have evolved by natural selection and should be studied as such. Possibly the most important idea in the study of reproductive allocation, though, has been the concept of reproductive effort in Fisher 1930, later “refined” in Williams 1966b. The authors of these divided an organism’s total reproductive fitness into fitness accruing from the current brood versus potential fitness from future broods (a term known as residual reproductive value, or RRV; see Fisher 1930). RRV is a crucial concept; in his model, Williams proposed that animals should allocate their effort to the current brood in an inverse proportion to their RRV. Thus, longer-lived organisms (or more precisely, those that have more broods in their lifetime) should invest less proportional effort into each brood. This idea has framed virtually all subsequent debate in the now familiar terms of reproductive costs and trade-offs. These ideas appeared as part of a wider trend that became the formal study of life history, namely, the appreciation that survival and fecundity change as an animal ages, and that this leads to knock-on changes in a whole suite of reproductive traits. For examples, see Williams 1966a, Gadgil and Bossert 1970, and Charnov and Schaffer 1973. In ensuing decades, the literature focused largely on how previous approaches were inadequate (see Complexities). More modern approaches are explicitly quantitative- or population-genetic (see Brown and Sibly 2006, cited in Other Questions, for examples).

                                    • Charnov, Eric L., and W. M. Schaffer. 1973. Life history consequences of natural selection: Cole’s result revisited. American Naturalist 107:791–793.

                                      DOI: 10.1086/282877Save Citation »Export Citation »E-mail Citation »

                                      Theoretical. Twenty years on, the authors incorporate realistic assumptions about adult and juvenile mortality to resolve Cole’s paradox (see Cole 1954) and explain why animals often breed repeatedly. Available online for purchase or by subscription.

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                                      • Cody, Martin L. 1966. A general theory of clutch size. Evolution 20:174–184.

                                        DOI: 10.2307/2406571Save Citation »Export Citation »E-mail Citation »

                                        Theoretical. One of the first formal life history models, incorporating the idea of allocation trade-offs within the reproductive lives of animals (coined as the “principle of allocation”). Available online for purchase or by subscription.

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                                        • Cole, Lamont C. 1954. The population consequences of life history phenomena. Quarterly Review of Biology 29.2: 103–137.

                                          DOI: 10.1086/400074Save Citation »Export Citation »E-mail Citation »

                                          Theoretical. Starting from the observation that all populations must replace themselves in order not to go extinct, Cole presents the idea that an integrated strategy of reproduction is a trait that is itself under selection. Cole’s models predict that no species should breed more than once—known as “Cole’s Paradox,” this was not resolved until twenty years later in Charnov and Schaffer 1973. Available online for purchase or by subscription.

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                                          • Fisher, Ronald A. 1930. The genetical theory of natural selection. Oxford: Clarendon.

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                                            One of the most influential works in evolutionary biology, Fisher’s work stands at the heart of the “Modern Synthesis”—the ideological successor to Darwin—in introducing mathematical rigor into evolutionary biology. However, by no stretch of the imagination is it an easy read. Rather, the reader is advised to explore Fisher’s ideas through the writings of his followers, for example, Krebs and Davies 1993 (cited under General Overviews).

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                                            • Gadgil, Madhav, and William H. Bossert. 1970. Life historical consequences of natural selection. American Naturalist 104.935: 1–24.

                                              DOI: 10.1086/282637Save Citation »Export Citation »E-mail Citation »

                                              Theoretical. The authors apply the then-recent developments of Fisher, Williams, and Cody to formulate a classic model of optimal life history strategies. Available online for purchase or by subscription.

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                                              • Williams, George C. 1966a. Adaptation and natural selection: A critique of some current evolutionary thought. Princeton, NJ: Princeton Univ. Press.

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                                                Williams provides characteristically sharp and insightful criticism of current evolutionary thinking, including what we now call “pan-adaptationism” and group selection. Very easy to read. Chapter 6 is most relevant for reproductive allocation.

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                                                • Williams, George C. 1966b. Natural selection, the cost of reproduction, and a refinement of Lack’s principle. American Naturalist 100:687–690.

                                                  DOI: 10.1086/282461Save Citation »Export Citation »E-mail Citation »

                                                  Theoretical. Very short, highly readable, and profoundly insightful—Williams’s refinement has formed the basis for whole fields of future research. Available online for purchase or by subscription.

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                                                  Offspring Size and Number

                                                  Studies of offspring size and number can be thought of as having begun in the 1940s with the work of British ornithologists, most notably Reginald Moreau, Alexander Skutch, and, in particular, David Lack. Ricklefs 2000 provides a review of the development of life history thinking based upon their ideas. The “Lack clutch,” introduced in Lack 1947, has formed the basis for most thinking about clutch sizes and, indeed, the optimality of most behavior, to date. Although long appreciated as a concept, the classic model of the trade-off between size and number came several decades later, with the influential paper of Smith and Fretwell 1974.This fundamental model has since been expanded and developed in a plethora of ways. For example, Parker and Begon 1986 and Shine 1978 variously incorporate body size, environmental quality, and parental care into Smith and Fretwell’s equations, and Godfray, et al. 1991 adds sibling competition.

                                                  Empirical Studies

                                                  Classic empirical studies of reproductive allocation began in the early 1980s. See, for instance, the work of Tessier, et al. 1983 on Daphnia (water fleas); Montague, et al. 1981 on the Hawaiian Drosophila (fruit flies); papers by David Reznick on guppies (e.g., Reznick 1983; and the classic Reznick, et al. 1990, cited in Mortality), and articles by Timothy Clutton-Brock on red deer (e.g., Clutton-Brock, et al. 1982 and Clutton-Brock, et al. 1983). Although studies of this nature continue to appear, beginning in the 1980s research became focused on genetic bases for allocation decisions and how these vary across environments (see Trade-Offs). The first of these to appear were predictably in Drosophila. For good examples, see the exceptionally thorough work of Michael Rose (e.g., Rose and Charlesworth 1981). Another classic system for such studies is the seed beetle Callosobruchus maculatus (see, e.g., Messina and Fry 2003, cited in Habitat Quality and Resource Availability).


                                                  A substantial volume of the life history literature, especially from the late 1980s and early 1990s, is devoted to pointing out that “the situation is not as simple as it seems.” First, trade-offs in reproductive allocation are often described as arising due to “constraints,” but many kinds of constraint can act to produce a trade-off. Initially, constraints were assumed to arise from simple division of resources (leading to what we call a physiological trade-off). However, Williams 1957, with characteristic foresight, points out that trade-offs can also operate under genetic constraints (leading to microevolutionary trade-offs). Arnold 1992 is an influential review that draws together decades of thinking about different kinds of constraints. The different forms of trade-offs, and their functional bases, are reviewed in great detail in Zera and Harshman 2001. Roff and Fairbairn 2007 provides a short, updated, and insightful review of current thinking on trade-offs and constraints, and their various mechanisms. One other way that “the situation is not as simple as it seems” is that apparently convincing evidence for trade-offs is often deceptively ambiguous. First, in van Noordwijk and de Jong 1986, a highly influential paper, the authors point out that variation in individuals’ total resource budgets (e.g., in body size) often obscures variation we are trying to observe in allocation among different traits, such that we may often observe positive correlations between traits even when a trade-off is present. An excellent summary of this model can be found in Reznick, et al. 2000. Second, Sgrò and Hoffmann 2004 points out and reviews the evidence that gene-environment interactions may obscure the trade-off we are trying to observe—if genes coding for the traits we want to observe have different effects in different environments. The work of Michael Rose’s lab on Drosophila provides a salient example (Leroi, et al. 1994); see also Messina and Fry 2003 (cited in Habitat Quality and Resource Availability). Finally, an excellent, if somewhat sobering, analysis in Tuomi, et al. 1983 points out that allocation trade-offs are certainly not the only forces shaping reproductive strategies: Selection operates “on whole organisms throughout their entire life history” (p. 25), which can obscure the predicted effects of trade-offs in isolation.

                                                  • Arnold, Stevan J. 1992. Constraints on phenotypic evolution. American Naturalist 140:S85–S107.

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                                                    Review. Excellent but long and involved synthetic review (containing minimal mathematics) of contemporary thinking with regard to evolutionary constraints. Available online for purchase or by subscription.

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                                                    • Leroi, Armand M., Adam K. Chippindale, and Michael R. Rose. 1994. Long-term laboratory evolution of a genetic life-history trade-off in Drosophila melanogaster. 1. The role of genotype–environment interaction. Evolution 48:1244–1257.

                                                      DOI: 10.2307/2410382Save Citation »Export Citation »E-mail Citation »

                                                      Experimental. Long and characteristically thorough article detailing the trade-off between early reproduction and three variables—later reproduction, starvation resistance, and longevity—showing that the trade-offs appear only in certain environments. Available online for purchase or by subscription.

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                                                      • Reznick, David, Leonard Nunney, and Alan Tessier. 2000. Big houses, big cars, superfleas and the costs of reproduction. Trends in Ecology & Evolution 15:421–425.

                                                        DOI: 10.1016/S0169-5347(00)01941-8Save Citation »Export Citation »E-mail Citation »

                                                        Review. Short and highly readable review of the difficulties of measuring the costs of reproduction, organized around a famous prior discovery of “superfleas” with apparently no reproductive costs. Available online for purchase or by subscription.

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                                                        • Roff, D. A., and D. J. Fairbairn. 2007. The evolution of trade-offs: Where are we? Journal of Evolutionary Biology 20.2: 433–447.

                                                          DOI: 10.1111/j.1420-9101.2006.01255.xSave Citation »Export Citation »E-mail Citation »

                                                          Review. Short and readable up-to-date review of problems in current thinking with respect to constraints and trade-offs.

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                                                          • Sgrò, C. M., and A. A. Hoffmann. 2004. Genetic correlations, tradeoffs and environmental variation. Heredity 93:241–248.

                                                            DOI: 10.1038/sj.hdy.6800532Save Citation »Export Citation »E-mail Citation »

                                                            Review. Explains and summarizes evidence that genetically mediated trade-offs are often different in different environments, but are generally only investigated in one, weakening the conclusions of most studies.

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                                                            • Tuomi, Juha, Tuomo Hakala, and Erkki Haukioja. 1983. Alternative concepts of reproductive effort, costs of reproduction, and selection in life-history evolution. American Zoologist 23.1: 25–34.

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                                                              Review. Points out weaknesses of considering life history forces in isolation from other selective pressures, shows that optimization approaches to life history often fail with real data, and suggests an alternative that might be termed “anti-optimization,” since natural selection operates by killing off less well-adapted individuals, leaving those that are “good enough.” Available online for purchase or by subscription.

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                                                              • van Noordwijk, A. J., and G. de Jong. 1986. Acquisition and allocation of resources: Their influence on variation in life-history tactics. American Naturalist 128:137–142.

                                                                DOI: 10.1086/284547Save Citation »Export Citation »E-mail Citation »

                                                                Theoretical. This seminal model served as a warning that, in order to demonstrate the presence or absence of a trade-off between two traits, it is inadequate simply to show a negative or positive correlation between them, respectively. Long and involved, but well summarized in many other sources, such as Reznick et al. 2000 and Krebs and Davies 1993 (see General Overviews). Available online for purchase or by subscription.

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                                                                • Williams, George C. 1957. Pleiotropy, natural selection and the evolution of senescence. Evolution 11:398–411.

                                                                  DOI: 10.2307/2406060Save Citation »Export Citation »E-mail Citation »

                                                                  Theoretical. Williams points out that the evolution of aging may come about because natural selection is blind to negative effects of genes after we have reproduced, so genes beneficial early in life will be selected even if they bring about negative effects in old age. Available online for purchase or by subscription.

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                                                                  • Zera, Anthony J., and Lawrence G. Harshman. 2001. The physiology of life history trade-offs in animals. Annual Review of Ecology and Systematics 32:95–126.

                                                                    DOI: 10.1146/annurev.ecolsys.32.081501.114006Save Citation »Export Citation »E-mail Citation »

                                                                    Review. Excellent and thorough review of the functional basis of life history trade-offs—provides detailed physiological context to many of the concepts discussed in this article.

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                                                                    The key concepts of reproductive allocation boil down to a core set of trade-offs, discussed in depth in Stearns 1992 (cited in General Overviews). Historically, these have been viewed separately, although this may not be the best approach (for example, see Tuomi, et al. 1983, cited in Complexities, and Winkler and Wallin 1987, cited in Total Reproductive Effort). Because each of these trade-offs has its own distinct literature, we will discuss each in a separate section. Given a limited resource budget over its lifetime, an animal must, first, decide how much to invest in reproduction in total (see Total Reproductive Effort). Then, current reproduction must be balanced against investment in survival to reproduce and, for some organisms, against further growth (see Reproduction versus Survival and Growth). An organism must then decide how to divide up reproductive effort among successive breeding attempts (see the Current/Future Reproduction Trade-off). Finally, within a brood, reproductive effort must be divided between many small offspring or few large ones (see Offspring Size versus Offspring Number).

