Associations between early life experience, chronic HPA axis activity, and adult social rank in rhesus monkeys

Early life experience and socioeconomic status (SES) are well-established predictors of health outcomes in people. Both factors likely influence health outcomes via hypothalamic-pituitary-adrenal (HPA) axis regulation. However, it is unclear how early experience and HPA axis activity influence adult social status. We studied differentially reared female rhesus monkeys (Macaca mulatta, N = 90) as models to test the hypothesis that chronic HPA axis activity assessed via hair cortisol concentrations (HCCs) mediated the relationship between early life experience and adult social rank. We found that mother-peer-reared (MPR) monkeys acquired higher social ranks than either of the two nursery-reared (NR) groups (peer-reared, PR, or surrogate-peer-reared, SPR monkeys) (β = -0.07, t₍₈₉₎ = -2.16, p = 0.034). We also found that MPR HCCs were lower during the juvenile period at 18 months (F_(2,25) = 3.49, p = 0.047). Furthermore, for MPR but not NR monkeys, changes in HCCs from 18 to 24 months (r_(s) = -0.627, p = 0.039) and adult HCCs (r_(s) = -0.321, p = 0.03) were negatively correlated with adult social rank. These findings suggest that chronic HPA axis regulation in juvenility, and perhaps in adulthood, may influence adult social status for primates that experience typical early rearing. However, early life adversity may result in dissociation between neuroendocrine stress regulation and adult social competence, which may be risk factors for adverse health outcomes.