Female Friendships: Social Network Analysis as a tool to understand intra-group affiliation in semi-commensal lion-tailed macaques (Macaca silenus)

Abstract

The lion-tailed macaque is a gregarious, rainforest-adapted species, that has, in certain locations across its natural distribution, recently begun to explore and utilise surrounding human-dominated habitats. In many primate species, exposure to novel human-use food resources and potential provisioning has previously been associated with changes in intra-group social structure, often categorised by increased aggression and, more importantly, increased affiliation, possibly as a means of reconciliation. Here we quantify the changing affiliative relationships among the female members of two groups with varying degrees of habituation, using social network analysis. We examine frequencies of pair-wise affiliation between the female members of each group, ranked within the prevailing linear hierarchy, to measure individual attributes or the local importance of individuals in a network, and group attributes or the global role of all individuals in the whole network. We found that the subordinate individuals in the less habituated group maintained a higher number of connections with group members, an expected outcome, as key affiliative behaviours such as grooming are known to be directed upwards in the hierarchy. This pattern was observed to be inverse in the highly habituated group, with dominant individuals maintaining more connections, suggesting that under the conditions of increased competition for the novel food resource, dominance rank was highly contested. In support of this theory, we also found multiple fluctuations in dominance rank over time for this highly habituated group, with nearly no fluctuations in the less habituated group. This study demonstrates that varied intensities of human presence and dependence on human-use foods have differential effects on the intra-group sociality of lion-tailed macaques.