KABELIK, D.*; WEISS, S.L.; MOORE, M.C.; Arizona State University; Arizona State University; Arizona State University: Testosterone, aggression, and amygdala morphology in the tree lizard, Urosaurus ornatus

How steroid hormones affect aggression is not well understood. Recent years have seen increased attention paid to the effect of steroids on plasticity of gross neural morphology, both during ontogeny and in adulthood. To differentiate between such organizational and activational effects, we examined a model system with alternative male phenotypes, the tree lizard (Urosaurus ornatus). In addition to being sexually dimorphic, tree lizards also express different male morphological phenotypes (morphs) that result from ontogenetic hormone differences. Additionally, hormone levels in adult tree lizards vary between breeding and non-breeding seasons, though not between morphs. We can thus compare sexes and morphs to examine organization, while comparing breeding states to examine activation. In this experiment across seasons, we compared free-living animals of different sexes and morphs in hormone level, aggression, and neural morphology. Our results indicate that males of the traditionally more aggressive morph type have larger amygdalas than males of the less aggressive morph type or females, which are not aggressive. However, unlike in previous years, a behavioral difference between morph types was not detected. This lack of a behavioral difference may be due to a facultative response to an ongoing severe drought. Finally, although male morph types did not differ in adult steroid levels, the intensity and frequency of aggressive display was positively correlated to testosterone level across seasons. These data are consistent with early developmental effects of steroid hormones on morphology of the amygdala and an activational effect of testosterone on aggression.