                                                                    Total Reproductive Effort

                                                                    Initial theories such as Lande 1982 modeled reproductive allocation decisions as a two-step process with “total reproductive effort” (TRE) as the first decision, followed by considerations about how to apportion effort among reproductive attempts and between offspring size and number. Winkler and Wallin 1987 predicts that these two steps are not independent. Caley, et al. 2001 backs this up with evidence in copepods, and Schwarzkopf, et al. 1999 finds that selection on offspring size produces correlated shifts in TRE in Drosophila. Recently, Charnov, et al. 2007 predicts that TRE should be a life history invariant (see also The Search for Unifying Principles): Summed over their lifetime, all animals should in theory produce about 1.4 times their body mass in offspring, but this remains to be confirmed independently.

                                                                    • Caley, M. Julian, Lin Schwartzkopf, and Richard Shine. 2001. Does total reproductive effort evolve independently of offspring size? Evolution 55.6: 1245–1248.

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                                                                      Comparative. Across copepod species (aquatic invertebrates), the authors demonstrate that total reproductive effort and offspring size are related. Available online for purchase or by subscription.

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                                                                      • Charnov, Eric L., Robin Warne, and Melanie Moses. 2007. Lifetime reproductive effort. American Naturalist 170:E129–E142.

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                                                                        Theoretical. Uses the principle articulated in Williams 1966b (cited in Historical Background) to formulate prediction that all animals should produce the same relative weight in offspring. Available online for purchase or by subscription.

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                                                                        • Lande, Russell. 1982. A quantitative genetic theory of life history evolution. Ecology 63:607–615.

                                                                          DOI: 10.2307/1936778Save Citation »Export Citation »E-mail Citation »

                                                                          Theoretical. Provides a theoretical basis for incorporating quantitative genetics into life history studies rather than the prior “blind optimization” approach that ignored underlying mechanisms. Reasonably involved and mathematical. Available online for purchase or by subscription.

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                                                                          • Schwarzkopf, Lin, Mark W. Blows, and M. Julian Caley. 1999. Life-history consequences of divergent selection on egg size in Drosophila melanogaster. American Naturalist 154:333–340.

                                                                            DOI: 10.1086/303242Save Citation »Export Citation »E-mail Citation »

                                                                            Experimental. Natural selection experiment in Drosophila showing that total reproduction and offspring size are indeed linked, as predicted in Winkler and Wallin 1987. Available online for purchase or by subscription.

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                                                                            • Winkler, David W., and Kjell Wallin. 1987. Offspring size and number: A life history model linking effort per offspring and total effort. American Naturalist 129:708–720.

                                                                              DOI: 10.1086/284667Save Citation »Export Citation »E-mail Citation »

                                                                              Theoretical. Model predicting that total reproductive effort should be related to offspring size. Involved, but summarized well in Caley, et al. 2001. Available online for purchase or by subscription.

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                                                                              Reproduction versus Survival and Growth

                                                                              According to Williams 1966b (cited under Historical Background), survival should vary inversely with investment in reproduction. Kirkwood and Rose 1991 reviews hypotheses and evidence that the cost of reproduction is paid in terms of reduced survival late in life. In a recent review of available evidence, though, Harshman and Zera 2007 concludes that physiological bases for such costs are only just becoming apparent, even in model systems. Notwithstanding this, many studies have demonstrated at least phenotypic-level survival costs of reproduction; one classic example is Snell and King 1977. Studies documenting effects of experimentally preventing mating upon lifespan are listed in Stearns 1992 (cited under General Overviews). Brodie 1989 investigates whether snakes mitigate one particular survival cost of reproduction (increased predation risk) by modifying their behavior. However, Ricklefs and Cadena 2007 shows that, even at phenotypic levels, this trade-off is only apparent in relatively poor environments. Phenotypic trade-offs between growth and reproduction have been shown in a range of taxa and are reasonably unambiguous (many examples are listed in Appendix 2d of Stearns 1992; another classic study is Reznick 1983, cited under Empirical Studies). As another example, Helle 2008 shows that girls who begin reproduction earlier and have more babies grow to be shorter. Barnes 1962 shows that breeding in barnacles results in less calcium deposition and reduced growth. However, we are warned in Roff 2000 that clear genetic correlations are not yet discernible between growth rates and reproduction.

                                                                              The Current/Future Reproduction Trade-Off

                                                                              Williams 1966b (cited under Historical Background) predicts that reproduction now carries a cost to future reproduction. At the phenotypic level, mothers with experimentally increased clutch sizes usually show reduced reproduction subsequently— many examples are listed in Appendix 2c of Stearns 1992 (cited under General Overviews). In another example, Lummaa 2001 finds that human mothers of twins show reduced future reproduction compared with mothers of singletons. Conversely, mothers prevented from investing in reproduction often show increased reproduction later (see Zink 2003 for an excellent example). Selection experiments also show convincing negative genetic correlations between early and late reproduction; examples are listed in Appendix 1 of Stearns 1992. A second clear prediction is that animals should allocate more of their available resources to reproduction, and less to survival and maintenance, as they get older and their RRV declines (the terminal investment hypothesis (TIH), introduced and discussed at length in Clutton-Brock 1984). The TIH should not be confused with senescence, which is age-related deterioration in condition (discussed in Monaghan, et al. 2008). Senescence predicts that the total resources available to an organism decline over time, whereas the TIH predicts that an increasing fraction of those resources will be spent on reproduction as the animal ages. Velando, et al. 2006 discusses evidence for the TIH and provides strong experimental support. Cotter, et al. 2010 uses a clever experimental design to simulate the risk of death in burying beetles, again finding convincing evidence for the TIH. Langley and Clutton-Brock 1998, though, finds that neither reproductive allocation nor the absolute amount of reproduction changed over time in tsetse flies.

                                                                              Offspring Size versus Offspring Number

                                                                              Studies of offspring number have a long and venerable history (see Godfray, et al. 1991 for a detailed review). However, when we are thinking about fitness rather than merely offspring numbers, size of offspring becomes an important quantity because larger offspring typically increase the fitness of parents. For a detailed treatment of the ecological effects of offspring size, see Krist 2011 (birds) and Fox and Czesak 2000 (arthropods).Yet, because resources are limited, a mother cannot maximize both offspring fitness and number; the two trade off. On the basis of the model in Smith and Fretwell 1974 (see Offspring Size and Number), it is widely assumed that offspring size and number must trade off at some level, but this is, nevertheless, a very complex issue. In Bernardo 1996, a typical “things-are-not-so-simple” paper, the author points out that the complexity of the problem is compounded by the fact that all parents were once offspring themselves, so offspring size will have a double effect upon parents’ fitness: first, by affecting offspring fitness; second, by determining the parent’s own size, and hence parental fitness. Charnov and Ernest 2006 suggests that the trade-off is better expressed as offspring size versus offspring number per unit parental mass, but this consideration has yet to take hold among empirical biologists. Evidence from phenotypic-level correlations is both mixed and complicated (summarized in Appendix 2e of Stearns 1992; see General Overviews). Recently Walker, et al. 2008, looking at mammals, shows negative size-number correlations across increasingly finer scales of analysis from patterns across all mammals down to patterns among populations of humans. The most elegant and convincing phenotypic evidence comes from experimental studies of lizards: Sinervo and Licht 1991 (cited in Morphology) uses careful surgical follicle removal and hormone treatment to, respectively, reduce and increase clutch sizes of lizards. Fox, et al. 1997 estimates genetic correlations between egg size and number in the seed beetle Stator limbatus.

                                                                              • Bernardo, Joseph M. 1996. The particular maternal effect of propagule size, especially egg size: Patterns, models, quality of evidence and interpretations. American Zoologist 36:216–236.

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                                                                                Review. Critically examines models and evidence for evolution of offspring size, arguing that we must simultaneously model both mother and offspring fitness together if we are to understand the problem. Long but readable and even enjoyable, owing to its rather scathing criticisms. Available online for purchase or by subscription.

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                                                                                • Charnov, Eric L., and S. K. Morgan Ernest. 2006. The offspring‐size/clutch‐size trade‐off in mammals. American Naturalist 167.4: 578–582.

                                                                                  DOI: 10.1086/501141Save Citation »Export Citation »E-mail Citation »

                                                                                  Theoretical. Unusually concise and readable for this journal; very clearly explained and insightful short paper that suggests that the size/number trade-off should better be expressed as size/relative number per unit body mass. Available online for purchase or by subscription.

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                                                                                  • Fox, Charles W., and Mary Ellen Czesak. 2000. Evolutionary ecology of progeny size in arthropods. Annual Review of Entomology 45:341–369.

                                                                                    DOI: 10.1146/annurev.ento.45.1.341Save Citation »Export Citation »E-mail Citation »

                                                                                    Review. Long and detailed but readable review of the ecological and evolutionary consequences of egg size in insects and their allies. Includes discussion of most of the concepts addressed in this article, including size/number trade-offs and associated evidence.

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                                                                                    • Fox, Charles W., Monica S. Thakar, and Timothy A. Mousseau. 1997. Egg size plasticity in a seed beetle: An adaptive maternal effect. American Naturalist 149:149–163.

                                                                                      DOI: 10.1086/285983Save Citation »Export Citation »E-mail Citation »

                                                                                      Experimental. The authors use manipulative experiments to show that different egg sizes are favored on different host plants; female beetles modify egg size according to host plant, and egg numbers vary inversely. Available online for purchase or by subscription.

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                                                                                      • Godfray, H. C. J., L. Partridge, and P. H. Harvey. 1991. Clutch size. Annual Review of Ecology and Systematics 22:409–429.

                                                                                        DOI: 10.1146/ Citation »Export Citation »E-mail Citation »

                                                                                        Review. Excellent and extended review of studies of clutch size, including reviews of many of the concepts described elsewhere in this article. Available online for purchase or by subscription.

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                                                                                        • Krist, Miloš 2011. Egg size and offspring quality: A meta-analysis in birds. Biological Reviews 86:692–716.

                                                                                          DOI: 10.1111/j.1469-185X.2010.00166.xSave Citation »Export Citation »E-mail Citation »

                                                                                          Review. Very recent, long, and thorough review of the effects of offspring size upon survival and fitness in birds. Available online for purchase or by subscription.

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                                                                                          • Walker, Robert S., Michael Gurven, Oskar Burger, and Marcus J. Hamilton. 2008. The trade-off between number and size of offspring in humans and other primates. Proceedings of the Royal Society B: Biological Sciences 275:827–834.

                                                                                            DOI: 10.1098/rspb.2007.1511Save Citation »Export Citation »E-mail Citation »

                                                                                            Comparative. Careful, controlled, and thorough statistical analysis across multiple taxonomic levels of progeny size versus number.

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                                                                                            The Search for Unifying Principles

                                                                                            Attempts to fit a grand unifying theory across animal life histories have often foundered. Boggs 1992 suggests one possible explanation: Life history trade-offs are usually studied in isolation and rarely alongside important traits such as foraging efficiency (see also Tuomi, et al. 1983, cited under Complexities). For example, the idea of r and K selection, discussed in Pianka 1970, held traction for a long time in the 1970s and 1980s, but has now been rejected as simplistic. Another idea still under debate is that of life history invariants, namely, dimensionless quantities, often ratios, that appear to apply across all species and body sizes: In one famous example, all mammals from elephants to pipistrelle bats are predicted to have a roughly similar lifetime number of breaths. The eminent life historian Eric Charnov is a strong advocate of life history invariants (Charnov 1993), but their existence is under close scrutiny because they may, in fact, largely result from regressing variables against themselves or fractions of themselves (Nee, et al. 2005). Charnov and colleagues have responded vigorously to this criticism (Savage, et al. 2006). One seemingly unifying pattern that is gaining ground with many theorists is that many life history variables covary predictably among species according to a “fast-slow continuum” (Promislow and Harvey 1990). This theory, though, has also had problems explaining many patterns (Bauwens and Díaz-Uriarte 1997 is a good example in lizards). Recently, Bielby, et al. 2007 explores the idea of a fast-slow continuum in mammals, and the authors find that most variation in life histories is attributable to two main “axes of variation,” each of which collectively describes a number of life history variables, including traits involved in reproductive allocation.

                                                                                            • Bauwens, Dirk, and Ramón Dı́az-Uriarte. 1997. Covariation of life-history traits in lacertid lizards: A comparative study. American Naturalist 149:91–111.

                                                                                              DOI: 10.1086/285980Save Citation »Export Citation »E-mail Citation »

                                                                                              Comparative. Study of life history traits across lizard species; finds little support for the idea of a fast-slow continuum. Available online for purchase or by subscription.

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                                                                                              • Bielby, J., G. M. Mace, O. R. P. Beninda-Emonds, et al. 2007. The fast-slow continuum in mammalian life history: An empirical reevaluation. American Naturalist 169.6: 748–757.

                                                                                                DOI: 10.1086/516847Save Citation »Export Citation »E-mail Citation »

                                                                                                Comparative. Large and rigorous analysis of life history variation across mammals. Distills a large volume of information across hundreds of mammal species, using modern web-based data repositories, into two main axes of variation: timing of reproduction and the offspring size/number trade-off. Available online for purchase or by subscription.

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                                                                                                • Boggs, Carol L. 1992. Resource allocation: Exploring connections between foraging and life history. Functional Ecology 6:508–518.

                                                                                                  DOI: 10.2307/2390047Save Citation »Export Citation »E-mail Citation »

                                                                                                  Review. Carol Boggs wonders why foraging ecology, which is the principal determinant of an animal’s energy intake, has not been featured in life history models that, at the end of the day, focus on energy allocation. She proposes a method of incorporating foraging into existing models. Although widely cited, this idea has yet to take hold in practice. Available online for purchase or by subscription.

