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

Male vervet monkeys engage in cooperative behaviors, such as male services, which provide public goods. Recent studies suggest that these male services may serve multiple purposes - either as displays of competitive ability or as honest signals attracting female choice. To differentiate between these hypotheses, researchers examined the relationship between long-term testosterone and cortisol levels in hair samples with four male services: alarm calling, participation in between-group conflicts (BGCs), leading river crossings, and sentinelling.

As expected, testosterone and cortisol levels rose with the males' competitive status, based on dominance rank and recent rank changes, particularly during the mating season. This confirmed the endocrine profile linked to competition. However, unlike testosterone, most male services showed minimal correlation with hormone concentrations.

Specifically, participation in BGCs was linked to lower cortisol and, to a lesser extent, lower testosterone levels. Since BGC participation strongly predicts male mating frequency in this primate population, these findings imply that males contributing to BGCs have an endocrine profile distinct from the one related to competitive status.

Instead, these results support the idea that high-quality males can perform costly cooperative behaviors while sustaining relatively low physiological costs. Overall, these findings provide physiological evidence for reputation-based partner choice as a mechanism contributing to the maintenance of male services in primates.