Social development and behavioural reciprocity in young rhesus monkeys with their siblings and non-siblings
This study aimed to assess the influence of relationships with partners close in age on the development of social competence in immature monkeys. Social relationships between 28 sibling and non-sibling immature captive rhesus macaques, (Macaca mulatta), 4 to 40 months old, living in four social groups, were investigated. First, the characteristics of affiliative and agonistic aspects of those relationships were described. Then, the degree to which social behaviours were reciprocated in dyads of immatures, and possible factors influencing the reciprocity were examined. For affiliative behaviours like play, grooming and proximity, Interaction Reciprocity, reflecting the behaviour of two individuals during their interactions, and partner reciprocity reflecting the behaviour of two monkeys in relation to the behaviour of each individual towards other available partners, were assessed for each pair of siblings and of non-siblings. Interaction Reciprocity in play was based on the ratio of play initiations, in grooming on the ratio of time spent grooming, and in proximity on the ratio of approaches and leavings due to each partner. The affiliation levels between immature rhesus monkeys shown in grooming and proximity were most affected by their kinship. As such, they reflected the differences found in patterns of affiliation among adults. The differences shown in play were much less pronounced but they were also biased towards siblings, which were more frequent partners than non-siblings. Among non-siblings, age difference was the key factor influencing their affiliation: same-age partners were more affiliated than partners born in different years. High levels of affiliation did not preclude high levels of aggression. Siblings were more aggressive to each other than non-siblings, but their agonistic interactions involved less severe aggression and more reciprocity, in so far as immatures were more likely to respond aggressively when attacked by a sibling than by a non-sibling. Dominance ranks, which were fairly stable between pairs of immatures, seemed to influence patterns of play initiations, grooming and proximity to a certain extent. Interaction Reciprocity was higher among siblings than among non-siblings only in grooming. However, non-sibling grooming Interaction Reciprocity was higher between reciprocal partners than between non-reciprocal ones. Play Interaction Reciprocity was high between reciprocal play partners. In play, grooming and proximity siblings were more often reciprocal partners than non-siblings. Siblings were also more likely to be reciprocal partners in more than one behaviour than non-siblings. Immatures did not reciprocate agonistic aid by aiding the former supporter. However, coalitions and coalitions for aids or vice versa were reciprocated, and it happened more often in sibling than in non-sibling dyads. Generally, sibling relationships were more likely to be reciprocal than relationships between non-siblings. On the basis of this evidence, the concept of reciprocity is discussed as a useful framework for considering the patterns and quality of relationships between immature monkeys, as well as for mechanisms of their social development. The degree of reciprocity observed in immatures may have implications for the development of reciprocal exchange, observed in the behaviour of adult primates.