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                                                                                                  • Charnov, Eric L. 1993. Life history invariants: Some explorations of symmetry in evolutionary ecology. Oxford: Oxford Univ. Press.

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                                                                                                    Book based in mathematics explaining the idea of the life history invariant, a dimensionless quantity that appears to hold true for all animals. Charnov is a great proponent of invariants, which have been popular but that now are under debate (see Nee, et al. 2005).

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                                                                                                    • Nee, Sean, Nick Colegrave, Stuart A. West, and Alan Grafen. 2005. The illusion of invariant quantities in life histories. Science 309:1236–1239.

                                                                                                      DOI: 10.1126/science.1114488Save Citation »Export Citation »E-mail Citation »

                                                                                                      Theoretical. The authors argue clearly and succinctly that life history invariants may result from a passive artifact of the method used to detect them (log-log regression), and generate their own “invariant” using randomly generated data. See also supporting comment by Gerdien de Jong in the same journal issue, and reply in Savage, et al. 2006. Available online for purchase or by subscription.

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                                                                                                      • Pianka, Eric R. 1970. On r and K selection. American Naturalist 104:592–597.

                                                                                                        DOI: 10.1086/282697Save Citation »Export Citation »E-mail Citation »

                                                                                                        Theoretical. Expands and clarifies the idea of r and K selection, coined several years beforehand, describing a continuum between the two kinds of selection and illustrating each extreme with examples. Available online for purchase or by subscription.

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                                                                                                        • Promislow, D. E. L., and P. H. Harvey. 1990. Living fast and dying young: A comparative analysis of life history variation among mammals. Journal of Zoology (London) 220.3: 417–437.

                                                                                                          DOI: 10.1111/j.1469-7998.1990.tb04316.xSave Citation »Export Citation »E-mail Citation »

                                                                                                          Comparative. The authors conclude that mortality patterns are the best predictor of salient variation in mammalian life histories. Available online for purchase or by subscription.

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                                                                                                          • Savage, Van M., Ethan P. White, Melanie E. Moses, et al. 2006. Comment on the illusion of invariant quantities in life histories. Science 312.5771: 198b.

                                                                                                            DOI: 10.1126/science.1123679Save Citation »Export Citation »E-mail Citation »

                                                                                                            Theoretical. Charnov and his colleagues respond to the charge in Nee et al. 2005 that life history invariants are illusory artifacts.

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                                                                                                            The Problem of Variation

                                                                                                            All environments show at least a degree of variation, over space and over time; thus, there need not be a single best pattern of reproductive allocation for a given species, population, or even individual. For example, the authors of Mueller, et al. 1991 reared fruit fly lineages at high- and low-population densities: each lineage evolved to outcompete the other on its respective “home” density. What is optimal in one environment may not be optimal in another, or even in the same environment at a different time: Herbers and Banschbach 1998 reports different responses to the same treatment by the same population of ants in different years. Kaplan and Cooper 1984 shows that often there are not one but many “optimal” life histories, depending upon variation in the environment. Even the same environment can select for multiple simultaneous solutions to the same “problem” (Dunbar 1983, cited under Within-Individual Variation, discusses alternative options for an individual; Kaitaniemi, et al. 2011 discusses the idea that there can be multiple optimal solutions to the same problem). Accordingly, it is very difficult to make predictions about what kinds of traits are generally favored by certain environments (in keeping with the idea that “there are no predictions in life history”; see Stearns 1992, cited under General Overviews) and explanations for observed patterns must frequently be specific to the species, population, or even individual that is being studied. In this article, each of these levels of variation is discussed in a separate section (see Species-Level Variation, Population-Level Variation, and Within-Individual Variation).

                                                                                                            • Herbers, Joan M., and Valerie S. Banschbach. 1998. Food supply and reproductive allocation in forest ants: Repeated experiments give different results. Oikos 83.1: 145–151.

                                                                                                              DOI: 10.2307/3546555Save Citation »Export Citation »E-mail Citation »

                                                                                                              Experimental. The authors repeat a previous food supplementation study in the same population of ants and report significant changes in reproductive allocation, where the previous study had seen no effect of supplementation. Thus, they stress the potential time-dependence and nonrepeatability of many results. Available online for purchase or by subscription.

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                                                                                                              • Kaitaniemi, Pekka, Annette Scheiner, Tero Klemola, and Kai Ruohomäki. 2011. Multiobjective optimisation shapes ecological variation. Proceedings of the Royal Society B: Biological Sciences 279.1729: 820–825.

                                                                                                                DOI: 10.1098/rspb.2011.1371Save Citation »Export Citation »E-mail Citation »

                                                                                                                Theoretical/empirical. Multiobjective optimization is a new technique borrowed from economics, which the authors use to “solve” several life history trade-offs at once, in a species of moth for which we have a substantial amount of information. Reveals that the same environment can select simultaneously for a range of strategies in the same population.

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                                                                                                                • Kaplan, Robert. H., and William S. Cooper. 1984. The evolution of developmental plasticity in reproductive characteristics: An application of the “adaptive coin-flipping” principle. American Naturalist 123.5018: 393–410.

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                                                                                                                  Theoretical. Kaplan and Cooper explore the idea that variability within a population (specifically within a brood of offspring) can be advantageous per se; hence, allocation to size and number can be different for different offspring of the same parent. Available online for purchase or by subscription.

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                                                                                                                  • Mueller, L. D., P. Z. Guo, and F. J. Ayala. 1991. Density-dependent natural selection and trade-offs in life history traits. Science 253.5018: 433–436.

                                                                                                                    DOI: 10.1126/science.1907401Save Citation »Export Citation »E-mail Citation »

                                                                                                                    Experimental. Classic study of divergent selection on reproductive allocation and growth rate in Drosophila. From the same population, lineages reared at different densities evolved different patterns of growth and reproduction. At high density, the high-density–adapted flies had higher fitness than their low-density–adapted counterparts, but when they were swapped into low-density conditions, their reproduction was depressed. Available online for purchase or by subscription.

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                                                                                                                    Species-Level Variation

                                                                                                                    Different animal groups exhibit very different rules and patterns of reproductive allocation and life history. In a meta-analysis, Hendriks and Mulder 2008 summarizes results from many studies showing strong taxonomic differences in how reproductive allocation scales with body mass across species of different body masses. Berrigan 1991 is a similar analysis among orders of insects, noting similar kinds of differences among orders in scaling of egg size and number. This kind of dramatic variation among animal groups in reproductive allocation patterns has been explained in terms of broad-scale, macroevolutionary patterns in the ecological strategies of species. This section explores some of the explanations that have been offered for this large-scale variation.

                                                                                                                    Variation in How Reproduction Is Financed

                                                                                                                    Jönsson 1997 reviews how species “finance” their reproduction. Capital breeders lay down nutrient reserves in their bodies (or externally in “larders,” such as squirrels and acorn woodpeckers) prior to breeding, and then breed using those reserves. In contrast, income breeders constantly divert a fraction of energy from ingested food to reproduction, without relying on stored reserves. Jönsson, et al. 1995 discusses the implications of costs experienced before and after breeding for reproductive allocation. The authors of Sibly and Calow 1984 couch their analysis in terms of “direct” and “absorption” costing, terms used in economics, showing that, broadly speaking, capital breeders tend to allocate less to reproduction than income breeders because of the extra storage costs. Bonnet, et al. 1998 reviews capital breeding in ectotherms where the costs of storage are greatly reduced. However, predictably enough, Stephens, et al. 2009 points out that the capital/income distinction has proven to be a false dichotomy, which is better viewed as a continuum.

                                                                                                                    • Bonnet, Xavier, Don Bradshaw, and Richard Shine. 1998. Capital versus income breeding: An ectothermic perspective. Oikos 83.2: 333–342.

                                                                                                                      DOI: 10.2307/3546846Save Citation »Export Citation »E-mail Citation »

                                                                                                                      Review. Studies of capital/income breeding had previously focused on endotherms; the authors show that ectotherms have different energetic requirements and, therefore, the costs and benefits of capital and income breeding are very different: capital breeding is much more common in ectotherms. Available online for purchase or by subscription.

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                                                                                                                      • Jönsson, K. Ingemar. 1997. Capital and income breeding as alternative tactics of resource use in reproduction. Oikos 78.1: 57–66.

                                                                                                                        DOI: 10.2307/3545800Save Citation »Export Citation »E-mail Citation »

                                                                                                                        Review. Jönsson gives a clarification of capital and income breeding, discusses their relative costs and benefits and conditions selecting for each. Clear, readable and nonmathematical. Available online for purchase or by subscription.

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                                                                                                                        • Jönsson, K. Ingemar, Juha Tuomi, and Johannes Järemo. 1995. Reproductive effort tactics: Balancing pre- and post-breeding costs of reproduction. Oikos 74.1: 35–44.

                                                                                                                          DOI: 10.2307/3545672Save Citation »Export Citation »E-mail Citation »

                                                                                                                          Theoretical. A slightly more mathematical treatment dividing the costs of reproduction into components that are experienced before and after reproduction, and the relative effects of each component upon the optimal reproductive strategy. Available online for purchase or by subscription.

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                                                                                                                          • Sibly, Richard M., and Peter Calow. 1984. Direct and absorption costing in the evolution of life cycles. Journal of Theoretical Biology 111.3: 463–473.

                                                                                                                            DOI: 10.1016/S0022-5193(84)80234-9Save Citation »Export Citation »E-mail Citation »

                                                                                                                            Theoretical. Applies the principle of direct and absorption costing, borrowed from economics, to the evolution of reproductive allocation. Available online for purchase or by subscription.

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                                                                                                                            • Stephens, Philip A., Ian L. Boyd, John M. McNamara, and Alasdair I. Houston. 2009. Capital breeding and income breeding: Their meaning, measurement, and worth. Ecology 90:2057–2067.

                                                                                                                              DOI: 10.1890/08-1369.1Save Citation »Export Citation »E-mail Citation »

                                                                                                                              Review. Evaluates the usefulness of the concepts of capital and income breeding, pointing out difficulties in their classification and measurement, and, predictably, proposes instead a continuum representing the degree of reliance on capital reserves for breeding. Available online for purchase or by subscription.

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                                                                                                                              Determinate Versus Indeterminate Growth

                                                                                                                              In most animals (e.g., fish, amphibians, mollusks), reproduction begins at a size smaller than the eventual size and the animal thereafter grows while also reproducing. This is known as indeterminate growth. In a studying of water fleas, Taylor and Gabriel 1992 shows that the degree to which further growth is advantageous depends upon the relationship between body size and survival. Perrin 1989 formulates a model of reproductive allocation in a species with indeterminate growth. In contrast, birds, mammals, and insects show determinate growth, first allocating all energy to growth up to a threshold size, and thereafter investing no more in growth, instead focusing entirely on reproduction. Why birds, mammals, and insects should be different from other animals is not known, but Sibly, et al. 1985 argues that their strategy is probably theoretically optimal. Charnov, et al. 2001 puts this down to differences in the costs of maintaining a body—indeterminate growth may be the result when it is “simply not possible for all personal production to be funneled to reproduction” (p. 9464).

                                                                                                                              • Charnov, Eric L., Thomas F. Turner, and Kirk O. Winemiller. 2001. Reproductive constraints and the evolution of life histories with indeterminate growth. Proceedings of the National Academy of Sciences of the United States of America 98:9460–9464.

                                                                                                                                DOI: 10.1073/pnas.161294498Save Citation »Export Citation »E-mail Citation »

                                                                                                                                Theoretical. Short, technical but clear article, proposing a set of life history invariants for indeterminate growers, and arguing that costs of bodily maintenance may drive the evolution of indeterminate and determinate growth strategies.

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                                                                                                                                • Perrin, N. 1989. Reproductive allocation and size constraints in the cladoceran Simocephalus vetulus (Müller). Functional Ecology 3.3: 279–283.

                                                                                                                                  DOI: 10.2307/2389366Save Citation »Export Citation »E-mail Citation »

                                                                                                                                  Empirical/theoretical. Uses data from life history of an aquatic invertebrate to formulate a model of growth and reproductive allocation in a species with indeterminate growth. Available online for purchase or by subscription.

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                                                                                                                                  • Sibly, Richard, P. Calow, and N. Nichols. 1985. Are patterns of growth adaptive? Journal of Theoretical Biology 112.3: 553–574.

                                                                                                                                    DOI: 10.1016/S0022-5193(85)80022-9Save Citation »Export Citation »E-mail Citation »

                                                                                                                                    Theoretical. Technical and involved mathematical model of optimal growth strategies, exploring conditions in which determinate and indeterminate growers are favored. Available online for purchase or by subscription.

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                                                                                                                                    • Taylor, Barbara E., and Wilfried Gabriel. 1992. To grow or not to grow: Optimal resource allocation for Daphnia. American Naturalist 139:248–266.

                                                                                                                                      DOI: 10.1086/285326Save Citation »Export Citation »E-mail Citation »

                                                                                                                                      Theoretical. Uses the well-known life history patterns of water fleas to formulate a model of optimal allocation to growth and reproduction after maturity. Dense and slightly technical, but well integrated with biological detail. Available online for purchase or by subscription.

