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. Nevertheless, older people commonly have narrowing social networks, with influences from both cultural factors and the constraints of senescence. We evaluate evolutionarily-grounded models by studying social aging in wild chimpanzees, a system where such influences are more easily separated than in humans, and where individuals are long-lived and decline physically with age. Methodology We applied social network analysis to examine age-related changes in social integration in a 7+ year mixed-longitudinal dataset on 38 wild adult chimpanzees (22 F, 16 M). Metrics of social integration included social attractivity and overt effort (directed degree and strength), social roles (betweenness and local transitivity), and embeddedness (eigenvector centrality) in grooming networks. Results Both sexes reduced the strength of direct ties with age (males in-strength, females out-strength). However, males increased embeddedness with age, alongside cliquishness. These changes were independent of age-related changes in social and reproductive status. Both sexes maintained highly repeatable inter-individual differences in integration, particularly in mixed-sex networks. Conclusions and implications: As in humans, chimpanzees appear to experience senescence-related declines in social engagement. However, male social embeddedness and overall sex differences were patterned more similarly to humans in non-industrialized vs. industrialized societies. Such comparisons suggest common evolutionary roots to ape social aging, and that social isolation in older humans may hinge on novel cultural factors of many industrialized societies. Lastly, individual and sex differences are potentially important mediators of successful social aging in chimpanzees, as in humans.

Multi-group analysis of grooming network position in a highly social primate

“…Moreover, out-strength decreased with age as well but in a linear fashion, meaning that males also groom others less often as they get older. In chimpanzees, a similar pattern is found in females which has been related to their subordinate status, as they try to avoid falling victim to agonistic interactions [ 21 ]. As male bonobos, and especially older males, are generally subordinate to their female group members ( Fig 1F ) [ 32 , 39 , 47 ], this could also explain their reduced levels of affiliation in our study.…”

“…Both these direct and indirect grooming network measures are dependent on individual characteristics like sex [12,17,18], age [12,[19][20][21], and early social rearing conditions [22][23][24]. However, studies do not always yield consistent results for these associations, even when done on the same species.…”

Personality traits, rank attainment, and siring success throughout the lives of male chimpanzees of Gombe National Park

“…Although chimpanzees experience age-related changes in social behavior ( Rosati et al, 2020 ; Thompson González et al, 2021 ) and personality ( King, Weiss & Sisco, 2008 ; Haux et al, 2023 ) resembling aging patterns in humans, studies in both captive and wild chimpanzees suggests that individual behavioral phenotypes are stable ( King, Weiss & Sisco, 2008 ; Weiss et al, 2017 ; Tkaczynski et al, 2020 ; Thompson González et al, 2021 ). Thus, given the present findings of associations between personality traits and fitness, another productive avenue for future studies would be to identify which behaviors related to Dominance and Conscientiousness are responsible for higher rank and reproductive success in males.…”

Observational approaches to chimpanzee behavior in an African sanctuary: Implications for research, welfare, and capacity‐building

“…Our data can also be used to identify strong relationships between specific chimpanzees, which can inform best practices for their well‐being. For example, we used grooming data to examine how the sex of partners influenced grooming rates, but we can also use the same kinds of data to identify strong bond partners (Gilby & Wrangham, 2008; Machanda et al, 2013; Rosati et al, 2020) or social networks (Thompson Gonzalez et al, 2021). Systematically assessing strong bond partners for all individuals in the group in this way can help with identifying who might provide comfort if a chimpanzee is in distress, can help inform pairings such as sleeping room locations, and can be useful information for sanctuaries planning releases as a long term goal, an increasingly urgent issue for Africans sanctuaries as they reach capacity (Andre et al, 2008; Farmer, 2002; Humle et al, 2011; Stokes et al, 2017; Stokes et al, 2018).…”