If I were to take survey of Anole Annals readers regarding the factors that regulate aggressive and showy behaviors, I suspect that the vast majority of you would implicate testosterone as the primary culprit. Whether we are discussing humans or nearly any other vertebrate, there is a common societal notion that testosterone fuels these behaviors like oxygen fuels fire. The widespread belief is simple: individuals with more testosterone tend to exhibit more aggressive, ostentatious, and risky behaviors.
For decades researchers have investigated the link between testosterone and behavior in a variety of biological contexts – including different behaviors, experimental manipulations, environmental conditions, and life history parameters – but rarely in wild animals or within an evolutionary context. If the supposed testosterone-behavior correlation is extended to a broader, comparative context, it would suggest that aggressive species should also have higher levels of circulating testosterone than more placid species. But, in an upcoming paper, Husak and Lovern test the testosterone-behavior supposition among Anolis lizards and, quite frankly, turn it right on its head. To give away their conclusion at the outset, three of the four “aggressive” anole lineages examined have evolved this behavior without a clear correlation with circulating levels of testosterone.
Anolis lizards are renowned for their convergent anatomical evolution (reviewed in Lizards in an Evolutionary Tree), but these species have also independently evolved similar behaviors. In a study that was one of the first of its kind, Johnson et al. showed that the Anolis ecomorphs exhibit evolutionary convergence towards similar patterns of aggressive display and territorial behaviors. Trunk-ground anoles tended to be the most “aggressive” ecomorphs, consistently exhibiting higher display rates and territoriality than the trunk-crown, grass-bush, or twig ecomorphs. Twig species tended to exhibit the least aggressive behavior in the analysis. (Also see Ord et al. 2013 for a more fine-scale dissection of display behavior.) Using this pattern of convergent behavior as a foundation, Husak and Lovern predicted that trunk-ground anoles would have higher levels of circulating testosterone than other ecomorphs from the same island, twig anoles the least. The absolute levels of testosterone might vary depending on the specific lineage in question, but they predicted that the rank-order of testosterone on each island would follow the behavioral continuum described in Johnson et al. In total the authors surveyed circulating levels of testosterone and corticosterone, an adrenal steroid hormone associated with stress, in 18 Anolis species!
As I already stated, the authors found no support for the idea that elevated levels of circulating testosterone consistently drive aggressive behavior in Anolis lizards. Instead they found that three out of the four clades of trunk-ground anoles had the lowest levels of testosterone, the opposite pattern than would be predicted based on their behavior. Only on Puerto Rico did the trunk-ground species exhibit the highest levels of testosterone. In fact, testosterone levels are evolutionarily quite labile among anoles and also do not correlate with body size (snout-to-vent length), body mass, or gross levels of sexual dimorphism (1, 2). However, testosterone levels are not evolutionarily sloppy, showing rapid changes among closely-related species with similar biology; no differences in testosterone were found in either case where sister species were examined, the two Puerto Rican trunk-ground species and the trunk anoles A. brevirostris and A. distichus. Therefore, circulating levels of testosterone among anoles likely correlate with another behavior, evolve as a byproduct of selection for something unrelated to behavior, or might be functionally neutral whereby changes in behavior are mediated by other downstream factors (i.e., brain-specific expression of hormonal receptors) or other pathways altogether (i.e., arginine vastocin).
Despite our widespread societal ideas regarding the role of testosterone on behavior, data collected within the past few years is starting to push back against this idea. It is important to point out that many of the studies that previously examined the effects of testosterone compared castrated males with intact males or males with testosterone levels elevated beyond physiological norms. Studies like these may be ideal for understanding the physiological effects of testosterone, but are not well suited for understanding the proximate mechanisms of behavioral variation in a diverse natural population where multiple pathways can yield similar results (referred to as “many-to-one mapping” in Husak and Lovern). Johnson et al. (different paper than that referenced above) also found a negative correlation between plasma testosterone and aggressive displays within a wild population of A. carolinensis. Although additional data is clearly necessary, this observation provides an intriguing connection between intraspecific variation and the interspecific patterns of evolutionary divergence described by Husak and Lovern. Furthermore, the mental relationship between testosterone and aggressive behavior is also weakening in other vertebrate systems. For example, in humans a placebo elicited a greater aggressive response if the participants were told that the placebo was testosterone than in individuals actually administered testosterone. Likewise, following a review of testosterone data from 51 wild bird species, it has also become clear that variation in circulating testosterone levels cannot alone explain differences in mating system or secondary sexual trait expression, traits often strongly linked to testosterone in laboratory-based studies.
It is clear that testosterone is a potent physiological hormone that plays a critical role in sexual differentiation during development and in adulthood. But as we move forward in attempts to tease apart the proximate mechanisms of evolutionary divergence, it is clear that we will need to take a more careful approach to studying the effects of testosterone, embracing both experimental and comparative approaches and surveying circulating levels of hormone, the expression of tissue-specific receptors, and downstream effectors. In regard to anole biology, the forthcoming paper by Husak and Lovern is an important step in this direction.
Husak, JF and Lovern MB. In press Variation in steroid hormone levels among Caribbean Anolis lizards: Endocrine system convergence? Hormones and Behavior.