Intergroup interactions and conflicts in non-human primates: mechanisms and evolutionary pathways toward warfare

The question of the origin of human warfare has triggered much debate and conjecture. The study of intergroup conflicts in our closest living relatives, the non-human primates, allows to dispassionately address this question from an evolutionary perspective. Primatology research reveals the proximate and ultimate mechanisms at work in primate intergroup conflicts, which can be extremely variable. Here, we review these mechanisms, highlighting the socio-ecological conditions leading to an intensification of intergroup conflicts, such as resource distribution, population density, group composition and familiarity. In particular, the concept of intergroup dominance and the associated fitness benefits demonstrate that intergroup competition over territories and their resources motivates inter-group conflicts. However, individual participation impairs efficiency of group-level actions, creating collective action problems. In contrast, chimpanzee territorial behaviour implies group-level cooperation, but also strong out-group hostility. The imbalance of power explains the occurrence of intergroup killings in chimpanzees, and the parochial cooperation model, proposing that strong social ties cement group-level actions, explains to co-occurrence of cooperation and out-group hostility. Intergroup conflicts can then act as a selective pressure favouring in-group cooperation. Intergroup conflicts can escalate to violent forms as seen in chimpanzees and humans when certain social and ecological conditions are fulfilled, but the mobilisation of anciently preserved neurophysiological pathways explain how cooperation is maintained at the group-level and the xenophobic feeling of “us versus they”. It is therefore likely that humans share with chimpanzees some commonalities inherited from our last common ancestor, explaining similarities between chimpanzee and human warfare.