Personality characteristics and basal cortisol concentrations in adult male rhesus macaques (Macaca mulatta).

Although data show that psychosocial factors can regulate physiological processes, few data have been collected on normative populations. Studies in humans have suggested that personality characteristics might be related to regulation of the hypothalamic--pituitary--adrenal (HPA) axis. We explored the relationship between personality characteristics and plasma cortisol concentrations in adult male rhesus macaques. Two sets of blood samples were obtained from monkeys using a procedure with which they were very familiar; thus, cortisol concentrations reflected basal values. Analyses indicated high-excitable animals had lower basal cortisol concentrations during the afternoon period, and that low-confidence was associated with lower cortisol in the morning period, and lack of a circadian decline in the afternoon period. Sociability and equability were unrelated to cortisol levels. Our data confirm and extend some results found in human studies, and suggest that even in normal populations, personality characteristics are related to measures of HPA function. We propose that comparative studies of personality in nonhuman primates that parallel studies in humans can increase our understanding of mechanisms whereby personality may relate to mental and physical health outcomes.