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

Abstract

Abstract The immunocompetence handicap hypothesis (ICHH) postulates that testosterone supports the development of secondary sexual traits while simultaneously suppressing immune function, creating a trade‐off between trait quality and pathogen vulnerability. The nature of interactions between testosterone and immunity is complex. Conflicting patterns from the literature suggest that testosterone–immunity relationships are variable across immune measures and may be modified by factors both intrinsic and extrinsic to the organism. In this study, we tested the ICHH in free‐ranging American alligators Alligator mississippiensis and examined how both intrinsic (steroid hormone levels) and extrinsic (temperature) factors modulate the relationship between testosterone and immunity. Specifically, we quantified the simultaneous effects of testosterone and dehydroepiandrosterone (DHEA) on microbial killing capacity of three bacteria species (Escherichia coli, Salmonella typhimurium and Klebsiella pneumoniae) at two challenge temperatures (15°C and 30°C). We found that accounting for circulating levels of DHEA was important for predicting testosterone‐mediated effects on microbial killing capacity. We also found that testosterone‐mediated immunosuppression was dependent on temperature and bacteria species, with negative effects of testosterone present only for S. typhimurium at 15°C. 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 modulating testosterone‐immunity trade‐offs. A free Plain Language Summary can be found within the Supporting Information of this article.