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.

Turtles are in decline worldwide, and the magnitude and recent acceleration of population declines requires immediate action to inform conservation and management plans. Long-term studies of population trends and characteristics covering multiple populations across a range of environmental contexts are needed to guide the most effective management decisions, yet such studies are uncommon. Eastern box turtle (Terrapene carolina carolina) populations have declined and require conservation action throughout much of their range. We assessed sources of variation and temporal trends in T. c. carolina population characteristics from 39 sites spanning four ecoregions in North Carolina using capture-mark-recapture. Surprisingly, there was no evidence of population decline at any site over a ten-year period (2008-2017). Population densities ranged from 0.2 to 6.0 turtles per hectare, decreasing with more urban development and increasing with more wetland habitats in the surrounding landscape. Populations varied in demographic structure but had similar sex ratios and more adults than immature turtles in each ecoregion even after adjusting for variation in encounter probability among groups and body sizes. Survivorship was similar among life stages and sexes and only weakly related to body size. After adjusting for emigration, annual survival probability ranged from 90.7% to 96.8% for the various demographic groups and body sizes. Growth rates decreased with increasing body size, with rates ranging from approximately 15-mm carapace length per year in the smallest individuals to near zero in the largest individuals. No aspect of population demographic structure or vital rates was related to surrounding landscape context. Population trends and characteristics were consistent among ecoregions, suggesting no need for regionspecific conservation or management with respect to the population characteristics examined. However, given the high site-specific variability in nearly all estimated parameters, managers would benefit from targeting local threats such as urban land development and wetland destruction to ensure local population viability. The population characteristics reported here can serve as a useful baseline to compare population trends at these sites as monitoring continues. Our approach could be a useful model for other regions to follow in developing monitoring programs for this and other imperiled turtles.