Reproductive Allocation in Animals

by

James Gilbert

• LAST REVIEWED: 23 May 2012
• LAST MODIFIED: 23 May 2012
• DOI: 10.1093/obo/9780199830060-0064

Introduction

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.

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

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

  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.

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

  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.

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

  DOI: 10.1017/CBO9780511608483

  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.

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

  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.

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

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

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

  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.

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

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

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

  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.