Male morphs in tree lizards have different testosterone responses to elevated levels of corticosterone.

Changes in circulating glucocorticoid and androgen levels mediate agonistic behaviors in many vertebrates. Individual variation in the magnitude of the glucocorticoid response to stressful stimuli, the negative effects of elevated glucocorticoid levels on androgen levels, or both could mediate individual differences in subsequent agonistic behavior. In a series of previous studies, we found that both alternative male reproductive morphs in the tree lizard, Urosaurus ornatus, can exhibit elevated levels of plasma corticosterone following male-male encounters, but that the territorial morph appears less likely to exhibit coincident decreases in plasma testosterone. Two studies tested the hypothesis that the two morphs differ in the degree to which testosterone levels are influenced by elevated corticosterone levels. In the first study, physically restraining males elicited endogenous elevations of circulating corticosterone levels. Testosterone levels were significantly negatively correlated with corticosterone levels in the nonterritorial morph, but there was no correlation between levels of the two steroids in territorial males. In the second study, corticosterone levels were artificially elevated in free-living male tree lizards using a noninvasive dermal patch. This exogenous elevation of corticosterone significantly depressed testosterone levels in both morphs, but it produced a significantly greater depression in the nonterritorial morph. Nonterritorial males appear to be more sensitive than territorial males to the testosterone-suppressing effects of elevated circulating levels of corticosterone. This difference between the morphs in the effects of a stress hormone on the reproductive axis may be a fundamental part of the mechanism (1) underlying behavioral tactic switching within the nonterritorial morph or (2) contributing to behavioral differences between the morphs.