Hormones and dimorphism
In addition to understanding why males are larger than females (and vice versa), we also want to know how these two different phenotypes are produced from the same genome. In fence and spiny lizards (Sceloporus), the direction of sexual size dimorphism and underlying sex differences in growth vary across species, but circulating testosterone levels follow similar seasonal patterns in each species (top panels). Experiments on wild males show that testosterone stimulates growth in species where males are larger, but it inhibits growth when females are larger (bottom panels).
When these experimental results are combined with findings from other labs and put into a broader evolutionary context, a pattern begins to emerge. In squamate reptiles, the evolution of sexual size dimorphism in which females are the larger sex is repeatedly associated with the evolution of growth inhibition by testosterone, whereas species in which males are the larger sex exhibit a more classic pattern of growth stimulation by testosterone.
In addition to size and growth, we're also interested in how testosterone and other hormones regulate sex differences in coloration and social signals. The panel on the left shows that, when male eastern fence lizards are castrated, they develop female-like dorsal and ventral coloration, whereas treatment of castrated males with testosterone restores the bright blue/black belly patches and copper dorsal coloration typical of breeding males. Likewise, when juvenile male and female brown anoles are treated with testosterone, their dewlaps acquire dark coloration typical of adult males.
Relevant publications
Wittman, T.N., C.D. Robinson, J.W. McGlothlin and R.M. Cox. 2021. Hormonal pleiotropy structures genetic covariance. Evolution Letters.
Cox, R.M. 2020. Sex steroids as mediators of phenotypic integration, genetic correlations, and evolutionary transitions. Molecular and Cellular Endocrinology 502: 110668
Cox, R.M., C.L. Cox, J.W. McGlothlin, D. Card, A. Andrew, and T.A. Castoe. 2017. Hormonally mediated increases in sex-biased gene expression accompany the breakdown of between-sex genetic correlations in a sexually dimorphic lizard. The American Naturalist 189: 315-332.
Cox, C.L., A.F. Hanninen, A.M. Reedy, and R.M. Cox. 2015. Female anoles retain responsiveness to testosterone despite the evolution of androgen-mediated sexual dimorphism. Functional Ecology 29: 758-767.
Cox, R.M., D.S. Stenquist, and R. Calsbeek. 2009. Testosterone, growth and the evolution of sexual size dimorphism. Journal of Evolutionary Biology 22: 1586-98.
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.
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 in juvenile male eastern fence lizards (Sceloporus undulatus): Implications for energy allocation and sexual dimorphism. Physiological & Biochemical Zoology 78: 531-545.
Wittman, T.N., C.D. Robinson, J.W. McGlothlin and R.M. Cox. 2021. Hormonal pleiotropy structures genetic covariance. Evolution Letters.
Cox, R.M. 2020. Sex steroids as mediators of phenotypic integration, genetic correlations, and evolutionary transitions. Molecular and Cellular Endocrinology 502: 110668
Cox, R.M., C.L. Cox, J.W. McGlothlin, D. Card, A. Andrew, and T.A. Castoe. 2017. Hormonally mediated increases in sex-biased gene expression accompany the breakdown of between-sex genetic correlations in a sexually dimorphic lizard. The American Naturalist 189: 315-332.
Cox, C.L., A.F. Hanninen, A.M. Reedy, and R.M. Cox. 2015. Female anoles retain responsiveness to testosterone despite the evolution of androgen-mediated sexual dimorphism. Functional Ecology 29: 758-767.
Cox, R.M., D.S. Stenquist, and R. Calsbeek. 2009. Testosterone, growth and the evolution of sexual size dimorphism. Journal of Evolutionary Biology 22: 1586-98.
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.
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 in juvenile male eastern fence lizards (Sceloporus undulatus): Implications for energy allocation and sexual dimorphism. Physiological & Biochemical Zoology 78: 531-545.