Body size and canine size do not confer a competitive advantage in male rhesus macaques

Abstract

Male armaments are hypothesized to have evolved under intrasexual selection. Such traits may function as signals, weapons, or both, in male–male mating competition. Primate sexually dimorphic canine teeth and body size are two potentially weaponized traits whose function as a signal and/or weapon remains unclear, largely due to the difficulty of collecting detailed measurements of morphology on large free-ranging mammals. Rhesus macaques, Macaca mulatta, are an interesting study system in which to investigate how such traits function because they experience relatively low levels of direct male–male mating competition compared to other members of their subfamily. Furthermore, male dominance rank is largely based on a queuing system rather than on the outcome of intermale aggressive encounters. We leveraged a novel data set of behavioural observations and morphometric data from free-ranging rhesus macaques to investigate the function of sexually dimorphic canine teeth and body mass as weapons and/or signals. We tested whether canine height or body mass was correlated with dominance rank, whether similarity in any of these factors influenced the occurrence or outcome of agonistic interactions between male–male dyads and whether either of these traits predicted the likelihood of winning an agonistic interaction. Neither canine height nor body mass was related to dominance rank. Similarity in dominance rank, but not in morphology, predicted the occurrence of agonism between dyads. Agonistic encounters between males more similar in dominance rank were more likely to be characterized by aggression rather than submission. Dominance rank, but not canine height or body mass, predicted the likelihood of winning an agonistic interaction. Our results suggest that canine height and body mass do not confer a strong competitive advantage in male rhesus macaques and add to a growing body of evidence indicating that weaponized traits do not always seem to function either in fights or as signals in male–male combat.