| Summary: | Intrinsic stressors associated with life history stages may alter sensitivity of the hypothalamic-pituitary- adrenal (HPA) axis and the characteristics of responses to extrinsic stressors. How stress influences the physiology and metabolism of free-ranging adult female pinnipeds at critical life stages is poorly understood. We administered adrenocorticotropic hormone (ACTH) to 14 free-ranging adult female northern elephant seals (Mirounga angustirostris, NES) at two life-history stages: early and late in their molting period. During this time, NES fast completely from food and water and nutrient metabolism is supported exclusively from body reserves. Animals were blood sampled every 30 min for 2 h post-injection, then blood sampled again 24 h later. These stages were selected to investigate how the stress response is influenced by an energetically demanding fasting period. In response to ACTH injection, cortisol levels increased 8 – 10-fold and remained highly elevated compared to baseline at 24 h. Aldosterone levels increased 6-9-fold before returning to baseline at 24 h. The magnitude of the cortisol and aldosterone release were strongly associated, and both were greater during the late molt. On average, late molt animals had lower mass and body composition than early molt animals. We observed a strong inverse relationship between fat mass and the magnitude of cortisol and aldosterone responses, suggesting that body reserves influenced adrenal sensitivity. Sustained elevation in cortisol was associated with alterations in thyroid hormones; both tT3 and tT4 concentrations were suppressed at 24 h, while rT3 increased. Immune cytokine IL-1β and immunoglobulin IgG were also suppressed after 24 hrs of cortisol elevation. As the first study to describe stress responses in a free-ranging adult female pinniped, our data suggests that female NES are more sensitive to stress late in their fasting periods, and highlights the importance of considering life history stage when assessing impacts of anthropogenic stressors on ...
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