Examining variation in social behaviour and associated endocrine physiology across groups of the same species can help identify consistent hormone–behaviour interactions. We investigated differences in urinary oxytocin levels of individuals, of two neighbouring chimpanzee groups related to (a) socio-positive and -negative interaction frequencies, (b) within-group cooperation associated with between-group competition and (c) group-specific differences in urinary oxytocin reactivity of individuals in response to the same behavioural contexts. We found higher rates of cooperative group-level behaviours and larger relative party sizes in East Group males, while South Group males had higher non-directed aggression and copulation rates. Individuals of both groups showed consistent urinary oxytocin reactivity after the same behavioural contexts. However, East Group males had higher urinary oxytocin levels across contexts than South Group males, including higher baseline levels. Our results support the oxytocinergic system’s involvement in cooperation and gregariousness, and suggest an association between group-specific social dynamics and oxytocinergic profiles.

In a competitive environment, helping others at a cost presents a puzzle. Nonetheless, altruistic behaviour has been associated with human success as a species. Adoption, the provision of alloparental care to an orphan by an individual other than the biological mother, is a potential altruistic act in both humans and chimpanzees. We investigated potential benefits of adoption (like kin selection, improved reputation, or recruitment of allies) in a chimpanzee community with high adoption rates and alloparental care by adult males, who typically provide no paternal care to offspring. The probability of an adult male providing care to an orphan was connected to the grooming relationship/bond between male and mother before her passing and was not influenced by orphan sex or relatedness. The probability of positively interacting with orphans was negatively affected by the number of female bystanders. Results suggest that male–orphan interactions are not solely driven by reputation, ally recruitment, or kin selection and highlight the link between highly prosocial behaviours and social closeness.