Anolis sagrei has impressive sexual size dimorphism, but what causes it? (Photo by Bob Reed)
Sexual dimorphism is always a hot topic at SICB, and this year it was no exception for anoles (1, 2). Christian Cox, a postdoc in the laboratory of Bob Cox (no relation) at the University of Virginia, sought to explain how testosterone might lead to phenotypic divergence in a number of sexually dimorphic traits. As many of us are aware, sexual dimorphism varies widely among lizard species, and evolutionary shifts to and away from dimorphism are common, including in anoles. Testosterone has been shown to be an important regulator of growth in several lizard species, so Cox experimentally tested this effect in Anolis sagrei.
Both males and females were given a testosterone or blank implant and allowed to grow to maturation. One group was manipulated as juveniles, just as phenotypic divergence was beginning, and the other group was manipulated as subadults after divergence. Testosterone addition increased growth in body size and mass, increased metabolic rate, increased dewlap size, and changed dewlap coloration in both sexes and both juveniles and subadults. Fat storage was reduced as expected, in both sexes and age classes. These results are intriguing, because a sex difference in testosterone production may play a role in the degradation of between-sex genetic correlations. The next question is how that happens, as both sexes produce testosterone, just to different extents.