Why are males the larger sex in some species, yet females are the larger sex in others?

In some species, such as deep-sea anglerfishes, orb-weaving spiders, and marine barnacles, “giant” females are an order of magnitude larger than their “dwarf” male counterparts. In other species, such as elephant seals, males are several times heavier than females. Why has this incredible diversity of sexual size dimorphism (SSD) evolved? Squamate reptiles (lizards and snakes) provide an excellent group in which to address this question because SSD varies considerably both within and across lineages (Fig. 1).

Related Papers

Cox, R.M., D.S. Stenquist, and R. Calsbeek. 2009. Testosterone, growth, and the evolution of sexual size dimorphism. Journal of Evolutionary Biology 22: 1586-1598. Cover photo

Cox, R.M., M.A. Butler, and H.B. John-Alder. 2007. The evolution of sexual size dimorphism in reptiles. Chapter 4 in Sex, Size & Gender Roles: Evolutionary Studies of Sexual Size Dimorphism. Edited by D.J. Fairbairn, W.U. Blanckenhorn, and T. Szekely. Oxford University Press, Oxford, UK.

Cox, R.M. and H.B. John-Alder. 2007. Growing apart together: the development of contrasting sexual size dimorphisms in sympatric Sceloporus lizards. Herpetologica 63: 245-257. 

Cox, R.M. 2006. A test of the reproductive cost hypothesis for sexual size dimorphism in Yarrow's spiny lizard, Sceloporus jarrovii. Journal of Animal Ecology 75: 1361-1369.

Cox, R.M., S.L. Skelly, and H.B. John-Alder. 2003. A comparative test of adaptive hypotheses for sexual size dimorphism in lizards. Evolution 57: 1653-1669.

For a complete list of publications related to this project, check the Publications page.

Figure 1. Bars indicate the relative proportion of species exhibiting each pattern of sexual dimorphism (>5% difference in length) or monomorphism (<5% difference) within each major squamate lineage. Redrawn from Cox et al. (2009).

Comparative phylogenetic methods. Charles Darwin hypothesized that males are the larger sex when large size gives them an advantage in competition for mates (sexual selection). He also proposed that females are the larger sex when large size gives them an advantage in the number of offspring they can produce (fecundity selection). One way to determine whether these two evolutionary forces have shaped SSD is to look for correlations between SSD and measures of sexual or fecundity selection across a large number of species whose phylogenetic relationships are known (Fig. 1). In lizards, evolutionary increases in male aggression and other estimates of sexual selection are associated with evolutionary shifts toward male-biased SSD (Fig. 2). Likewise, evolutionary increases in clutch size and other estimates of fecundity selection are associated with evolutionary shifts toward female-biased SSD (Fig. 2).

Figure 2. Data from 497 lizard species. Top panels show evolutionary changes in SSD as a function of evolutionary changes in three correlates of intrasexual selection for large male body size. Bottom panels show changes in SSD as a function of changes in three correlates of fecundity selection for large female size. Each point is an “independent contrast” between two sister nodes on a phylogeny to correct for shared ancestry. Redrawn from Cox et al. (2003).

Figure 3. Sex differences in natural selection on body size. Adult survival as a function of body size in female (left) and male (right) brown anoles from the islands of Eleuthera (top) and Great Exuma (bottom). Fitness surfaces, estimated by cubic spline, illustrate stabilizing selection for intermediate size in females and directional selection for large size in males.