Although the Evolution meetings are coming to a close, we get to go out on a high note. Christian Cox gave one of the last talks of the day discussing the hormonal basis for gender differences in sexual size dimorphism in anoles. Sexual size dimorphism (SSD), or the tendency for the sexes to differ in the size of different traits, has been widely documented in nature. Usually the male exhibits comparatively larger features, such as bigger body size or larger ornaments. Anoles are an intriguing case of SSD, as the traits that can exhibit dimorphism can vary widely among species. Some species, such as Anolis carolinensis, exhibit SSD in multiple traits, including body size, head shape, and dewlap size. In contrast, other species exhibit minimal SSD. As an example, A. distichus from the Caribbean island of Hispaniola tends to show no SSD in body size or head shape, but has strong SSD in dewlap size.
Christian Cox and his collaborators posit that one mechanism underlying SSD may be a pleiotropic regulator that can couple and decouple dimorphism in different phenotypes and their candidate for this study was testosterone. They conducted experiments manipulating levels of testosterone in adult males and females of Anolis sagrei and assessed how body size, head shape, and dewlap traits changed. Anolis sagrei is a particularly good system for assessing the role of SSD in anoles. Male A. sagrei can be up to 50% larger and three times more massive than females.
To conduct the study, they took three year-old male and female lizards and gave them either testosterone or blank subdermal implants. They maintained lizards under laboratory conditions for two months and then gathered information on morphological dimensions and dewlap characteristics. Under testosterone treatment, males and females grew similarly, whereas males grew faster than females in the control group. This merits restating – they were able to make females grow like males just by applying testosterone! Clearly testosterone has strong effects on male-specific growth patterns.
To determine if testosterone affects metabolism, they measured metabolic rate using stop-flow respirometry. They found that testosterone treatment increased metabolic rate for males and females. Correspondingly, they found that visceral fat bodies were lower in testosterone treated animals, suggesting that increased growth is caused by shunting energy towards growth and away from storage metabolism. They further determined that testosterone treatment increased the size of the humerus and femur, but had no significant effect on jaw length and head width. Because this species exhibits little SSD with respect to head dimensions, perhaps this finding is not surprising, but I would be curious to know whether testosterone influences skull growth in species with SSD in head dimensions, such as A. carolinensis.
Finally, the authors found that testosterone led to increased dewlap size in both males and females. In fact, the dewlaps of testosterone-treated females were comparable in size to those of control males and eroded the sex differences that otherwise existed between them. Testosterone treatment decreased the saturation and brightness in the dewlap, leading the authors to suggest that it accelerates its development, as they posit that this color is representative of the fully developed dewlap in the wild.
Thus, they find strong evidence that testosterone plays a large role in modulating SSD in anoles. In particular, it abolishes differences in growth in various traits except for skull shape. And it can create male-like females as well as forge super-males. It would be interesting to see if, in addition to acquiring a male-like morphology, the females would tend to act like males, as well. Their next step is to conduct testosterone manipulation experiments in A. distichus, a species that has low SSD in body size and head shape, but strong SSD in dewlap size, to determine if the effects of testosterone are repeatable in a system exhibiting a pattern of SSD that is different from A. sagrei.
Extreme sex differences in the development of body size and sexual signals are mediated by hormonal pleiotropy in a dimorphic lizard. Authors: Cox, Christian L.; Hanninen, Amanda F; Cox, Robert M.