Love thy neighbour: behavioural and endocrine correlates of male strategies during intergroup encounters in bonobos

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Across group-living taxa, males act aggressively towards outgroup males because they represent a threat to reproduction with ingroup females. This intergroup aggression can entail agonistic coalition formation between group members, is typically associated with higher testosterone levels and prevents extended associations between members of different groups. However, in a few species, including humans, groups can interact peacefully, allowing the exchange of resources and information. To better understand how social relationships are manifested between groups and the involvement of males in these relationships, we investigated behavioural and endocrine correlates of male strategies during intergroup encounters in bonobos, Pan paniscus . We found that, despite overt intergroup aggression, males rarely engaged in coalitionary attacks against outgroup individuals and their testosterone levels did not rise during intergroup encounters. Also, males sacrificed the time available to affiliate with ingroup individuals and actively affiliated with outgroup individuals, especially outgroup males. The paucity of cooperative group defence and high levels of intergroup affiliation suggest that male bonobos play a role in maintaining tolerant intergroup relationships. We discuss how our findings relate to the establishment of male–male social relationships beyond the group level in other species. • Male bonobos rarely participated in coalitionary attacks against an outgroup. • Male testosterone levels did not vary in relation to intergroup encounters. • Males did not bias their grooming towards ingroup members during encounters. • Males actively groomed with outgroup individuals, especially males.