Associations between testosterone and immune activity in alligators depend on bacteria species and temperature

Males often use elaborate sexual traits to enhance reproduction, but can experience trade-offs between investment in these traits and other physiological needs. The immunocompetence handicap hypothesis (ICHH) postulates that testosterone supports development of sexual traits while also suppressing immunity. While the ICHH implicates testosterone-mediated immunosuppression as a key mechanism of honest signaling in males, conflicting patterns across studies suggest that testosterone-immunity interactions are complex. In this study, we test the ICHH in free-ranging alligators and examine how intrinsic (steroid hormone levels) and extrinsic (temperature) factors modulate testosterone-immunity relationships. Specifically, we quantified simultaneous effects of testosterone and dehydroepiandrosterone (DHEA) on killing capacity of plasma against three bacteria species at two challenge temperatures. We found that DHEA was important in predicting the effects of testosterone on killing capacity. Further, testosterone-mediated effects were dependent on both temperature and bacteria species. Our results highlight the context dependency of interactions between testosterone and immunity, and illustrate the importance of evaluating the ICHH in natural systems to identify key intrinsic and extrinsic factors mediating testosterone-immunity trade-offs.