Explanations of the evolution of canine tooth size sexual dimorphism in primates implicitly assume that male canine size is the primary target of natural or sexual selection, even though there is substantial unexplained variation in female canine size. Here we demonstrate that the canines of both males and females are relatively larger in species where agonistic interactions are more likely to escalate. Hence, the canine teeth of both sexes are influenced by selection for the development of weaponry, though the nature of the selection (natural or sexual) favoring development of the canines as weapons depends on the objects of competition (mates vs. food, for example). However, we also suggest that where the outcome of fights is typically determined between coalitions of individuals, selection for weaponry is reduced, even though agonistic competition can be very intense. Coalitionary aggression between males occurs in only a handful of anthropoid primates, but it is common among female anthropoids, and helps explain, in part, the extreme sexual dimorphism in the canine teeth of a number of catarrhine species. This is the first demonstration that cooperation among animals may affect their morphology. The lack of sexual dimorphism in canine size among prosimian primates is also consistent with the model. This analysis demonstrates that a better understanding of the interspecific variation in sexual dimorphism among primates is gained by considering behavioral variation in both sexes.
