Oxytocin reactivity during intergroup conflict in wild chimpanzees
Significance Warfare is one of the most pervasive problems among human societies, and understanding mechanisms involved in in group cooperation and favoritism is of paramount importance. Wild chimpanzees share key features of humans' intergroup conflict, in terms of in group coordination, coalitionary support, and out group hostility. The hormone oxytocin may regulate humans' intergroup conflict, although tests in natural settings are lacking. We found strong evidence that, like in humans, oxytocin is involved in chimpanzee intergroup conflict. Both intergroup conflict anticipation and participation involved high urinary oxytocin levels, irrespective of intragroup affiliations or potential threat by rivals. These results are indicative of similar physiological processes involved in intergroup violence and intragroup support in both species, likely supporting behavior that is adaptive during intergroup conflicts. Intergroup conflict is evident throughout the history of our species, ubiquitous across human societies, and considered crucial for the evolution of humans' large scale cooperative nature. Like humans, chimpanzee societies exhibit intragroup coordination and coalitionary support during violent intergroup conflicts. In both species, cooperation among group members is essential for individuals to gain access to benefits from engaging in intergroup conflict. Studies suggest that a contributive mechanism regulating in group cooperation during intergroup conflicts in humans involves the neuropeptide hormone oxytocin, known to influence trust, coordination, and social cognition, although evidence from natural settings is lacking. Here, applying a noninvasive method, we investigate oxytocinergic system involvement during natural intergroup conflicts in wild chimpanzees. We found that chimpanzees of both sexes had significantly higher urinary oxytocin levels immediately before and during intergroup conflict compared with controls. Also, elevated hormone levels were linked with greater cohesion during intergroup conflicts, rather than with the level of potential threat posed by rival groups, intragroup affiliative social interactions, or coordinated behavior alone. Thus, the oxytocinergic system, potentially engendering cohesion and cooperation when facing an out group threat, may not be uniquely human but rather a mechanism with evolutionary roots shared by our last common ancestor with chimpanzees, likely expediting fitness gains during intergroup conflict.
Proceedings of the National Academy of Sciences