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                                                                                                                                      Other Investments in Offspring Fitness

                                                                                                                                      Offspring size is not the only way of investing in offspring fitness. For example, Shine 1978 predicts that egg size should increase when species perform parental care (the “safe harbor” hypothesis). Sargent, et al. 1987 extends this model, adding more realistic assumptions about mortality. This hypothesis has received much attention, with mixed support. Summers, et al. 2006 finds supporting evidence in frogs. Gilbert and Manica 2010 finds no support in insects; instead, the authors find that the demands of parental care may impose a particular constraint upon offspring numbers, so that larger-bodied parents must have fewer offspring. In another example of alternative investments in offspring fitness, Podolsky 2004 investigates protective egg coatings in marine, broadcast-spawning species that buffer against variable environments, reducing variability in egg size.

                                                                                                                                      • Gilbert, James D. J., and Andrea Manica. 2010. Parental care trade-offs and life history relationships in insects. American Naturalist 176:212–226.

                                                                                                                                        DOI: 10.1086/653661Save Citation »Export Citation »E-mail Citation »

                                                                                                                                        Comparative. Examines the way parental care affects reproductive allocation and its scaling in insects. Egg sizes were identical regardless of parental care, refuting the safe harbor hypothesis. Egg numbers scaled dramatically differently under parental care, suggesting that parental care may fundamentally constrain reproductive allocation. Available online for purchase or by subscription.

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                                                                                                                                        • Podolsky, Robert D. 2004. Life-history consequences of investment in free-spawned eggs and their accessory coats. American Naturalist 163:735–753.

                                                                                                                                          DOI: 10.1086/382791Save Citation »Export Citation »E-mail Citation »

                                                                                                                                          Theoretical. Uses data from literature to formulate a model of how reproductive allocation should change when females invest in protective egg coating. Involved but well anchored in biological data, and very thorough in its treatment of a novel topic. Available online for purchase or by subscription.

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                                                                                                                                          • Sargent, Robert C., Peter D. Taylor, and Mart R. Gross. 1987. Parental care and the evolution of egg sizes in fishes. American Naturalist 129:32–46.

                                                                                                                                            DOI: 10.1086/284621Save Citation »Export Citation »E-mail Citation »

                                                                                                                                            Comparative/theoretical. The authors expand and augment the safe harbor hypothesis in Shine 1978, adding biologically realistic assumptions about mortality, and testing it across a dataset of fish species. Long, but clear and rewards the effort. Available online for purchase or by subscription.

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                                                                                                                                            • Shine, Richard. 1978. Propagule size and parental care: The “safe harbor” hypothesis. Journal of Theoretical Biology 75.4: 417–424.

                                                                                                                                              DOI: 10.1016/0022-5193(78)90353-3Save Citation »Export Citation »E-mail Citation »

                                                                                                                                              Review/Theoretical. Notes that many prior studies have found offspring size increases when parents care for offspring, and formulates a theoretical model based on contrasting patterns of juvenile and adult mortality. This hypothesis has received much attention. Surprisingly readable for this journal. Available online for purchase or by subscription.

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                                                                                                                                              • Summers, Kyle, Christian Sea McKeon, and Heather Heying. 2006. The evolution of parental care and egg size: A comparative analysis in frogs. Proceedings of the Royal Society B: Biological Sciences 273:687–692.

                                                                                                                                                DOI: 10.1098/rspb.2005.3368Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                Comparative. The authors test the safe harbor hypothesis (Shine 1978) across a large dataset of frog species, finding strong support. They also found that large egg size precedes parental care evolution, so it is likely that large eggs require care, rather than care selecting for large eggs.

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                                                                                                                                                It has been proposed that shape may constrain reproductive allocation in some species: The classic example is Congdon and Gibbons 1987 on turtles, but see also Sinervo and Licht 1991 on lizards. Bowden, et al. 2004 discusses whether or not the matter is as simple as has been previously assumed. In tiny Callitrichid primates (marmosets and tamarins), which usually have twins, Leutenegger 1979 and Ford 1980 both argue that extreme reduction in body size may limit the size of offspring in order to avoid obstetric problems, making twinning the optimal strategy.

                                                                                                                                                • Bowden, R. M., H. K. Harms, R. T. Paitz, and F. J. Janzen. 2004. Does optimal egg Size vary with demographic stage because of a physiological constraint? Functional Ecology 18.4: 522–529.

                                                                                                                                                  DOI: 10.1111/j.0269-8463.2004.00861.xSave Citation »Export Citation »E-mail Citation »

                                                                                                                                                  Empirical. Argues that size constraints may not be as important as previously supposed in the study of painted turtles in Congdon and Gibbons 1987; hormonal and physiological constraints are probably just as important. Available online for purchase or by subscription.

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                                                                                                                                                  • Congdon, Justin D., and J. Whitfield Gibbons. 1987. Morphological constraint on egg size: A challenge to optimal egg size theory?. Proceedings of the National Academy of Sciences of the United States of America 84.12: 4145–4147.

                                                                                                                                                    DOI: 10.1073/pnas.84.12.4145Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                    Comparative. Celebrated example of a possible constraint upon egg size arising from the shape of the pelvis in turtles. Short and readable, a good introductory example to the topic of constraints upon optimality.

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                                                                                                                                                    • Ford, S. M. 1980. Callitrichids as phyletic dwarfs, and the place of the Callitrichidae in Platyrrhini. Primates 21:31–43.

                                                                                                                                                      DOI: 10.1007/BF02383822Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                      Comparative/review. Advances the idea that the marmosets arose as a result of dwarfism in their lineage within the primates. Their unusual reproductive strategy is discussed in this context. Available online for purchase or by subscription.

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                                                                                                                                                      • Leutenegger, Walter. 1979. Evolution of litter size in primates. American Naturalist 114:525–531.

                                                                                                                                                        DOI: 10.1086/283499Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                        Comparative. Classic study of litter size and body size in primates, showing differences among groups in scaling and finding that twinning species have smaller offspring, evidence for a size/number trade-off. Available online for purchase or by subscription.

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                                                                                                                                                        • Sinervo, Barry, and Paul Licht. 1991. Proximate constraints on the evolution of egg size, number, and total clutch mass in lizards. Science 252:1300–1302.

                                                                                                                                                          DOI: 10.1126/science.252.5010.1300Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                          Experimental. Short, readable account of a very clever experiment in which egg numbers were surgically reduced and egg sizes increased in response. The authors infer, therefore, that the size of the reproductive tract may be a constraint on egg sizes in lizards much as in turtles (see Congdon and Gibbons 1987). Available online for purchase or by subscription.

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                                                                                                                                                          Population-Level Variation

                                                                                                                                                          At the level of the population, variation in life history is the substrate matter for natural selection (Fisher 1930, cited under Historical Background). Where environments vary in space, naturally occurring heritable genetic variation among individuals can lead to microevolutionary change, resulting in ecotypes with different patterns of reproductive allocation (see Hassall, et al. 2005). A large literature exists on selective pressures affecting reproductive allocation: If there are any generalities to be drawn at all, it might be noted that environments that impose high or unpredictable adult mortality favor increased reproductive allocation and early reproduction, whereas environments that impose high or unpredictable juvenile mortality favor reduced reproductive allocation and increased longevity of adults (Stearns 1983; McGinley, et al. 1987).

                                                                                                                                                          • Hassall, Mark, Alvin Helden, Andrew Goldson, and Alastair Grant. 2005. Ecotypic differentiation and phenotypic plasticity in reproductive traits of Armadillidium vulgare (Isopoda: Oniscidea). Oecologia 143:51–60.

                                                                                                                                                            DOI: 10.1007/s00442-004-1772-3Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                            Experimental. Very thorough investigation of differences in phenotypic plasticity in two ecotypes of woodlice with regard to variation in several environmental factors. Available online for purchase or by subscription.

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                                                                                                                                                            • McGinley, Mark A., David H. Temme, and Monica A. Geber. 1987. Parental investment in offspring in variable environments: Theoretical and empirical considerations. American Naturalist 130.3: 370–398.

                                                                                                                                                              DOI: 10.1086/284716Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                              Theoretical. In their model, the authors consider the evidence that variability in offspring size is a necessary outcome of variability in the environment (see Kaplan and Cooper 1984, cited under The Problem of Variation). They conclude that it is not, and discuss possible reasons for observed variation in offspring size. Available online for purchase or by subscription.

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                                                                                                                                                              • Stearns, Stephen C. 1983. A natural experiment in life-history evolution: Field data on the introduction of mosquitofish (Gambusia affinis) to Hawaii. Evolution 37:601–617.

                                                                                                                                                                DOI: 10.2307/2408273Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                Experimental. Stearns documents complex life history changes in mosquitofish following their introduction into a set of reservoirs in Hawaii, relating these changes to contemporary life history theory. The take-home message is that nothing is simple in life history: exceptions seem to outnumber rules. Available online for purchase or by subscription.

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                                                                                                                                                                Habitat Quality and Resource Availability

                                                                                                                                                                Resource availability is key in determining reproductive allocation. For example, Hassall, et al. 2005 (cited under Population-Level Variation) shows that woodlice reared on a site with good growth conditions evolved to allocate a higher proportion of total resources to reproduction, and had both more and larger offspring. Ellers and Jervis 2003 shows, using simulations that parasitoid wasps raised in high habitat quality allocate more resources to early reproduction (i.e., emerge as adults with more of their eggs mature). In a thorough study of seed beetles, Messina and Fry 2003 finds that low resource availability (seeds for breeding) induces a threefold reduction in fecundity and a concomitant increase in life span. However, the authors also find that across different conditions of resource availability, fecundity and life span are not necessarily as tightly coupled as is usually assumed.

                                                                                                                                                                • Ellers, Jacintha, and Mark A. Jervis. 2003. Body size and the timing of egg production in parasitoid wasps. Oikos 102.1: 164–172.

                                                                                                                                                                  DOI: 10.1034/j.1600-0706.2003.12285.xSave Citation »Export Citation »E-mail Citation »

                                                                                                                                                                  Theoretical. Uses a dynamic programming model to test the hypothesis that the timing of egg production in wasps depends on features of the host. Habitat quality and habitat unpredictability are found to be the main predictors of reproductive allocation early in life (a quantity they call “ovigeny”). Available online for purchase or by subscription.

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                                                                                                                                                                  • Messina, Frank J., and J. D. Fry. 2003. Environment-dependent reversal of a life history trade-off in the seed beetle Callosobruchus maculatus. Journal of Evolutionary Biology 16.3: 501–509.

                                                                                                                                                                    DOI: 10.1046/j.1420-9101.2003.00535.xSave Citation »Export Citation »E-mail Citation »

                                                                                                                                                                    Experimental. Very thorough quantitative-genetic study of how life history trade-offs vary across different levels of availability of seeds for breeding. Finds that fecundity and life span are both dependent on resources, but that they are not particularly tightly linked to each other. Available online for purchase or by subscription.

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                                                                                                                                                                    Ectothermic (cold-blooded) animals have metabolic rates that vary with ambient temperature, and reproductive allocation varies accordingly. Females should produce fewer, larger eggs when the environment is colder (Perrin 1988, Yampolsky and Scheiner 1996), but temperature also affects adult size, which in turn affects offspring size. These complex predictions are explored and tested in Angilletta, et al. 2006. Drosophila egg size was found to be different in populations from different regions, but egg sizes in all populations responded similarly to manipulation of temperature (Azevedo, et al. 1996). One current paradox is that ectotherms tend to mature at larger sizes when growth is limited by low temperature, but smaller sizes when growth is limited by food (Berrigan and Charnov 1994).

                                                                                                                                                                    • Angilletta, Michael J., Jr., Christopher E. Oufiero, and Adam D. Leache. 2006. Direct and indirect effects of environmental temperature on the evolution of reproductive strategies: An information-theoretic approach. American Naturalist 168:E123–E135.

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                                                                                                                                                                      Comparative. Discusses and explores the multifarious predicted effects of temperature upon offspring size and number using information theory, a way of testing many models against each other. Available online for purchase or by subscription.

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                                                                                                                                                                      • Azevedo, Ricardo B. R., Vernon French, and Linda Partridge. 1996. Thermal evolution of egg size in Drosophila melanogaster. Evolution 50:2338–2345.

                                                                                                                                                                        DOI: 10.2307/2410702Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                        Experimental. Analysis of egg size with respect to temperature in laboratory populations and among ecotypes along climatic gradients. Available online for purchase or by subscription.

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                                                                                                                                                                        • Berrigan, D., and E. L. Charnov. 1994. Reaction norms for age and size at maturity in response to temperature: A puzzle for life historians. Oikos 70.3: 474–478.

                                                                                                                                                                          DOI: 10.2307/3545787Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                          Theoretical. The authors use the Bertalanffy growth equation to attempt to explain why low temperature and low food availability seem to have opposing effects upon life histories. They reason that these two factors are linked in different ways to fundamental parameters of the Bertalanffy equation, such as the growth coefficient and the asymptotic size. Available online for purchase or by subscription.

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                                                                                                                                                                          • Perrin, N. 1988. Why are offspring born larger when it is colder? Phenotypic plasticity for offspring size in the cladoceran Simocephalus vetulus (Müller). Functional Ecology 2:283–288.

                                                                                                                                                                            DOI: 10.2307/2389399Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                            Experimental. Reviewing models and predictions, Perrin confirms that the slower juvenile growth rate that we observe at lower temperatures does indeed result in the production of larger offspring, supporting existing models. Note that his model indicates that phenotypic plasticity is favored with respect to temperature (see Phenotypic Plasticity). Available online for purchase or by subscription.

