The Dynamics of Sociality and Glucocorticoids in Wild Male Assamese Macaques

Abstract

For males of gregarious species, dominance hierarchy status and the strength of their affiliative relationships can have major fitness consequences. Social dynamics also impose costs by affecting glucocorticoids, key mediators of homeostasis and indicators of the physiological response to challenges and within-group competition. We investigated the relationships between dominance rank, social bonds, seasonal challenges, and glucocorticoids in wild Assamese macaques (Macaca assamensis) at Phu Khieo Wildlife Sanctuary, Thailand, combining behavioural data with more than 4,100 faecal samples from 62 adult males collected over 15 years. Our previous work on this population suggested that increased competition during the mating season induced elevated glucocorticoids and that, unusually for male primates, lower rank position correlated with higher glucocorticoids (cost of subordination). With a much larger dataset and dynamic measures of sociality, we re-examined these relationships and tested for an association between social bonding and attenuated glucocorticoids (social buffering hypothesis). Contrary to our prediction and previous work, dominance rank had a positive relationship with glucocorticoids, as high status correlated with elevated glucocorticoids. We demonstrated a social buffering effect in males and showed that dynamics in affiliative relationships can correlate with dynamics in physiological responses. Glucocorticoids were upregulated at the onset of the mating season, replicating our previous study. Taken together, our results suggest that even in a relaxed competitive regime, high dominance status can impose physiological costs on males that may potentially be moderated by social relationships. We highlight the need to consider the dynamics of sociality and competition that influence hormonal processes.