Adrenal response to ACTH challenge alters thyroid and immune function and varies with body reserves in molting adult female northern elephant seals.

Intrinsic stressors associated with life-history stages may alter the responsiveness of the hypothalamic-pituitary-adrenal axis and responses to extrinsic stressors. We administered adrenocorticotropic hormone (ACTH) to 24 free-ranging adult female northern elephant seals (NES) at two life-history stages: early and late in their molting period and measured a suite of endocrine, immune and metabolite responses. Our objective was to evaluate the impact of extended, high-energy fasting on adrenal responsiveness. Animals were blood sampled every 30 min for 120 min post-ACTH injection, then blood sampled 24 hrs later. In response to ACTH injection, cortisol levels increased 8-10-fold and remained highly elevated compared to baseline at 24 hr. Aldosterone levels increased 6-9-fold before returning to baseline at 24 hr. The magnitude of cortisol and aldosterone release were strongly associated, and both were greater after extended fasting. We observed an inverse relationship between fat mass and the magnitude of cortisol and aldosterone responses, suggesting that body reserves influenced adrenal responsiveness. Sustained elevation in cortisol was associated with alterations in thyroid hormones; both tT3 and tT4 concentrations were suppressed at 24 hr, while rT3 increased. Immune cytokine IL-1β was also suppressed after 24 hrs. of cortisol elevation, and numerous acute and sustained impacts on substrate metabolism were evident. Our data suggest that female NES are more sensitive to stress after the molt fast and that acute stress events can have important impacts on metabolism and immune function. These findings highlight the importance of considering life-history context when assessing the impacts of anthropogenic stressors on wildlife.