Relationship Between Social Rank and Cortisol and Testosterone Concentrations in Male Cynomolgus Monkeys (Macaca fascicularis)

In nonhuman primate social groups, biological differences related to social status have proven useful for investigating the mechanisms of sensitivity to various disease states. Physiological and neurobiological differences between dominant and subordinate monkeys have been interpreted in the context of chronic social stress. The present experiments were designed to investigate the relationships between basal cortisol and testosterone concentrations and the establishment and maintenance of the social hierarchy in male cynomolgus monkeys. Cortisol concentrations were measured at baseline and following suppression with dexamethasone (DEX) and subsequent administration of adrenocorticotrophic hormone (ACTH) while monkeys were individually housed (n = 20) and after 3 months of social housing (n = 4/group), by which time dominance hierarchies had stabilised. Cortisol was also measured during the initial 3 days of social housing. Neither pre-social housing hormone concentrations, nor hypothalamic-pituitary-adrenal (HPA) axis sensitivity predicted eventual social rank. During initial social housing, cortisol concentrations were significantly higher in monkeys that eventually became subordinate; this effect dissipated within 3 days. During the 12 weeks of social housing, aggressive and submissive behaviours were observed consistently, forming the basis for assignment of social ranks. At this time, basal testosterone and cortisol concentrations were significantly higher in dominant monkeys and, after DEX suppression, cortisol release in response to a challenge injection of ACTH was significantly greater in subordinates. These results indicate that basal cortisol and testosterone concentrations and HPA axis function are state variables that differentially reflect position in the dominance hierarchy, rather than trait variables that predict future social status.