Sexual dichromatism. In many species, males and females also differ in coloration, which is termed sexual dichromatism. In the lizard Sceloporus undulatus, males possess bright blue and black patches on their throats and abdomens, but females lack this bright coloration. When males are castrated, their black coloration disappears and their blue coloration fades (Fig. 4). Testosterone induces blue and black patches in females or castrated males (Fig. 4). In the closely related S. virgatus, testosterone fails to induce the blue ventral coloration, which is naturally absent in both sexes In another congener, S. jarrovii, both sexes exhibit blue throat coloration, although it is more intense in males. As in S. undualatus, castration diminishes the intensity of blue and black pigmentation, and testosterone restores its expression (Fig. 5). These examples illustrate how the evolution of sexual dichromatism can be linked to its underlying regulation by testosterone.

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., V. Zilberman, and H.B. John-Alder. 2008. Testosterone stimulates the expression of a social color signal in Yarrow's spiny lizard, Sceloporus jarrovii. Journal of Experimental Zoology 309A: 505-514.

Cox, R.M. and H.B. John-Alder. 2005. Testosterone has opposite effects on male growth in lizards (Sceloporus spp.) with opposite patterns of sexual size dimorphism. Journal of Experimental Biology 208: 4679-4687. Featured Inside JEB

Cox, R.M., S.L. Skelly, A. Leo, and H.B. John-Alder. 2005. Testosterone regulates sexually dimorphic coloration in the eastern fence lizard, Sceloporus undulatus. Copeia 2005: 597-608.

Cox, R.M., S.L. Skelly, and H.B. John-Alder. 2005. Testosterone inhibits growth of juvenile male eastern fence lizards (Sceloporus undulatus): implications for energy allocation and sexual size dimorphism. Physiological and Biochemical Zoology 78: 531-545.

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

Figure 5. Ventral surfaces of male and female Yarrow’s spiny lizards, illustrating the presence of a blue throat patch in both sexes. Castration reduces the intensity of blue coloration and eliminates the surrounding black coloration that is typical of males, but these colors are restored in castrated males by exogenous testosterone (T). From Cox et al. (2008).

Figure 4. Ventral and dorsal surfaces of eastern fence lizards, illustrating the blue and black ventral patches and orange dorsal coloration that are expressed only in males. Castration prevents the development of sexual dichromatism, but testosterone restores coloration of castrated males. From Cox et al. (2005).

Growth and body size. Sceloporus lizards provide an interesting study system because related species have very different patterns of growth and sexual size dimorphism (SSD). For example, Sceloporus jarrovii males grow more quickly and average 10% larger than females. However, females grow more quickly and average 10% larger than males in two related species, S. virgatus and S. undulatus (Fig. 1). Circulating testosterone levels are always higher in males than in females, particularly when they peak during the reproductive season (Fig. 1). Therefore, if testosterone mediates the development of the different patterns of SSD found in these lizards, then its effects on growth must differ between species.

Figure 3. In squamate reptiles, evolutionary shifts in SSD are associated with changes in the effect of testosterone on growth. Testosterone stimulates growth in species with male-biased SSD and inhibits growth in species with female-biased SSD. Modified from Cox et al. (2009).

Figure 1. Top panels show sex differences in asymptotic growth trajectories for wild populations of three species with different patterns of SSD. Inserts show absolute growth rates (mm/day) for critical periods in the development of SSD. Bottom panels show plasma testosterone levels throughout ontogeny. In each species, plasma testosterone levels are higher in males than in females and peak during the 1st and 2nd breeding seasons. See Cox and John-Alder (2005) and (2007) and John-Alder et al. (2007) for details.

Testosterone Manipulations. One way to determine whether testosterone influences growth is to remove endogenous testosterone by surgical castration, then replace it with an exogenous hormone implant. Testosterone manipulations in wild populations of these same three Sceloporus species show that the effects of testosterone on growth differ across species. In the species with male-biased SSD, testosterone stimulates growth, whereas in species with female-biased SSD, testosterone inhibits growth (Fig. 2).

Figure 2. Growth rate for free-living male lizards of three species. CAST = surgical castration, placebo implant; CON = sham surgery, placebo implant; TEST = castration surgery, testosterone implant. See Cox et al. (2005), Cox and John-Alder (2005) and John-Alder et al. (2007) for further details.

How do hormones regulate sex differences in growth, body size, and coloration?

Even when selection favors different traits in males and females, the evolution of sexual dimorphism is constrained by the fact that the sexes share most of their genome. In order for sexual dimorphism to evolve, the genes for sexually dimorphic traits must become sex-limited in their inheritance or in their expression. How does this occur? One mechanism that may be of general importance is the differential regulation of shared genes by sex-specific modifiers. Sex steroids are excellent candidates for differential regulation because they are produced and secreted in sex-specific fashion by the gonads.