COMPETITION, COALITIONS AND CANINE SIZE IN PRIMATES

Explanations of the evolution of canine tooth size sexual dimorphism i n primates implicitly assume that male canine size is the primary targ et of natural or sexual selection, even though there is substantial un explained variation in female canine size. Here we demonstrate that th e canines of both males and females are relatively larger in species w here agonistic interactions are more likely to escalate. Hence, the ca nine teeth of both sexes are influenced by selection for the developme nt 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 sugges t that where the outcome of fights is typically determined between coa litions of individuals, selection for weaponry is reduced, even though agonistic competition can be very intense. Coalitionary aggression be tween males occurs in only a handful of anthropoid primates, but it is common among female anthropoids, and helps explain, in part, the extr eme sexual dimorphism in the canine teeth of a number of catarrhine sp ecies. This is the first demonstration that cooperation among animals may affect their morphology. The lack of sexual dimorphism in canine s ize among prosimian primates is also consistent with the model. This a nalysis demonstrates that a better understanding of the interspecific variation in sexual dimorphism among primates is gained by considering behavioral variation in both sexes.