Alliances II. Rates of encounter during resource utilization: a general model of intrasexual alliance formation in fission-fusion societies

Ecological explanations of sex-specific patterns of bond formation have focused primarily on resource defence and predation. Resource defence models of alliance formation had not, until recently, explicitly considered encounter rates between competing individuals. Here we present a general model for alliance formation in fission-fusion societies based upon the rates at which individuals encounter each other in competition for resources. Our model applies to both territorial and nonterritorial species. Given the prevalence of stronger bonds among female mammals, the occurrence of prominent male-male alliances in phylogenetically distant species with a fission-fusion grouping pattern is striking (e.g. chimpanzees, Pan troglodytes; bottlenose dolphins, Tursiops spp.). In our model, a sex difference in alliance formation emerges, even when encounter rates are the same for each sex, if there is a sex difference in the duration of resource defence. Thus, if the primary resources for which males compete (oestrous females) are defended for longer periods than the primary resources for which females compete (food), male alliance formation is expected to occur at lower encounter rates than female alliances.