Oxytocin activity is not linked to out-group prosociality in wild bonobos.

In many group-living species, cooperative group defense is crucial to the reproduction and survival of group members. In humans and chimpanzees, this adaptive behavior is regulated by oxytocin, a highly conserved neurohormone. In humans, oxytocin can also enhance prosocial attitudes towards out-group individuals and reduces xenophobia. While the role of oxytocin in supporting cooperative group defense is likely evolutionarily ancient, it is unclear to what extent oxytocin's role in promoting out-group prosociality is conserved. Bonobos, our closest living relatives together with chimpanzees, can provide valuable insights into this question, because they are not known to engage in collective group defense but instead exhibit tolerance and prosocial behaviors across groups. Through examining variation in bonobo cooperative behavior, specifically coalition formation, we reinforce the idea that bonobo coalitions do not serve as a form of group defense. Despite increased competition, bonobos formed fewer coalitions in the presence of out-groups. Further, bonobo coalitions included both in- and out-group partners, reflecting reduced xenophobia and between-group cooperation. Physiologically, neither females nor males showed increased oxytocin activity with out-group presence. This suggests that, unlike in humans, oxytocin is not involved in regulating out-group prosociality in bonobos.