Age-related change in adult chimpanzee social network integration

Abstract

Background: Social isolation is a key risk factor for the onset and progression of age-related disease and mortality in humans, yet older people commonly have narrowing social networks. Few models explain why human networks shrink with age, despite the risk that small networks and isolation pose. We evaluate models grounded in a life history perspective by studying social aging in wild chimpanzees, which are long-lived and show physical decline with age. Methodology: We applied social network analysis to examine age-related changes in social integration in a 7+ year mixed-longitudinal dataset comprised of 38 wild adult chimpanzees (22 F, 16 M) in the Kanyawara community in the Kibale National Park, Uganda. Metrics of social integration included social attractivity and overt effort (directed degree and strength), gregariousness (undirected strength), social roles (betweenness and local transitivity), and embeddedness (eigenvector centrality) in grooming and spatial association networks. Results: Males reduced overt social effort yet increased in attractivity, roles in cliques, and embeddedness. Females were overall less integrated than males, and their decreased integration with age suggested social avoidance. Effects of age were largely independent of rank. Both sexes maintained highly repeatable inter-individual differences in several aspects of integration, particularly among mixed-sex partners. Conclusions and implications: As in humans, chimpanzees experience age-related declines in social effort. However, important facets of integration aged more similarly to humans in non-industrialized vs. industrialized societies, suggesting an evolutionary social mismatch between conserved declines in effort and dynamics of industrialized society. Lastly, individual and sex differences have the potential to be important mediators of successful social aging in chimpanzees, as in humans.