In This Article Sexual Size Dimorphism

  • Introduction
  • Historical Overview
  • Morphology, Body Size, and Sexual Size Dimorphism (SSD)
  • Rensch’s Rule
  • Explanations for Rensch’s Rule
  • The Empirical Measurement of Dimorphism: Statistical Issues
  • Ultimate (Evolutionary) Causes of Dimorphism: The Differential Equilibrium Model
  • Ultimate (Evolutionary) Causes of Dimorphism: The Dimorphic Ecological Niche Model
  • Proximate Causes of Dimorphism: Development
  • Proximate Causes of Dimorphism: Physiology
  • Proximate Causes of Dimorphism: Genetics

Evolutionary Biology Sexual Size Dimorphism
Wolf Blanckenhorn
  • LAST MODIFIED: 27 June 2018
  • DOI: 10.1093/obo/9780199941728-0110


Sexual reproduction is common in nature, especially among larger organisms (animals and plants), albeit not ubiquitous. Although the evolution of sex remains somewhat enigmatic to this day, an obvious consequence is the existence of, typically, two types of gametes (eggs and sperm, i.e., anisogamy) and their bearers (females and males). This raises the very general and interesting evolutionary question of why and how the sexes can evolve independently despite sharing a large part of their genome. In many species the two sexes indeed look substantially different, giving rise to the prominent phenomenon of sexual dimorphism, of which a special case is sexual size dimorphism (SSD), the difference between the sexes in body size and associated morphological traits, the focus of this article. The behavioral, ecological, and evolutionary causes and consequences of sexual dimorphism are conspicuous and therefore a central focus of biological research.

Historical Overview

Strictly speaking, sexual dimorphism refers to differences in morphology between the sexes, thus primarily concerning secondary sexual traits and not the primary sexual traits defining the sexes (eggs, sperm, gonads). However, dimorphism frequently encompasses differences in non-morphological traits such as coloration, but also life history characters such as development time, although these are often referred to by other specific terms: thus Badyaev and Hill 2003 talks about sexual dichromatism, and Stamps and Krishnan 1997 refers to sexual bimaturity, respectively. Interest in sexual dimorphism dates all the way back to Darwin 1981 (originally published 1871). Earlier reviews, such as Hedrick and Temeles 1989 or Fairbairn 1997, clearly reveal that dimorphism research is mainly focused on animals, most of which have separate sexes (i.e., are gonochoristic). However, Barret and Hough 2013 shows that dimorphism is also important in plants if they are dioecious (i.e., have separate sexes), and even in monoecious plants, then referring to various flower characteristics (petals, stamen, pollinia, scent, etc.). Historically, early work, such as Selander 1972, Ralls 1977, or Leutenegger and Kelly 1977, in the context of sociobiology and mating systems predominately investigated birds, mammals, and, particularly, primates, plus some prominent ectothermic (cold-blooded) vertebrates, as in Shine 1979; invertebrates were studied only later, for instance in Fairbairn 1997. This early focus on endothermic (warm-blooded) vertebrates did introduce some biases, for instance the term “reversed sexual size dimorphism” in the bird literature, which assumed male-biased size dimorphism as the standard, even though most animals, namely invertebrates, actually feature female-biased dimorphism as the standard. However, such biases are largely erased lately as ever-more taxa have been considered in this context.

  • Badyaev, Alexander V., and Geoffrey E. Hill. 2003. Avian sexual dichromatism in relation to phylogeny and ecology. Annual Review of Ecology, Evolution and Systematics 34:27–49.

    DOI: 10.1146/annurev.ecolsys.34.011802.132441E-mail Citation »

    A review of the phenomenon of, primarily, dichromatism in bird plumage, covering phylogenetic patterns as well as their underlying ecological, developmental, and physiological mechanisms. The article discusses why there is sometimes no concordance between ecological conditions and sexual dichromatism even though it is expected as a result of sexual selection.

  • Barret, Spencer C. H., and Josh Hough. 2013. Sexual dimorphism in flowering plants. Journal of Experimental Botany 64.1: 67–82.

    DOI: 10.1093/jxb/ers308E-mail Citation »

    A competent recent review of sexual dimorphism in angiosperm plants, covering the genetic and evolutionary processes that produce divergence between female and male phenotypes as well as the ecological consequences. The authors discuss why sexual dimorphism in plants is generally less well developed than in many animal groups.

  • Darwin, Charles R. 1981. The descent of man, and selection in relation to sex. Princeton, NJ: Princeton Univ. Press.

    DOI: 10.5962/bhl.title.70891E-mail Citation »

    Originally published in 1871 (London: John Murray). First coverage and original definition of the phenomenon of sexual dimorphism.

  • Fairbairn, Daphne J. 1997. Allometry for sexual size dimorphism: Pattern and process in the coevolution of body size in males and females. Annual Review of Ecology, Evolution and Systematics 28:659–687.

    DOI: 10.1146/annurev.ecolsys.28.1.659E-mail Citation »

    An integrated and thorough conceptual discussion of the evolutionary causes of sexual size dimorphism. The role of allometry for understanding dimorphism is developed, including methodological aspects of statistical treatment and a formal definition of Rensch’s Rule.

  • Hedrick, Ann V., and Ethan J. Temeles. 1989. The evolution of sexual dimorphism in animals: Hypotheses and tests. Trends in Ecology & Evolution 4.5: 136–138.

    DOI: 10.1016/0169-5347(89)90212-7E-mail Citation »

    An early integrated conceptual discussion of the evolutionary factors and hypotheses underlying sexual size dimorphism in animals.

  • Leutenegger, Walter, and James T. Kelly. 1977. Relationship of sexual dimorphism in canine size and body size to social, behavioral, and ecological correlates in anthropoid primates. Primates 18.1: 117–136.

    DOI: 10.1007/BF02382954E-mail Citation »

    An early comparative study of sexual size dimorphism in the age of sociobiology, showing that both body and canine size dimorphism correlate with sexual selection by male-male competition depending on the social organization of primate species.

  • Ralls, Katherine. 1977. Sexual dimorphism in mammals: Avian models and unanswered questions. American Naturalist 111.981: 917–938.

    DOI: 10.1086/283223E-mail Citation »

    Another early sociobiological discussion that developed how fundamental differences in social organization affect the evolution of sexual size dimorphism in mammals relative to birds.

  • Selander, Robert K. 1972. Sexual selection and dimorphism in birds. In Sexual selection and the descent of man, 1871–1971. Edited by Bernard G. Campbell, 180–230. Chicago: Aldine.

    E-mail Citation »

    A prominent early comparative study of sexual size dimorphism in birds that helped resolve its evolutionary causes and advanced the field.

  • Shine, Richard. 1979. Sexual selection and sexual dimorphism in the amphibia. Copeia 1979.2: 297–306.

    DOI: 10.2307/1443418E-mail Citation »

    An early comparative study of sexual size dimorphism in amphibians, an ectothermic (cold-blooded) vertebrate group predominantly featuring females that are larger than males. The study provided a counterpoint to studies of warm-blooded mammals and birds, nevertheless confirming that strong sexual selection on males repeatedly lead to the evolution of larger males in frogs and newts.

  • Stamps, Judy, and Vaidyanad V. Krishnan. 1997. Sexual bimaturation and sexual size dimorphism in animals with asymptotic growth after maturity. Evolutionary Ecology 11.1: 21–39.

    DOI: 10.1023/A:1018479312191E-mail Citation »

    The first systematic theoretical treatment of the relationship between size dimorphism and sexual differences in development, i.e., maturation time.

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