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                                                                                                                                                                            • Yampolsky, Lev Y., and Samuel M. Scheiner. 1996. Why larger offspring at lower temperatures? A demographic approach. American Naturalist 147:86–100.

                                                                                                                                                                              DOI: 10.1086/285841Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                              Theoretical/comparative. The authors formulate and test a model of offspring size and number at varying temperatures, confirming their predictions with literature data from populations of Daphnia. Note that their model indicates that phenotypic plasticity is favored with respect to temperature (see Phenotypic Plasticity). Available online for purchase or by subscription.

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                                                                                                                                                                              Michod 1979 points out that the age at which mortality has its greatest effect is critical. If mortality increases in one age class, then it becomes optimal to invest more into reproduction before that age and less important after it. The classic experimental demonstration of this is in guppies (Reznick, et al. 1990). In another example, Wellborn 1994 finds that different age-specific mortality patterns generated different reproductive ecotypes in amphipods. If adult mortality increases, then individuals can expect their fitness to decline more sharply with age. Thus, in keeping with Williams 1966b (cited under Historical Background) and the terminal investment hypothesis, generally high adult mortality is expected to select for earlier reproduction in fewer attempts, and higher reproductive allocation per attempt (Charlesworth 1980; see also Stearns 1983, cited under Population-Level Variation). Fishing pressure, for example, leads to the evolution of earlier reproduction in cod (Olsen, et al. 2004). Jones, et al. 2008 finds that Tasmanian devil populations plagued by an infectious cancer have evolved to concentrate all their reproduction into fewer early bouts. In a particularly thorough study, Kinnison, et al. 2001 shows that salmon experimentally forced to migrate—a risky strategy—laid more, smaller eggs as a result.

                                                                                                                                                                              • Charlesworth, Brian. 1980. Evolution in age-structured populations. Cambridge, UK: Cambridge Univ. Press.

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                                                                                                                                                                                Book-length account of theory and evidence, both phenotypic and genetic, concerning the importance of including age structure into population-based studies of evolution. Contains an in-depth section on life history evolution, including many topics covered here, approached from a population-genetic standpoint. Charlesworth’s pedigree in studies of Drosophila is impeccable and this is a classic text, although unashamedly mathematical and fairly dense.

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                                                                                                                                                                                • Jones, Menna E., Andrew Cockburn, Rodrigo Hamede, et al. 2008. Life-history change in disease-ravaged Tasmanian devil populations. Proceedings of the National Academy of Sciences of the United States of America 105:10023–10027.

                                                                                                                                                                                  DOI: 10.1073/pnas.0711236105Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                  Empirical/experimental. The authors make use of a natural experiment in mortality, the origin of a novel infectious skin cancer among Tasmanian devils, to study effects upon life history evolution. They find that, as predicted by theory, the devils have responded to this devastatingly high adult mortality by evolving to concentrate reproduction into fewer and earlier bouts with higher reproductive allocation.

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                                                                                                                                                                                  • Kinnison, Michael T., Martin J. Unwin, Andrew P. Hendry, and Thomas P. Quinn. 2001. Migratory costs and the evolution of egg size and number in introduced and indigenous salmon populations. Evolution 55:1656–1667.

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                                                                                                                                                                                    Experimental. The authors conduct an experiment in which they translocated groups of Chinook salmon upstream, forcing them to migrate. This extra energetic cost and risk induced a change in egg size and number. They compare this result to natural variation among populations of Pacific salmon with relation to their migratory strategies.

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                                                                                                                                                                                    • Michod, Richard E. 1979. Evolution of life histories in response to age-specific mortality factors. American Naturalist 113:531–550.

                                                                                                                                                                                      DOI: 10.1086/283411Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                      Theoretical. Rigorous mathematical model incorporating the idea of age-specific mortality into contemporary life history models (e.g., Schaffer 1974, cited under Environmental Variability, and Gadgil and Bossert 1970, cited under Historical Background). Explicitly mathematical (refers the reader elsewhere for biological motivation!), thus dense and difficult. Available online for purchase or by subscription.

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                                                                                                                                                                                      • Olsen, Esben M., Mikko Heino, George R. Lilly, et al. 2004. Maturation trends indicative of rapid evolution preceded the collapse of northern cod. Nature 428:932–935.

                                                                                                                                                                                        DOI: 10.1038/nature02430Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                        Empirical/experimental. The authors use data from fisheries to model changes in life history of cod up to and following a moratorium on fishing. The data are strongly in agreement with life history theory, suggesting that mortality from fishing limits the maturation strategy and reproductive allocation of the fishes. Available online for purchase or by subscription.

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                                                                                                                                                                                        • Reznick, David A., Heather Bryga, and John A. Endler. 1990. Experimentally induced life-history evolution in a natural population. Nature 346:357–359.

                                                                                                                                                                                          DOI: 10.1038/346357a0Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                          Experimental. A classic of the life history literature. In this clever experiment, the authors translocated guppies among pools containing predators that specifically target different age classes of prey. Life history in the different experimental populations diverged in the manner predicted by theory. Available online for purchase or by subscription.

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                                                                                                                                                                                          • Wellborn, Gary A. 1994. Size-biased predation and prey life histories: A comparative study of freshwater amphipod populations. Ecology 75:2104–2117.

                                                                                                                                                                                            DOI: 10.2307/1941614Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                            Empirical. Clever study using natural variation in predator species between lakes to study size-biased mortality from predation on Daphnia. Analyses stomach contents of predators to assess size bias in the range of Daphnia prey taken, and matches this to life history shifts in the prey. Available online for purchase or by subscription.

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                                                                                                                                                                                            Environmental Variability

                                                                                                                                                                                            Where environments vary stochastically over time (i.e., fluctuate), this has complex effects: It can select either for earlier reproduction (Ellers and Jervis 2003, cited under Habitat Quality and Resource Availability), for greater reproductive effort in fewer broods (McGinley, et al. 1987, cited under Population-Level Variation), or for variability of offspring size as a bet-hedging strategy (Capinera 1979). Support for Capinera’s prediction is provided in Crump 1981. More constant environments often tend to favor deferred reproduction, long life spans, and reduced effort per brood (Real and Ellner 1992). Again, however, it is not simple, for example, Schaffer 1974 points out that this pattern depends crucially upon the age at which environmental fluctuations are most important for survival.

                                                                                                                                                                                            • Capinera, John L. 1979. Qualitative variation in plants and insects: Effect of propagule size on ecological plasticity. American Naturalist 114.3: 350–361.

                                                                                                                                                                                              DOI: 10.1086/283484Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                              Theoretical. Starting from standard contemporary models of life history (Smith and Fretwell 1974, cited under Offspring Size and Number), suggests that variable environments may select for a range of offspring sizes in order to hedge bets. Available online for purchase or by subscription.

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                                                                                                                                                                                              • Crump, Martha L. 1981. Variation in propagule size as a function of environmental uncertainty for tree frogs. American Naturalist 117:724–737.

                                                                                                                                                                                                DOI: 10.1086/283755Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                Empirical. Studies differences in egg size in frogs living in temporary (and therefore fluctuating) pools of water versus permanent pools. Finds support for the prediction in Capinera 1979 that fluctuating environments may select for a range of offspring sizes. Available online for purchase or by subscription.

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                                                                                                                                                                                                • Real, Leslie A., and Stephen Ellner. 1992. Life history evolution in stochastic environments: A graphical mean-variance approach. Ecology 73.4: 1227–1236

                                                                                                                                                                                                  DOI: 10.2307/1940671Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                  Theoretical. Analyzes the effect of variation in environmental parameters upon optimal growth and reproduction strategies. Dense and mathematical. Available online for purchase or by subscription.

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                                                                                                                                                                                                  • Schaffer, William M. 1974b. Optimal reproductive effort in fluctuating environments. American Naturalist 120:787–815.

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                                                                                                                                                                                                    Theoretical. Relaxes the previous simplifying assumption that environments are constant. Explores the effects of environmental variability affecting survival before and after breeding. Notes that these result in opposite predicted effects. Fairly dense. Available online for purchase or by subscription.

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                                                                                                                                                                                                    Other Ecological Factors

                                                                                                                                                                                                    Two other factors that clearly affect the evolution of reproductive allocation are crowding and chemical toxicity. Broadly speaking, crowding is predicted to have similar evolutionary consequences to those observed under high adult mortality. Hassall, et al. 2005 (cited under Population Variation) shows that populations of woodlice reared in crowded conditions produced smaller, but not more, offspring. Coulson, et al. 2004 examines the effects of a natural experiment changing population density in a red deer population. The authors discuss the extended effects upon a red deer population released from culling, from changes in reproductive allocation up to population dynamics. Posthuma, et al. 1993 shows that cadmium-adapted springtails (primitive insects) evolved to reproduce earlier and grow faster, but clutch sizes were not affected. Similar adaptations were found in Tranvik, et al. 1993, although the authors found that metal concentrations also reduced reproductive rates. In frog populations raised in acidic breeding sites, clutch size increased and total reproductive output decreased, and the egg size/clutch size trade-off was stronger than controls raised in less acidic sites (Räsänen, et al. 2008).

                                                                                                                                                                                                    • Coulson, Tim, Fiona Guinness, Josephine Pemberton, and Timothy Clutton-Brock. 2004. The demographic consequences of releasing a population of red deer from culling. Ecology 85.2: 411–422.

                                                                                                                                                                                                      DOI: 10.1890/03-0009Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                      Empirical/experimental. The red deer population on the Isle of Rhum, Scotland, was released from hunting pressure in 1972—a natural experiment that the authors use to model long-term changes associated with this change in population density. Available online for purchase or by subscription.

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                                                                                                                                                                                                      • Posthuma, Leo, Rudo A. Verweij, Budi Widianarko, and Cor Zonneveld. 1993. Life-history patterns in metal-adapted Collembola. Oikos 67.2: 235–249.

                                                                                                                                                                                                        DOI: 10.2307/3545468Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                        Empirical/experimental. Life history patterns of individuals from metal-adapted populations were different from those in normal environments, and the authors calculate that natural selection is sufficient to explain the divergence patterns. Available online for purchase or by subscription.

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                                                                                                                                                                                                        • Räsänen, Katja, Fredrik Söderman, Anissi Laurila, and Juha Merilä. 2008. Geographic variation in maternal investment: Acidity affects egg size and fecundity in Rana arvalis. Ecology 89:2553–2562.

                                                                                                                                                                                                          DOI: 10.1890/07-0168.1Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                          Experimental. Comprehensive and well-written study of effects of acidity of rearing conditions upon divergence in life history parameters in frog populations. Available online for purchase or by subscription.

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                                                                                                                                                                                                          • Tranvik, Lena, Göran Bengtsson, and Sten Rundgren. 1993. Relative abundance and resistance traits of two Collembola species under metal stress. Journal of Applied Ecology 30.1: 43–52.

                                                                                                                                                                                                            DOI: 10.2307/2404269Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                            Experimental. Compares life history and reproductive allocation under different levels of metal concentrations in Collembola collected from populations adapted to high metal concentrations and nonadapted populations, respectively. Metals caused reduced growth and reproduction, but adapted populations were more resistant than nonadapted populations. Available online for purchase or by subscription.

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                                                                                                                                                                                                            Within-Individual Variation

                                                                                                                                                                                                            Individual organisms usually carry adaptations that allow them to detect and respond appropriately to environmental variation or fluctuation (see Houston and McNamara 1999). Reproductive allocation is no different and organisms are expected to alter patterns of reproductive allocation in response to a changing environment (reviewed at length in Giesel 1976). On the one hand, animals can facultatively adjust their reproductive allocation on each breeding attempt to suit prevailing conditions, i.e., a given individual can choose one of a set of life history tactics, depending on a range of conditions (Dunbar 1983). On the other hand, young animals can also choose different developmental options based upon cues giving information about how the environment will be in future (known as phenotypic plasticity). Phenotypic plasticity is discussed separately (see Phenotypic Plasticity).

                                                                                                                                                                                                            • Dunbar, R. I. M. 1983. Life history tactics and alternative strategies of reproduction. In Mate choice. Edited by Patrick Bateson, 423–447. Cambridge, UK: Cambridge Univ. Press.

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                                                                                                                                                                                                              Theoretical. Dunbar formulates a theory of the evolution of alternative reproductive options available to a (male) individual, combining these ideas with contemporary life history literature.

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                                                                                                                                                                                                              • Giesel, James T. 1976. Reproductive strategies as adaptations to life in temporally heterogeneous environments. Annual Review of Ecology and Systematics 7:57–79.

                                                                                                                                                                                                                DOI: 10.1146/ Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                Review. Long and very detailed review of variation in reproductive (i.e., life history) strategies, covering many of the ideas explored throughout this article, with a focus on alternative strategies available to cope with a changing environment. Available online for purchase or by subscription.

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                                                                                                                                                                                                                • Houston, Alasdair I., and John M. McNamara. 1999. Models of adaptive behaviour: An approach based on state. Cambridge, UK: Cambridge Univ. Press.

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                                                                                                                                                                                                                  Heavily mathematical book addressing theoretical bases behind alternative tactics and the time-courses of decisions made by animals based on their current state. Chapter 8 deals with state-dependent life history theory, but it is recommended only for those who are mathematically orientated.

