Diel and annual variation in plasma cortisol concentrations among wild and captive harbor seals (Phoca vitulina)

Annual and diel changes in plasma cortisol concentrations were investigated among wild and captive harbor seals (Phoca vitulina) of various sex and age classes. No significant effects of age, sex, or season were found in captive animals. However, significant inter-individual differences between two juvenile males were noted during both the breeding/molt (p = 0.041) and postbreeding/postmolt (p = 0.001) seasons. Seasonal and sex- and age-related differences were found among wild harbor seals. Across all sex and age categories, mean cortisol concentrations during the postbreeding and prebreeding seasons were significantly different from those measured during the breeding season (p = 0.014 and p = 0.038, respectively), but did not differ from each other. When each sex and age group was examined separately, seasonal effects were significant for mature females (p = 0.009) and mature males (p = 0.048). Differences in plasma cortisol concentration between captive and wild animals of the same sex during the same seasons were highly significant, particularly in mature animals (between p < 0.0001 and p = 0.035). There was a diel pattern of plasma cortisol concentrations in samples collected from captive animals over a 24-h period. Mean concentrations differed between samples collected during the hours of light and dark, being highest at night (p = 0.009), peaking around 01:00, and dropping again at around 13:00. Annual and diel patterns of plasma cortisol concentrations occur in harbor seals, and seasonal differences may relate to important physiological and behavioral phases in the harbor seals' annual cycle.