Flexibility in the social structure of male chimpanzees (Pan troglodytes schweinfurthii) in the Budongo Forest, Uganda

Abstract

Individuals of social species face a trade-off between the competitive costs and social benefits of group living. Species show a range of social strategies to deal with this trade-off, for example atomistic fission-fusion dynamics in which temporary social groups of varying size and membership form and re-form; or molecular fission-fusion dynamics which contain stable sets of multilevel nested subgroups. Chimpanzees are considered an archetypical atomistic fission-fusion species, using dynamic changes in day-to-day association to moderate the costs of within-group competition. It has been argued that humans’ highly flexible social organisation allows us to live in extremely large groups. Using four years of association data from two neighbouring communities of East African chimpanzees (Pan troglodytes schweinfurthii), we describe new levels of flexibility in chimpanzee social organisation and confirm the presence of subgrouping in a second, large community of chimpanzees. We show that males from the larger Waibira community (N males 24-31) exhibited additional levels of semi-stable subgrouping, while males from the smaller Sonso community (N males 10-13) did not. Subgroup membership showed stability across some years, but flexibility across others. Our data support the hypothesis that chimpanzees can incorporate strategies other than fission-fusion to overcome costs of social living, and that their social organisation may be closer to that of modern humans than previously described.SIGNIFICANCE STATEMENTSocial living offers benefits and costs; groups can more easily locate and defend resources, but experience increased individual competition. Many species, or individuals, flexibly adjust their social organization when faced with different competitive pressures. It is argued that humans are unique among primates in combining multigroup social organisation with fission-fusion dynamics flexibly within and across groups, and that doing so allows us to live in extremely large groups. Using four-years of association data we show new levels of flexibility in chimpanzee social organization. Males from a typically-sized community employed atomistic fission-fusion dynamics, but males in an unusually large community incorporated additional semi-stable levels of subgrouping. Our data support the hypothesis that chimpanzee males combine social organization strategies, and that doing so may allow them, like humans, to mitigate individual costs at larger community sizes.

Flexibility in the social structure of male chimpanzees ( Pan troglodytes schweinfurthii ) in the Budongo Forest, Uganda

“…Data accessibility. The data and code used for this study are available from the GIThub repository https://github.com/ Wild-Minds/Multi-level-social-structure_BudongoChimpanzees and have been archived within the Zenodo repository https://zenodo.org/record/7023496#.Ywe2mOzMI0Q [146].…”

“…The data and code used for this study are available from the GIThub repository and have been archived within the Zenodo repository [ 146 ].…”

Dead-infant carrying by chimpanzee mothers in the Budongo Forest

“…Observations of wild chimpanzees in dense secondary forest are often challenging, and individuals in highly fission-fusion East African chimpanzee communities may not be observed for days or weeks (Badihi et al 2021). In addition, we often choose not to follow, or to follow only at an extended distance, individuals who are experiencing highstress events, such as maternal loss.…”