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                                                                                                                                                                                                                  Quality of Breeding Partner

                                                                                                                                                                                                                  Individuals are expected to invest more when breeding with a good quality partner (providing the partner’s “quality” affects offspring fitness). This was first proposed by Nancy Burley in her work on zebra finches (Burley 1986). This and other models and evidence are reviewed in Sheldon 2000. Most evidence to date comes from birds (see, e.g., Burley 1986; Uller, et al. 2005), but studies have shown similar results in fish (Kolm 2001, Loscatello and Neat 2005) and frogs (Reyer, et al. 1999).

                                                                                                                                                                                                                  • Burley, Nancy. 1986. Sexual selection for aesthetic traits in species with biparental care. American Naturalist 127.4: 415–445.

                                                                                                                                                                                                                    DOI: 10.1086/284493Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                    Experimental. Classic study of zebra finches by Nancy Burley in which she lays out the differential allocation hypothesis, namely, the idea that individuals may be selected to invest in reproduction to different degrees depending on the quality of the mate with whom they are currently reproducing. Available online for purchase or by subscription.

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                                                                                                                                                                                                                    • Kolm, Niclas. 2001. Females produce larger eggs for large males in a paternal mouth brooding fish. Proceedings of the Royal Society B: Biological Sciences 268:2229–2234.

                                                                                                                                                                                                                      DOI: 10.1098/rspb.2001.1792Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                      Experimental. First test of the differential allocation hypothesis in fish, where male parental care is often important for offspring survival. Niclas Kolm finds that females lay larger (but not more) eggs when they are breeding with larger males.

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                                                                                                                                                                                                                      • Loscatello, Lisa, and Francis C. Neat. 2005. Reproductive allocation in Aidablennius sphynx (Teleostei, Blenniidae): Females lay more eggs faster when paired with larger males. Journal of Experimental Zoology Part A: Comparative Experimental Biology 303A.10: 922–926.

                                                                                                                                                                                                                        DOI: 10.1002/jez.a.204Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                        Experimental. Short and readable study showing that females invest more in reproduction when paired with large males. Available online for purchase or by subscription.

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                                                                                                                                                                                                                        • Reyer, Heinz-Ulrich, Gerhard Frie, and Christian Som. 1999. Cryptic female choice: Frogs reduce clutch size when amplexed by undesired males. Proceedings of the Royal Society B: Biological Sciences 266:2101–2107.

                                                                                                                                                                                                                          DOI: 10.1098/rspb.1999.0894Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                          Experimental. In a neat experiment, the authors show that female water frogs lay fewer eggs when breeding with hybrid males rather than purebred males of their own species. The authors call this cryptic female choice—a term from the large literature on postcopulatory sexual selection—but this term and differential allocation are essentially the same concept.

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                                                                                                                                                                                                                          • Sheldon, Ben C. 2000. Differential allocation: Tests, mechanisms and implications. Trends in Ecology & Evolution 15.10: 397–402.

                                                                                                                                                                                                                            DOI: 10.1016/S0169-5347(00)01953-4Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                            Review. Short, engaging and highly readable review of the theory and evidence behind the differential allocation hypothesis. Recommended starting point for readers interested in this topic. Available online for purchase or by subscription.

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                                                                                                                                                                                                                            • Uller, Tobias, Johan Eklöf, and Sofia Andersson. 2005. Female egg investment in relation to male sexual traits and the potential for transgenerational effects in sexual selection. Behavioral Ecology and Sociobiology 57:584–590.

                                                                                                                                                                                                                              DOI: 10.1007/s00265-004-0886-2Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                              Experimental. The authors find that egg sizes laid by female Chinese quail vary with the size of male sexual ornaments, and also with male testis size. They show that the effect may be mediated by hormones added to eggs by females. Available online for purchase or by subscription.

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                                                                                                                                                                                                                              The amount and composition of food resources have a profound effect upon decisions made by individuals about reproductive allocation because they partly determine the budget available to an organism. For example, female reindeer that foraged naturally allocated less to their calves the following year than females fed supplementary food (Bårdsden, et al. 2008). In Daphnia, mothers raised at low food availability had smaller clutches of larger offspring than lineages raised on plentiful food (Gliwicz and Guisande 1992; see also Burns 1995, cited under Other Ecological Factors), although Tessier and Consolati 1991 finds complex results in a similar experiment on Daphnia. Female zebra finches on high-quality diets lay both more eggs and larger eggs than those on low-quality diets (Williams and Christians 2003). Given a choice of diets, flies will choose the diet that maximizes lifetime egg production, while diets imbalanced toward protein or carbohydrate shift allocation toward faster egg-laying or longer life, respectively (Lee, et al. 2008). Despite these findings, studies of life history have been generally slow to integrate foraging ecology into their models (Boggs 1992, cited under The Search for Unifying Principles).

                                                                                                                                                                                                                              • Bårdsden, Bård–Jørgen, Per Fauchald, Torkild Tveraa, et al. 2008. Experimental evidence of a risk-sensitive reproductive allocation in a long-lived mammal. Ecology 89:829–837.

                                                                                                                                                                                                                                DOI: 10.1890/07-0414.1Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                                Experimental. The authors show season-to-season differences in reproductive allocation in response to changing patterns of feeding in reindeer. Supplementary feeding resulted in females allocating more resources to their next-born calf. Available online for purchase or by subscription.

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                                                                                                                                                                                                                                • Gliwicz, Z. Maciej, and Castor Guisande. 1992. Family planning in Daphnia: resistance to starvation in offspring born to mothers grown at different food levels. Oecologia 91:463–467.

                                                                                                                                                                                                                                  DOI: 10.1007/BF00650317Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                                  Experimental. Daphnia mothers grown at high-food conditions produced larger clutches of smaller eggs than counterparts grown at low-food availability.Available online for purchase or by subscription.

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                                                                                                                                                                                                                                  • Lee, Kwang Pum, Stephen J. Simpson, Fiona J. Clissold, et al. 2008. Lifespan and reproduction in Drosophila: New insights from nutritional geometry. Proceedings of the National Academy of Sciences of the United States of America 105:2498–2503.

                                                                                                                                                                                                                                    DOI: 10.1073/pnas.0710787105Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                                    Experimental. In a classic paper that is already highly cited, the authors use a clever new technique called “nutritional geometry” to investigate the effect of nutrient balances on ageing. They find no support for the “caloric restriction” hypothesis, instead interpreting their result in terms of ratios of macronutrients in the diet.

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                                                                                                                                                                                                                                    • Tessier, Alan J., and Nina L. Consolati. 1991. Resource quantity and offspring quality in Daphnia. Ecology 72.2: 468–478.

                                                                                                                                                                                                                                      DOI: 10.2307/2937188Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                                      The authors conduct a thorough study of variation in offspring characteristics across genotypes and species in relation to food availability. Species showed different reactions to varying food availability and the response in terms of offspring mass was different from the response in terms of offspring length. Thus, individual responses to differing food availability are not simple. Available online for purchase or by subscription.

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                                                                                                                                                                                                                                      • Williams, Tony D., and Julian K. Christians. 2003. Experimental dissociation of the effects of diet, age and breeding experience on primary reproductive effort in zebra finches Taeniopygia guttata. Journal of Avian Biology 34.4:379–386.

                                                                                                                                                                                                                                        DOI: 10.1111/j.0908-8857.2003.03080.xSave Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                                        Experimental. The authors use a clever experimental design to disentangle several possible effects upon reproductive allocation in zebra finches. The effects of diet and age were distinct and different. Available online for purchase or by subscription.

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                                                                                                                                                                                                                                        Other Ecological Factors

                                                                                                                                                                                                                                        Other factors that might affect season-to-season patterns of reproductive allocation are crowding, temperature, and the availability of resources such as nest sites. Burns 1995 shows complex effects of crowding in water fleas: As crowding increased, egg sizes reached a peak and then declined, but overall reproductive allocation remained steady. Female zebra finches incurred substantially higher costs of egg production when housed at low temperatures, and reproductive output declined (Salvante, et al. 2007), but, similarly, low temperature conditions did not affect seasonal testis growth in male great tits (Caro and Visser 2009). In hermit crabs from the Bay of Panama, growth and reproductive allocation are both limited by the availability of empty shells in which to live: Crabs given an artificially poor supply of shells reproduce at smaller sizes, more frequently, and with larger clutches (Bertness 1980).

                                                                                                                                                                                                                                        Phenotypic Plasticity

                                                                                                                                                                                                                                        Animals can also use environmental cues to anticipate future conditions by switching to alternative patterns of growth and development (reviewed in Via, et al. 1995), a phenomenon known as phenotypic plasticity. Many workers have predicted, and identified, plasticity in reproductive allocation (see, for example, Caswell 1983, Stearns and Koella 1986, and Lloyd 1987; see also Hassall, et al. 2005, under Population-Level Variation). For a more modern perspective on phenotypic plasticity, see Piersma and Drent 2003.

                                                                                                                                                                                                                                        • Caswell, Hal. 1983. Phenotypic plasticity in life-history traits: Demographic effects and evolutionary consequences. American Zoologist 23:33–46.

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                                                                                                                                                                                                                                          Review/theoretical. Excellent review of contemporary evidence for phenotypic plasticity in life history variables; constructs models to set these in an adaptive context.Available online for purchase or by subscription.

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                                                                                                                                                                                                                                          • Lloyd, David G. 1987. Selection of offspring size at independence and other size-versus-number strategies. American Naturalist 129:800–817.

                                                                                                                                                                                                                                            DOI: 10.1086/284676Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                                            Theoretical. Modifies the model in Smith and Fretwell 1974 (cited under Offspring Size and Number), incorporating terms to accommodate various different kinds of environments. Predicts the emergence of alternative strategies to respond to environmental changes.Available online for purchase or by subscription.

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                                                                                                                                                                                                                                            • Piersma, Theunis, and Jan Drent. 2003. Phenotypic flexibility and the evolution of organismal design. Trends in Ecology & Evolution 18:228–233.

                                                                                                                                                                                                                                              DOI: 10.1016/S0169-5347(03)00036-3Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                                              Review. Updated review of phenotypic plasticity drawing together concepts from recent reviews and books, setting it in the context of other forms of “phenotypic flexibility.” Short and readable. Available online for purchase or by subscription.

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                                                                                                                                                                                                                                              • Stearns, Stephen C., and Jacob C. Koella. 1986. The evolution of phenotypic plasticity in life-history traits: Predictions of reaction norms for age and size at maturity. Evolution 40.5: 893–913.

                                                                                                                                                                                                                                                DOI: 10.2307/2408752Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                                                Review/theoretical. Based on a review of contemporary evidence, the authors construct several life history models based on different assumptions to predict how age and size at maturity, with associated changes in reproductive allocation, should change with changing environments. Illustrated using evidence from well-known systems such as red deer. Available online for purchase or by subscription.

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                                                                                                                                                                                                                                                • Via, Sara, Richard Gomulkiewicz, Gerdien de Jong, et al. 1995. Adaptive phenotypic plasticity: Consensus and controversy. Trends in Ecology & Evolution 10:212–217.

                                                                                                                                                                                                                                                  DOI: 10.1016/S0169-5347(00)89061-8Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                                                  Review. Short and accessible review of the contemporary issues surrounding phenotypic plasticity and the concept of “reaction norms.” For an updated review, see Piersma and Drent 2003. Available online for purchase or by subscription.

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                                                                                                                                                                                                                                                  Predation Risk

                                                                                                                                                                                                                                                  Environmental cues that signal a predator is present often cause life history shifts in their prey, which typically mature earlier and grow faster as a result. This has been shown in snails (Crowl and Covich 1990) and water fleas (Weider and Pijanowska 1993), and it has been extensively studied in the tadpoles of various amphibians. Tollrian and Harvell 1999 contains a review of the relevant literature. Relyea 2005 expands the study of predator-induced developmental shifts to a quantitative-genetic level, investigating the heritability of plastic responses.

                                                                                                                                                                                                                                                  • Crowl, Todd A., and Alan P. Covich. 1990. Predator-induced life-history shifts in a freshwater snail. Science 247:949–951.

                                                                                                                                                                                                                                                    DOI: 10.1126/science.247.4945.949Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                                                    Experimental. Classic, very short, and easily accessible article describing a neat experiment. Snails altered their patterns of growth and reproduction in response to juvenile-specific predation by crayfish. Available online for purchase or by subscription.

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                                                                                                                                                                                                                                                    • Relyea, R. A.2005. The heritability of inducible defenses in tadpoles. Journal of Evolutionary Biology 18.4: 856–866.

                                                                                                                                                                                                                                                      DOI: 10.1111/j.1420-9101.2005.00882.xSave Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                                                      Experimental. Assesses the “evolvability” (i.e., heritability) of plastic responses to predation and found substantial genetic variation in the capacity for phenotypic plasticity, but also finding that heritability is variable for different traits. Development of survival-related morphological traits in the predator environment was associated with growth and development.

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                                                                                                                                                                                                                                                      • Tollrian, Ralph, and C. Drew Harvell. 1999. The ecology and evolution of inducible defenses. Princeton, NJ: Princeton Univ. Press.

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                                                                                                                                                                                                                                                        Edited volume of articles about induced defenses. Covers plants as well as animals. Chapter 17 is a useful review summarizing allocation trade-offs associated with predator-induced developmental plasticity.

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                                                                                                                                                                                                                                                        • Weider, Lawrence J., and Joanna Pijanowska. 1993. Plasticity of Daphnia life histories in response to chemical cues from predators. Oikos 67.3: 385–392.

