Sex-specific effects of social bonds on glucocorticoids in wild chimpanzees

Across social species, strong social bonds are linked to health benefits, including reduced disease risk and increased survival. Chronic glucocorticoid exposure, frequently associated with negative health outcomes, may mediate this relationship, as bonds may reduce stressor exposure or buffer physiological responses. Among primates, these relationships have mostly been studied in female-bonded species, where kinship shapes cooperation and access to resources. Whether bonds confer simsilar benefits in species with different social structures - especially bonds between non-kin - remains unclear. We tested whether strong affiliative bonds were associated with glucocorticoid production in wild adult chimpanzees using 22 years of behavioural and urinary cortisol data. Bonds were quantified for same-sex adult dyads using an index of party association, proximity, and grooming. Bond strength was measured by averaging each individuals top three bonds per year. Stronger bonds predicted lower cortisol in females but higher cortisol in males, a pattern that persisted after accounting for dominance rank, aggression, and contextual stressors. In this male-bonded species, stronger male bonds predicted higher physiological costs, while stronger female bonds, though weaker, appeared to attenuate stress. Our results challenge the assumption that social bonds are universally health-promoting, and suggest their physiological consequences vary with sex and social organization.