Readers of Anole Annals are likely familiar with the amazing convergent evolution of habitat use and morphology in Caribbean anoles, but the corresponding divergent and convergent evolution of social behavior has recently captured the interest of anolologists. The species differences in social behavior would seem to be due to differences in how much testosterone, a steroid hormone that regulates behavior in many other vertebrates, but this does not appear to be the case. Bonnie Kircher, formerly of Michele Johnson’s lab at Trinity University and currently at the University of Florida, examined what other aspects of hormone signaling might be responsible for the diversity of social behavior seen in Hispaniolan anoles. Since hormones can only act on tissues that have receptors for them, it is possible that variation in hormone receptors might explain behavioral differences independent of hormone levels circulating in the blood. Since the behavioral differences in anoles involve variation in pushup displays and dewlap extensions, it seems intuitive that there may be differences in receptors for testosterone (androgen receptors) in the muscles responsible for these displays.
Bonnie studied six species of anoles that vary in pushup and dewlap display frequency: A. bahorucoensis, A. brevirostris, A. carolinensis, A. coelestinus, A. cybotes, and A. olssoni. After measuring display frequencies in these six species, the investigators quantified the number of androgen receptors in two muscles that are important for pushup displays (biceps) and dewlap displays (ceratohyoid). As predicted, the results showed that species with higher rates of pushup displays have more androgen receptors in their biceps than species with lower pushup frequencies. Interestingly, this was not the case for the ceratohyoid muscle, which controls dewlap extensions. There was no relationship between androgen receptor density of the ceratohyoid and dewlap display frequency. These results are a tantalizing clue to the still-enigmatic mechanism(s) that underlies anole behavioral diversity.