Between-group conflict participation is associated with lower long-term cortisol in wild vervet monkeys

Male-biased cooperative behaviours that produce public goods (male services) are widespread in primates. Recent behavioural studies suggest that male services may function beyond paternal investment, either as displays of competitive ability or as honest signals eliciting female choice. Because these hypotheses make contrasting predictions about the physiological state of service providers, endocrine measures offer a complementary approach for distinguishing between them. We investigated whether long-term testosterone and cortisol concentrations, measured from hair samples, were associated with four male services in wild vervet monkeys: alarm calling, participation in between-group conflicts (BGCs), leading river crossings, and sentinelling. Testosterone and cortisol concentrations increased with male competitive status, measured as a combination of dominance rank and recent rank trajectory, particularly during the mating period, establishing the endocrine profile associated with competition. In contrast, most male services showed few relationships with hormone concentrations: participation in BGCs was associated with lower cortisol concentrations and, to a lesser extent, lower testosterone concentrations. Because BGC participation is the strongest predictor of male mating frequency in this population, these findings indicate that males contributing to BGCs present an endocrine profile distinct from that associated with competitive state and instead support the prediction that high-quality males, irrespective of their dominance rank, can perform costly cooperative behaviours while maintaining relatively low physiological costs. More broadly, our results provide physiological support for reputation-based partner choice as a mechanism contributing to the maintenance of male services in primates.