Low heritability and high phenotypic plasticity of salivary cortisol in response to environmental heterogeneity in fur seals

Individuals are unique in how they interact with and respond to their environment. Correspondingly, unpredictable challenges or stressors often produce an individualized response of the hypothalamic-pituitary-adrenal axis and its downstream effector cortisol. We used a fully crossed, repeated measures design to investigate the factors shaping individual variation in baseline cortisol and cortisol responsiveness in Antarctic fur seal pups and their mothers. Saliva samples were collected from focal individuals at two breeding colonies, one with low and the other with high population density, during two consecutive years of contrasting food availability. Mothers and pups were sampled concurrently at birth and shortly before weaning, while pups were additionally sampled every 20 days. We found that heritability was low for both baseline cortisol and cortisol responsiveness, while within-individual repeatability and among-individual variability were high. A substantial proportion of the variation in baseline cortisol could be explained in pups and mothers by a combination of intrinsic and extrinsic factors including sex, weight, day, season, and colony of birth. However, the same variables explained little of the variation in cortisol responsiveness. Our findings provide detailed insights into the individualization of endocrine phenotypes in a wild pinniped. Lay summaryStress is ubiquitous, but individuals differ. In Antarctic fur seals, individual stress levels can be explained by sex, weight, day, density and food availability. Stress can influence how an individual behaves and interacts with its environment, so gaining a better understanding of those factors that explain variation in the stress response is important for population monitoring and conservation.