Seasonal and predator influences on adrenal function in adult Steller sea lions: Gender matters

Chronically heightened adrenal activity indexed by fecal corticosteroids has been shown to be a valid descriptor of stress in many species. As part of an ongoing investigation of adrenal activity in Steller sea lions (SSL), adrenocorticotropic hormone (ACTH) challenges were performed during the summer at the Alaska SeaLife Center (ASLC). These results were compared to earlier data from winter months. Additionally, adrenal response of free-ranging females SSL to a presumed in situ stressor, pup predation by killer whales (Orcinus orca), was evaluated as a field trial of developed methodologies. Summer ACTH results indicated that gender-dependent differences in baseline fecal corticosterone concentrations exist, with summer baseline fecal corticosterone concentrations higher in males than in females, based on season when compared with previously reported values in winter ACTH trials for this species. ACTH trials in the male in the summer resulted in 2468 ng/g basal to 10,937 ng/g maximal fecal corticosterone concentrations, a fourfold change. Female 1 exhibited a 30.5-fold increase 24 h post-ACTH stimulation (27.9–852.0 ng/g dry weight), with a return to just above baseline concentrations by hour 25. Additionally, female 2 exhibited a 64.4-fold increase at 25 h post-stimulation (31.7–2042.0 ng/g dry weight), with a return to just above baseline concentrations by hour 45. In situ female fecal corticosterone tripled 24 h subsequent to orca predation on pups (54.6 ± 18.5 ng/g mean pre-attack, 542.5 ± 252.7 ng/g mean post-attack) during the field trial. Data from both the summer ACTH and subsequent field trial underscore the necessity of gender determination of collected scat source and knowledge of conditions at collection sites for proper interpretation of fecal corticosterone data, particularly in studies that focus on population-wide stress. With elimination of gender bias and delineation of acute response to natural stimuli, corticosterone data can now be evaluated within context and provide meaningful information about stress and potentially reproductive physiology in free-ranging Steller sea lions.