Facial width-to-height ratio predicts aggression and contest outcomes in wild golden snub-nosed monkeys

Animals evolve behavioural and anatomical traits to avoid physical conflict by assessing visual cues, and primates’ facial width-to-height ratio (fWHR) is linked to aggression, though relevant empirical evidence in primates is inconsistent across species. We studied the associations of fWHR with aggressiveness and contest outcomes in wild golden snub-nosed monkeys, finding age-specific sexual dimorphism in fWHR and a significant positive correlation between adult males’ fWHR and their aggressive initiation frequency as well as contest winning probability, with male age and first-mover advantage also predicting contest success. No direct evidence was found that resident males initiate more aggression toward conspecifics with smaller fWHR in this species. Our findings confirm fWHR as a signal of male aggressiveness and fighting ability in golden snub-nosed monkeys, while further detailed ethological research is needed to verify if it acts as a deterrent signal to opponents.