                                                                                                                                                                                                                                                          DOI: 10.2307/3545351Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                                                          Experimental. Daphnia that were experimentally exposed to water from predator-infested lakes matured earlier and at a smaller size. Available online for purchase or by subscription.

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                                                                                                                                                                                                                                                          The effect of starvation, or a restricted diet, upon patterns of reproductive allocation is well documented and is reviewed in some detail in Kirkwood and Shanley 2005. For example, in mice, experimentally restricting the diet results in increased activity of cell maintenance and repair processes and in a reduction or complete halt to fertility (Shanley and Kirkwood 2000, Merry 2004). However, a highly regarded recent paper, Lee, et al. 2008 (cited under Within-Individual Variation: Food), provides a critique of a purely quantity-centric view of this result, arguing that, instead, it is the nutritional composition of the diet that effects a shift in resource allocation.

                                                                                                                                                                                                                                                          Maternal Effects and the Thrifty Phenotype

                                                                                                                                                                                                                                                          Phenotypic plasticity is often affected (or deliberately induced) via maternal effects, or the physical, nongenetic influence of the mother upon the offspring. Bateson, et al. 2004 provides a review. The environment in the womb, for example, can be a “weather forecast” of the conditions an offspring can expect to experience as an adult, and, therefore, what kind of developmental pathway to take. An early and prescient study, Huck, et al. 1987 finds that hamsters raised on restricted diets produce daughters that allocate less to reproduction, that is, have smaller litters and relatively fewer sons—even though the daughters themselves were raised on unrestricted diets. However, when environments change from those experienced in the womb, the “weather forecast” can be wrong. This can lead to a thrifty phenotype, or an offspring that emerges prepared for a harsh environment (i.e., allocating less to reproduction and more to survival) when, in fact, the environment is benign. For examples of the thrifty phenotype hypothesis, especially as applied to humans, see many recent papers by Jonathan Wells (e.g., Wells 2007).


                                                                                                                                                                                                                                                          Males are relatively neglected in the literature on reproductive allocation. Males experience many similar trade-offs as those experienced by females: for example, the classic study of Partridge and Farquhar 1981 shows that reproduction reduces the life span of male fruit flies. Recent evidence confirms that males also incur a size/number trade-off among sperm (Immler, et al. 2011). However, since males tend to compete for females, reproductive allocation in males is usually less geared toward improving offspring survival and more toward winning contests with other males, whether directly in fights or by competing for the attention of females. This results in interesting differences in the trade-offs experienced by males and females. In red deer, Yoccoz, et al. 2002 finds that males allocate resources to reproduction differently according to whether or not they are reproductively dominant (the “alpha male”): dominant males accordingly suffer much greater costs of reproduction per attempt than do subordinate males. The same authors find no support for the terminal investment hypothesis in male red deer. Hunt, et al. 2004 demonstrates a clear reproduction/survival trade-off in male crickets investing in different reproductive strategies. Gomendio and Roldan 1991 (and references therein) discusses the evidence that increased levels of sperm competition (i.e., sperm of different males competing within the reproductive tract of a promiscuous female) select for increased allocation to testes, increased numbers of sperm, and also increased size of sperm. The animal with the largest sperm is a species of fruit fly, Drosophila bifurca, whose sperm are more than twenty times its body length, but which can produce only a few sperm (tens) per copulation (Pitnick 1996). Vahed, et al. 2010 recently awarded the world record for investment in testis size to the tuberous bush cricket Platycleis affinis, whose testes make up 14 percent of its body weight (in a human being the equivalent would be testes the size of two human heads!), and which allow it to copulate several times in rapid succession.


                                                                                                                                                                                                                                                          It is frequently in one animal’s interest to manipulate the reproductive allocation of another. The result is a conflict of interest over reproductive allocation that can result in suboptimal investment by either or both individuals. Such conflicts are widely studied throughout behavioral ecology, but, in the case of reproductive allocation, the most common and well-studied cases are Parent-Offspring Conflict and Sexual Conflict (although see also Social Species: Conflict.

                                                                                                                                                                                                                                                          Parent-Offspring Conflict

                                                                                                                                                                                                                                                          The literature on parent-offspring conflict is enormous. Perhaps the most important paper is Trivers 1974, which outlines the main principles. Mock and Forbes 1992 provides an excellent early review, and a more thorough updated treatment can be found in Mock 2004. Parent-offspring conflict is considered to have a strong effect upon the life histories of parents, particularly where parental care is involved and where siblings compete for resources provided by parents (Godfray, et al. 1991 [cited under Offspring Size and Number], Godfray and Parker 1992) or where mothers and offspring are in prolonged contact, for example, through a placenta (Crespi and Semeniuk 2004). Brown boobies, for example, always have a brood of two chicks of which one is without exception killed by the other, a result that appears to be suboptimal both for parents and offspring (Anderson 1990). Forbes 1993 wonders whether offspring are indeed frequently at odds with parents, given the negotiating position of each.

                                                                                                                                                                                                                                                          • Anderson, David J. 1990. Evolution of obligate siblicide in boobies. 2: food limitation and parent-offspring conflict. Evolution 44:2069–2082.

                                                                                                                                                                                                                                                            DOI: 10.2307/2409616Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                                                            Experimental. By experimentally preventing siblicide in brown boobies, Anderson calculates that both parents and offspring would benefit if one offspring did not always kill the other. He suggests possible resolutions to this paradox. Available online for purchase or by subscription.

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                                                                                                                                                                                                                                                            • Crespi, Bernard, and Christina Semeniuk. 2004. Parent-offspring conflict in the evolution of vertebrate reproductive mode. American Naturalist 163:635–653.

                                                                                                                                                                                                                                                              DOI: 10.1086/382734Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                                                              Review/theoretical. Advances the idea that parent-offspring conflict over reproductive allocation has been a major driver behind the evolution of the placenta, a highly novel and interesting idea. Available online for purchase or by subscription.

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                                                                                                                                                                                                                                                              • Forbes, L. Scott. 1993. Avian brood reduction and parent-offspring “conflict.” American Naturalist 142:82–117.

                                                                                                                                                                                                                                                                DOI: 10.1086/285530Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                                                                Theoretical. Models considering the negotiating position of parents and offspring and whether this resolves the idea of parent-offspring conflict. Suggests that, because behavioral decisions by parents and offspring are not made simultaneously, both parties end up with restricted options and there is often little potential for conflict. Available online for purchase or by subscription.

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                                                                                                                                                                                                                                                                • Godfray, H. C. J., and G. A. Parker. 1992. Sibling competition, parent-offspring conflict and clutch size. Animal Behaviour 43.3: 473–490.

                                                                                                                                                                                                                                                                  DOI: 10.1016/S0003-3472(05)80106-XSave Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                                                                  Theoretical. Expands and extends the model of Godfray et al 1991 (cited under Offspring Size and Number), exploring the effects of different kinds of sibling competition upon the fitness of parents and proposing ways of testing between them. Available online for purchase or by subscription.

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                                                                                                                                                                                                                                                                  • Mock, Douglas W. 2004. More than kin and less than kind: The evolution of family conflict. Cambridge, MA: Belknap.

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                                                                                                                                                                                                                                                                    Very thorough and well-written popular account of theory surrounding parent-offspring conflict. Recommended reading for any student interested in this topic.

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                                                                                                                                                                                                                                                                    • Mock, Douglas W., and L. Scott Forbes. 1992. Parent-offspring conflict: A case of arrested development. Trends in Ecology & Evolution 7:409–413.

                                                                                                                                                                                                                                                                      DOI: 10.1016/0169-5347(92)90022-4Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                                                                      Review. Comprehensive and highly cited review of the state of parent-offspring conflict research at the time. Very readable even its treatment of mathematical models. Available online for purchase or by subscription.

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                                                                                                                                                                                                                                                                      • Trivers, Robert L. 1974. Parent-offspring conflict.American Zoologist 14:249–264.

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                                                                                                                                                                                                                                                                        Theoretical. Seminal, highly influential work describing disagreement between parents and offspring over distribution and amount of parental care. Mothers are related equally to all offspring so are selected to invest equally; each recipient offspring is most closely related to itself, so is selected to manipulate parent(s) into investing in it alone at the expense of siblings. Available online for purchase or by subscription.

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                                                                                                                                                                                                                                                                        Sexual Conflict

                                                                                                                                                                                                                                                                        Sexual conflict is introduced in a very readable way in Krebs and Davies 1993 (cited under General Overviews), and explored in much more depth in Arnqvist and Rowe 2005. Sexual conflict can increase the costs of reproduction in the short term—and therefore push both males and females away from their optimal patterns of reproductive allocation (Promislow 2003). In one celebrated example, in fruit flies, a very clever experimental design allowed experimenters to prevent females from evolving to respond to male manipulation. When females were forced for many generations to mate monogamously with random males, but males were allowed to choose females freely, female life span declined and females remated less frequently, investing more than was optimal in the current brood (Holland and Rice 1999). The reverse effect is seen in males: Males forced to mate monogamously show reduced courtship and are less effective in reducing female remating. This effect is brought about by toxic chemicals in the males’ semen that force females to lay more eggs than is optimal (Wolfner, et al. 1997).

                                                                                                                                                                                                                                                                        Social Species

                                                                                                                                                                                                                                                                        In eusocial species, such as naked mole-rats, ants and bees, which can be viewed as functioning as a “superorganism” (Hölldobler and Wilson 2008), the problem of reproductive allocation is just as relevant. In this case, though, “allocation to reproduction” equates to producing sexual offspring (queens and drones) whereas allocation to somatic tissue equates to producing reproductively suppressed workers. Biology of social colonies is dealt with extensively in Wilson 1971. Many of the above principles of reproductive allocation apply to social colonies (Michener 1964). For example, in clonal colonies (where all members are genetically identical), just as in unitary animals, the optimal allocation pattern is predicted to be to concentrate on workers for a set period of time and then concentrate exclusively on sexuals (Macevitz and Oster 1976, Oster and Wilson 1978). Also, ant colonies produce sexuals according to a roughly fixed proportion of their worker biomass, and, among the sexuals they produce, incur a size/number trade-off (Shik 2008). Karsai and Wenzel 1998 proposes that large colony sizes, which are associated with low per capita reproduction, may buffer against environmental changes—much as large body size does in unitary organisms. The response of social organisms to changing environments is often similar to those of unitary organisms as well. For example, when supplemented with food, ant colonies tend to allocate a higher proportion of available resources to reproduction (Herbers and Banschbach 1998, cited under The Problem of Variation).

                                                                                                                                                                                                                                                                        • Hölldobler, Bert, and Edward O. Wilson. 2008. The superorganism: The beauty, elegance, and strangeness of insect societies. New York: W. W. Norton.

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                                                                                                                                                                                                                                                                          The successor to these authors’ celebrated classic The Ants (Cambridge, MA: Belknap, 1990), this volume incorporates much social insect research done over the past few decades, including an exploration of the parallels among unitary, colonial, and social animals. The authors are among the most vigorous proponents of multilevel selection (i.e., group selection) and point out many similarities between colonies and multicellular bodies.

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                                                                                                                                                                                                                                                                          • Karsai, István K., and John W. Wenzel. 1998. Productivity, individual-level and colony-level flexibility, and organization of work as consequences of colony size. Proceedings of the National Academy of Sciences of the United States of America 95.15: 8665–8669.

                                                                                                                                                                                                                                                                            DOI: 10.1073/pnas.95.15.8665Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                                                                            Empirical/comparative. The authors consider why large colonies exist when per capita reproduction is tiny, proposing that larger colonies provide better buffers against environmental changes.

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                                                                                                                                                                                                                                                                            • Macevicz, S., and G. Oster. 1976. Modeling social insect population. II. Optimal reproductive strategies in annual eusocial insect colonies. Behavioral Ecology and Sociobiology 1.3: 265–282.

                                                                                                                                                                                                                                                                              DOI: 10.1007/BF00300068Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                                                                              Theoretical. The authors construct a model of optimal allocation to reproduction versus growth in social insects, concluding much the same as Sibly et al. 1985 (cited in Determinate versus Indeterminate Growth), that the optimal strategy is to grow for a set period and then concentrate exclusively on reproduction. Available online for purchase or by subscription.

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                                                                                                                                                                                                                                                                              • Michener, Charles D. 1964. Reproductive efficiency in relation to colony size in hymenopterous societies. Insectes Sociaux 11:317–341.

                                                                                                                                                                                                                                                                                DOI: 10.1007/BF02227433Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                                                                                Comparative/theoretical. Michener elucidates and discusses allometric scaling relationships in insect societies. Available online for purchase or by subscription.

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                                                                                                                                                                                                                                                                                • Oster, George F., and Edward O. Wilson. 1978. Caste and ecology in social insects. Princeton, NJ: Princeton Univ. Press.

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                                                                                                                                                                                                                                                                                  Textbook reviewing contemporary thought on the evolution of castes. Chapter 6 covers reproductive allocation in social species. As with Wilson 1971, should be balanced against more modern approaches to the subject.

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                                                                                                                                                                                                                                                                                  • Shik, J. Z. 2008. Ant colony size and the scaling of reproductive effort. Functional Ecology 22.4: 674–681.

