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.Pan troglodytes | cooperation | group cohesion | neuropeptide | parochial altruism R ecent evolutionary models suggest that parochial altruism,

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Grooming interactions benefit groomers, but may have negative consequences for bystanders. Grooming limits bystanders' grooming access and ensuing alliances could threaten the bystander's hierarchy rank or their previous investment in the groomers. To gain a competitive advantage, bystanders could intervene into a grooming bout to increase their own grooming access or to prevent the negative impact of others' grooming. We tested the impact of dominance rank and social relationships on grooming intervention likelihood and outcome in two sympatric primate species, Western chimpanzees (Pan troglodytes verus) and sooty mangabeys (Cercocebus atys atys). In both species, rather than increasing their own access to preferred partners, bystanders intervened mainly when an alliance between groomers could have a negative impact on them: when the lower-ranking groomer was close to the bystander in rank, when either groomer was an affiliation partner whose services they could lose, or the groomers were not yet strongly affiliated with each other. Thus, bystanders in both species appear to monitor grooming interactions and intervene based on their own dominance rank and social relationships, as well as triadic awareness of the relationship between groomers. While the motivation to intervene did not differ between species, mangabeys appeared to be more constrained by dominance rank than chimpanzees.