Male-inflicted wounds have opposite effects on hair cortisol for captive male and female rhesus macaques (Macaca mulatta) following new group formation.

Sexual dimorphism in body size, aggression, and dispersal patterns may affect the degree to which males and females perceive aggression from either sex as stressful. Whereas male macaques typically disperse to new groups at maturity, thus encountering many unfamiliar individuals of both sexes, females are philopatric, usually only encountering unfamiliar males who transfer into their natal groups. In rare circumstances, however, group fusions can expose both males and females to many novel individuals, which often increases aggression. Here, we use a captive new group formation of rhesus macaques (Macaca mulatta) as a model of social instability during fusions and examine differences in male and female chronic stress responses to male-pattern and female-pattern trauma (i.e., trauma inflicted by males or by females, respectively). We found that male- but not female-pattern traumas predicted hair cortisol concentrations during the first 9 months after new group formation, but in opposite ways for males and females. A greater number of male-pattern traumas was linked to elevated hair cortisol concentrations in females but slightly lower hair cortisol concentrations in males. We suggest that the apparent importance of male-pattern trauma, but not female-pattern-trauma, in predicting higher hair cortisol concentrations in females can be attributed to the more acutely intense but less persistent nature of male aggression toward females.