WEISS*, S.L.; MOORE, M.C.: Testosterone and progesterone activate aggression in male tree lizards

Neuroendocrine mechanisms regulating aggressive behavior are complex, and often no direct relationship between circulating hormone levels and aggression is found. For instance, in the face of identical testosterone levels, male tree lizards (Urosaurus ornatus) express one of two behaviorally-distinct reproductive strategies: “orange” dewlapped males are non-territorial, and “orange-blue” dewlapped males aggressively defend territories and behaviorally dominate orange males. A possible explanation is that aggression of adult male tree lizards is unaffected by testosterone. Alternatively, testosterone may affect aggression, yet the two morphs may differ in their sensitivities to the steroid. We examined whether aggression can be promoted by testosterone, as well as progesterone. Progesterone was included because previous studies suggest it plays a role in organizing tree lizard morphs, but its affect on adult aggression is unknown. Orange and orange-blue males were castrated; given blank, testosterone-filled, or progesterone-filled implants; and tested for their response to orange-blue stimulus males. Relative to castrates, both testosterone- and progesterone-implanted males had reduced latencies to display, and responded with displays of higher intensity. Additionally, testosterone-implanted males performed significantly more aggressive displays and bites relative to castrates; progesterone-implanted males performed intermediate levels. Thus, both testosterone and progesterone can promote aggression in adult male tree lizards. Orange and orange-blue males responded similarly to the high, exogenous hormone treatments. We suggest that naturally-occurring behavioral differences between morphs may be maintained by differing sensitivities to relatively low, endogenous levels of these hormones.