The evolution of intergroup tolerance in non-human primates and humans

Primate individuals use a variety of strategies in intergroup encounters, from aggression to tolerance; however, recent focus on the evolution of either warfare or peace has come at the cost of characterizing this variability. We identify evolutionary advantages that may incentivize tolerance toward extra-group individuals in humans and non-human primates, including enhanced benefits in the domains of transfer, mating, and food acquisition. We highlight the role these factors play in the flexibility of gorilla, chimpanzee, bonobo, and human behavior. Given humans have an especially broad range of intergroup behavior, we explore how the human foraging ecology, especially large geographic and temporal fluctuations in resource availability, may have selected for a greater reliance on tolerant between-community relationships – relationships reinforced by status acquisition and cultural institutions. We conclude by urging careful, theoretically-motivated study of behavioral flexibility in intergroup encounters in humans and the non-human great apes.