Dominance, cortisol and stress in wild chimpanzees ( Pan troglodytes schweinfurthii )

Abstract

Field studies of endocrine function in a range of social mammals suggest that high dominance rank is commonly associated with elevated glucocorticoid production. This is puzzling, because in stable dominance hierarchies, high status is normally associated with social control and predictability, key predictors of low psychological stress. One solution to this problem may be that high rank is commonly associated with elevated energetic expenditure, leading to increased metabolic stress and glucocorticoid secretion. We conducted behavioural observations and non-invasive hormone sampling of male chimpanzees in Kibale National Park, Uganda, to examine the relationship between cortisol, dominance and stress in wild chimpanzees. Results indicate that male dominance rank positively correlated with urinary cortisol excretion in a stable dominance hierarchy. Cortisol excretion also correlated positively with rates of male aggression. We suggest that the relationship between cortisol and rank in chimpanzees may be driven by energetic factors rather than psychosocial ones. This interpretation is supported by the observation that urinary cortisol levels correlated negatively with food availability. These findings suggest that dominant chimpanzees experience significant metabolic costs that must be set against the presumed reproductive benefits of high rank. Metabolic stress may mediate the relationship between rank and cortisol in other social mammals.