                                                                                                                                                                                                                                                                                    DOI: 10.1111/j.1365-2435.2008.01428.xSave Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                                                                                    Comparative. Shik finds that reproductive effort scales more steeply in ants than in unitary organisms, but his results also suggests that ants and unitary organisms experience much the same size-number trade-off among their (sexual) offspring.

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                                                                                                                                                                                                                                                                                    • Wilson, Edward O. 1971. The insect societies. Cambridge, MA: Belknap.

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                                                                                                                                                                                                                                                                                      Original classic work on insect societies (see Hölldobler and Wilson 2008 for an update). Very easy to read for pleasure, fascinating, and recommended for any student of this topic; however, it should be remembered that ideas have moved on substantially in this fast-moving field and readers should consult work done more recently by a range of authors for an idea of the current state of the art.

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                                                                                                                                                                                                                                                                                      Because social colonies are nonunitary, and colony members are not genetically identical, the potential also exists for evolutionary effects not seen in other organisms. For example, either one of the queen or the workers can invest in sexuals earlier than the other, leading to conflict over reproductive allocation (Bulmer 1981, Pamilo 1991, Ohtsuki and Tsuji 2009). (The equivalent in a unitary organism would be a conflict between cells in the gonads and the somatic cells such as muscle and bone.) Intriguingly, some ants steal workers from other species as “slaves”; in these species, such conflicts are predicted not to occur, because slaves have no genetic stake in the sexual offspring (Trivers and Hare 1976). Subsequent data have confirmed that slaves have no control over reproductive allocation by slave-makers, but data also reveal complex conflicts between queens and their own (few) slave-maker workers (Herbers and Stuart 1998). Intricate and complex conflicts of interest are also observed within colonies over sex allocation, a topic that is beyond the scope of this article, but the reader is referred to Hölldobler and Wilson 2008 (cited under Social Species) for a review.

                                                                                                                                                                                                                                                                                      • Bulmer, M. G. 1981. Worker-queen conflict in annual social Hymenoptera. Journal of Theoretical Biology 93.1: 239–251.

                                                                                                                                                                                                                                                                                        DOI: 10.1016/0022-5193(81)90068-0Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                                                                                        Theoretical. Builds on the classic model in Trivers and Hare 1976 by incorporating into a formal model the effect of worker-laid male eggs in social insect colonies, and the implications for conflict between queens and workers over who gets to lay the eggs that become reproductive males. Available online for purchase or by subscription.

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                                                                                                                                                                                                                                                                                        • Herbers, Joan M., and Robin J. Stuart. 1998. Patterns of reproduction in slave-making ants. Proceedings of the Royal Society B: Biological Sciences 265:875–887.

                                                                                                                                                                                                                                                                                          DOI: 10.1098/rspb.1998.0373Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                                                                                          Comparative. Provides a rigorous test of the clever prediction in Trivers and Hare 1976 that slave-making ant queens should have exclusive control of reproductive allocation. Finds qualified support for this hypothesis, but also identifies new conflicts in these species that are complex and interesting in their own right. Available online for purchase or by subscription.

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                                                                                                                                                                                                                                                                                          • Ohtsuki, Hisashi, and Kazuki Tsuji. 2009. Reproduction schedule as a cause of worker policing in social Hymenoptera: A dynamic game analysis. American Naturalist 173:747–758.

                                                                                                                                                                                                                                                                                            DOI: 10.1086/598488Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                                                                                            Theoretical. Uses sophisticated modeling to elucidate how worker reproduction and worker “policing” (that is, workers preventing each other reproducing) are affected by the conflicts over colony reproductive allocation, sex allocation, and male production, considered simultaneously. Available online for purchase or by subscription.

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                                                                                                                                                                                                                                                                                            • Pamilo, Pekka. 1991. Evolution of colony characteristics in social insects. II. Number of reproductive individuals. American Naturalist 138:412–433.

                                                                                                                                                                                                                                                                                              DOI: 10.1086/285224Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                                                                                              Theoretical. Constructs models considering the effects of multiple queens, multiple mating by queens, and worker reproduction upon conflicts of interest within social insect colonies. Ohtsuki and Tsuji 2009 provides an updated perspective on this body of work. Available online for purchase or by subscription.

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                                                                                                                                                                                                                                                                                              • Trivers, Robert L., and H. Hare. 1976. Haplodiploidy and the evolution of the social insect. Science 191:249–263.

                                                                                                                                                                                                                                                                                                DOI: 10.1126/science.1108197Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                                                                                                Theoretical/comparative. Classic model predicting a conflict of interest between queens and workers in social colonies over the sex ratio produced by the queen. Predicts, and finds, that workers are ultimately in control of the sex ratio of reproductives raised by the colony. Also cleverly predicts that this conflict should not be evident in slave-making ants. Available online for purchase or by subscription.

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                                                                                                                                                                                                                                                                                                Humans pose a unique set of challenges for life history and reproductive allocation. One challenge in particular is that, as resources get more abundant, human fertility paradoxically declines (the demographic transition). There is ample evidence for this phenomenon but few convincing explanations. Hypotheses for the decline in fertility in more affluent societies have mostly been in terms of humans trying to maximize the wealth inherited by each offspring, although this does not stand up to scrutiny (reviewed in Mace 2000). More likely this is a response to increasing costs of raising a child (Hoem 1992; Conrad, et al. 1996; Mace 2000). A second challenge is that female humans stop investing in reproduction well before the end of their lives (undergo menopause). The evolution of menopause, which humans share only with pilot whales, killer whales, and almost no other animals, has been debated for decades. The “grandmother hypothesis” suggests that investing in fitness of existing offspring (i.e., helping raise grandchildren) brings greater fitness rewards to relatively old women than attempting to have more babies (Williams 1957 [cited under Complexities], Hamilton 1966, Mace 2000). In contrast, the “antagonistic pleiotropy hypothesis” holds that menopause results from the accumulation of many negative effects of traits that have positive effects earlier in life (when they are more important for natural selection) (Williams 1957, Hamilton 1966; Wood, et al. 2001). Neither hypothesis explains why menopause occurs particularly in these groups of animals. It is argued in Johnstone and Cant 2010 that special social structures in these animals may result in the formation of groups of highly related females, thus creating unique conditions that favor the grandmother hypothesis.

                                                                                                                                                                                                                                                                                                • Conrad, Christoph, Michael Lechner, and Welf Werner. 1996. East German fertility after unification: Crisis or adaptation? Population Development Review 22:331–358.

                                                                                                                                                                                                                                                                                                  DOI: 10.2307/2137438Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                                                                                                  Theoretical/empirical. East Germany suffered a fertility crash following reunification; the authors explore possible reasons for this, concluding that mothers were behaving rationally in the face of economic uncertainty. Cited in Mace 2000. Available online for purchase or by subscription.

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                                                                                                                                                                                                                                                                                                  • Hamilton, W. D. 1966. The moulding of senescence by natural selection. Journal of Theoretical Biology 12.1: 12–45.

                                                                                                                                                                                                                                                                                                    DOI: 10.1016/0022-5193(66)90184-6Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                                                                                                    Theoretical. Aging is cast as the accumulation of mutations that have negative effects late in life, but are not removed by natural selection because, by then, the individual has already reproduced. Menopause is described in a similar light. Available online for purchase or by subscription.

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                                                                                                                                                                                                                                                                                                    • Hoem, Jan M. 1992. Social policy and recent fertility change in Sweden. Population Development Review 16:735–748.

                                                                                                                                                                                                                                                                                                      DOI: 10.2307/1972965Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                                                                                                      Theoretical/empirical. Relates a recent rise in fertility among Swedish families to a social policy that rewards births that are closely spaced. Cited in Mace 2000. Available online for purchase or by subscription.

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                                                                                                                                                                                                                                                                                                      • Johnstone, Rufus A., and Michael A. Cant. 2010. The evolution of menopause in cetaceans and humans: The role of demography. Proceedings of the Royal Society B: Biological Sciences 277:3765–3771.

                                                                                                                                                                                                                                                                                                        DOI: 10.1098/rspb.2010.0988Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                                                                                                        Theoretical. The authors conclude that most recent theories of the evolution of menopause, including one they have themselves published, are inadequate to explain observed patterns. They propose an alternative model based on local genetic relatedness patterns that are caused by unique patterns of dispersal (i.e., species-typical male and female movement patterns) in humans and whales.

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                                                                                                                                                                                                                                                                                                        • Mace, Ruth. 2000. Evolutionary ecology of human life history. Animal Behaviour 59:1–10.

                                                                                                                                                                                                                                                                                                          DOI: 10.1006/anbe.1999.1287Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                                                                                                          A thorough and very well-written review of intriguing patterns in human life history that presents challenges to the evolutionary biologist. Covers all stages of the human lifecycle with special emphasis on menopause and the demographic transition. Available online for purchase or by subscription.

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                                                                                                                                                                                                                                                                                                          • Wood, James W., Kathleen A. O’Connor, Darryl J. Holman, et al. 2001. The evolution of menopause by antagonistic pleiotropy. Seattle: Department of Anthropology and Center for Studies in Demography and Ecology, University of Washington.

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                                                                                                                                                                                                                                                                                                            Review/theoretical. A mechanistic treatment of the antagonistic pleiotropy hypothesis for the evolution of menopause, proposing a mechanism to do with the hormonal process of follicular depletion.

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                                                                                                                                                                                                                                                                                                            Other Questions

                                                                                                                                                                                                                                                                                                            The scope of life history research (and the breadth of subjects touched on in the study of reproductive allocation) is enormous, with far too much to cover in detail here. One particularly interesting development, which we have not covered here, is the recent “metabolic theory of ecology” (Brown, et al. 2004), which has deep implications for the effects of body size upon life history evolution and reproductive allocation (Brown and Sibly 2006). Another is the evolution of cooperative breeding, a situation in which some members of a social group forgo all reproduction (i.e., have zero reproductive allocation) and focus instead on helping other members reproduce (typically close relatives). This has a truly enormous literature, but excellent (although bird-centric) overviews can be found in Cockburn 1998 and Koenig and Dickinson 2004. Sober and Wilson 1998 is a well-written, popular treatment of this topic. Yet another is the problem of modes of reproduction: Most models have centered on the most familiar of reproductive modes: (bi)sexual, diploid organisms, and that is what we have concentrated on here. However, reproductive allocation is predicted (and found) to be just as complex in groups with other reproductive modes—for example, Michiels 1998 explores patterns of reproductive allocation in hermaphrodites, where reproductive allocation also involves decisions about what proportion of time to spend as a female.

                                                                                                                                                                                                                                                                                                            • Brown, James H., James F. Gillooly, Andrew P. Allen, Van M. Savage, and Geoffrey B. West. 2004. Toward a metabolic theory of ecology. Ecology 85.7: 1771–1789.

                                                                                                                                                                                                                                                                                                              DOI: 10.1890/03-9000Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                                                                                                              Theoretical. The authors set forth their theory that many processes in ecology, including life history, should be able to be derived from first principles concerning organismal metabolism. Necessarily mathematical but truly astounding in scope, this article is well worth the return for the interested scholar. Available online for purchase or by subscription.

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                                                                                                                                                                                                                                                                                                              • Brown, James H., and Richard M. Sibly. 2006. Life-history evolution under a production constraint. Proceedings of the National Academy of Sciences of the United States of America 103.47: 17595–17599.

                                                                                                                                                                                                                                                                                                                DOI: 10.1073/pnas.0608522103Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                                                                                                                Review. Examines life history evolution in the context of the metabolic theory of ecology (see Brown, et al. 2004). Contains an excellent short review of some of the concepts included in this article from a metabolic perspective and is highly recommended reading.

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                                                                                                                                                                                                                                                                                                                • Cockburn, Andrew. 1998. Evolution of helping behavior in cooperatively breeding birds. Annual Review of Ecological Systems 29.1: 141–177.

                                                                                                                                                                                                                                                                                                                  DOI: 10.1146/annurev.ecolsys.29.1.141Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                                                                                                                  Review. Cockburn’s classic critical review of contemporary thinking about the selective pressures contributing to the evolution of cooperative breeding in birds. Available online for purchase or by subscription.

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                                                                                                                                                                                                                                                                                                                  • Koenig, Walter D., and Janis L. Dickinson. 2004. Ecology and evolution of cooperative breeding in birds. Cambridge, UK: Cambridge Univ. Press.

                                                                                                                                                                                                                                                                                                                    DOI: 10.1017/CBO9780511606816Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                                                                                                                    An edited volume containing an updated and comprehensive view of cooperative breeding in birds, providing a modern perspective on many of the topics discussed in Cockburn 1998.

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                                                                                                                                                                                                                                                                                                                    • Michiels, Nico K. 1998. Mating conflicts and sperm competition in simultaneous hermaphrodites. In Sperm competition and sexual selection. Edited by Tim R. Birkhead and Anders P. Møller, 219–254. London: Academic Press.

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                                                                                                                                                                                                                                                                                                                      Review. A long and well-written review summarizing the life history compromises involved in being a hermaphrodite, as well as sexual selection and conflict over mating.

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                                                                                                                                                                                                                                                                                                                      • Sober, Elliott, and David Sloan Wilson. 1998. Unto others: The evolution and psychology of unselfish behaviour. Cambridge, MA: Harvard Univ. Press.

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                                                                                                                                                                                                                                                                                                                        A popular and well-written volume that explains most of the evolutionary theory surrounding the evolution of altruistic behavior, including reproductive altruism and cooperative breeding. An excellent starting point for those interested in this topic.

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