The evolution of social bonds in primate males

Social bonds, here defined as strong, equitable and enduring social relationships, increase fitness in both male and female primates irrespective of their dispersal regime. Despite the benefits they carry for some, social bonds evolved more often among female than among male primates which is thought to be caused by the unsharable nature of males' limiting resource, fertilizations. Here we present a structured review of variation in primate male social relationships, mating systems, and social organization. In addition to classical socio-ecological reasoning and recent models on the evolution of male coalitions, we consider the phylogenetic history of species living in multi-male groups and alternative evolutionary routes to male co-residency, which may constrain the evolution of male social bonds in some cases. We summarize our results in a conceptual framework that represents the effects of male contest competition within and between groups on male social organization, affiliation and cooperation. We conclude that male social bonds evolved as long-term alliances that gain their adaptive function in within group contests and, thus, that the evolution of male social bonds is driven by variation in within group contest competition. Between group contest competition may select for large male group size but in the end it is the narrow window of medium to low within group contest competition that promotes the evolution of political coalitions and thus is responsible for the rarity of social bonds among primate males.