Associations between fecal glucocorticoid levels and social bonds vary with relatedness in juvenile rhesus macaques

Strong social bonds in gregarious adult animals have been associated with lower levels of glucocorticoids. However, similar research is lacking for juvenile primates. We examined relationships between social bonds and mean concentrations of fecal glucocorticoid metabolites (fGCMs) in 44 free-ranging juvenile rhesus macaques (Macaca mulatta) on Cayo Santiago, Puerto Rico. We measured frequencies of affiliative behavior (grooming, play, approaches and proximity) with other same-sex, same-aged juveniles (peers) and the total number of affiliative peer relationships. We found a positive relationship between fGCMs and grooming frequencies. Females that spent more time in proximity to peers also had higher fGCMs. In contrast, among juveniles with more closely related peers, those with more affiliative peers or more frequent play bouts had lower fGCMs. However, strong peer bonds in most juveniles did not appear to be associated with reduced glucocorticoid levels. fGCMs were higher for females than males, but were unassociated with physical activity, aggression, or peer seeking tendencies. We propose that the establishment and navigation of some peer bonds at this life stage may involve increased metabolic demand.