Selective social tolerance drives differentiated relationships among wild female chimpanzees

Abstract

Strong, affiliative bonds often function to facilitate social competition through cooperative defence of resources, but the benefits of social bonds may be low when direct competition is less intense or less beneficial. In such cases, one possible outcome is that relationships are weak and undifferentiated. Alternatively, negotiating stable, selectively tolerant relationships may be a strategy to mitigate the costs and risks of sharing space when direct competition is undesirable. We investigated dyadic social tolerance among wild adult female eastern chimpanzees, Pan troglodytes schweinfurthii, who engage in low rates of affiliation and aggression with one another. While females associate with one another at different rates, these patterns could reflect shared patterns of behaviour (e.g. ranging) rather than social preference or variation in relationship quality. We first determined whether patterns of dyadic spatial association (5 m proximity) were differentiated and stable over time. To assess whether dyadic spatial association reflected preference and variation in social tolerance, we tested whether spatial association was actively maintained by waiting and following behaviour and associated with decreased aggression and increased co-feeding. Spatial associations were differentiated, and stronger associations were more stable. Frequent associates used following and waiting behaviour to actively maintain associations. Association positively predicted time co-feeding and negatively predicted aggression. These patterns were true among related and unrelated dyads. Among unrelated females, dyads with stronger associations maintained proximity more mutually. This study highlights social tolerance as a stable relationship attribute that can predict and explain patterns of behaviour and social network structure, distinct from, or in the absence of, affiliation.