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

Abstract

Individuals of social species experience competitive costs and social benefits of group living. Substantial flexibility in humans' social structure and the combination of different types of social structure with fission–fusion dynamics allow us to live in extremely large groups—overcoming some of the costs of group living while capitalizing on the benefits. Non-human species also show a range of social strategies to deal with this trade-off. Chimpanzees are an archetypical fission–fusion species, using dynamic changes in day-to-day association to moderate the costs of within-group competition. Using 4 years of association data from two neighbouring communities of East African chimpanzees ( Pan troglodytes schweinfurthii ), we describe an unexplored level of flexibility in chimpanzee social structure. We show that males from the larger Waibira community ( N = 24–31) exhibited additional structural levels of semi-stable core–periphery society, while males from the smaller Sonso community ( N = 10–13) did not. This novel core–periphery pattern adds to previous results describing alternative modular social structure in other large communities of chimpanzees. Our data support the hypothesis that chimpanzees can incorporate a range of strategies in addition to fission–fusion to overcome costs of social living, and that their social structures may be closer to that of modern humans than previously described.

Dialects in leaf-clipping and other leaf-modifying gestures between neighbouring communities of East African chimpanzees

“…While Sonso and Waibira occupy neighbouring (and partly overlapping) territories in the same forest and experience regular migration, there are some differences between them that may be salient for gesture production. The Waibira community is approximately double the size of Sonso with a considerably higher male to female adult sex ratio (Waibira = 1:1, Sonso = 1:2), which likely increases mating competition between males 44 . Higher mating competition between Waibira males may provide an explanation for the apparent delay in the onset of these gestures’ use, particularly in sexual solicitation, as immature individuals are not yet able to compete for access to sexually receptive females 55 , 56 .…”

“…While Sonso represent a typically sized community, Waibira is a relatively large community 43 . Furthermore, male mating competition in Waibira is likely considerably higher than in Sonso as the Waibira community have an almost equal male:female adult sex ratio but Sonso have a (more typical) female biased sex ratio with around double the number of adult females to adult males 44 . Both communities are followed daily by a team of trained field assistants who conduct daily focal follows on all independent individuals (those regularly observed in parties without their mother) within the community.…”

Camera traps unveil demography, social structure, and home range of six unhabituated Western chimpanzee groups in the Moyen Bafing National Park, Guinea

“…Such male‐biased sex ratios can occur in more forested areas but appeared to be contextual, for example resulting from group fission (Feldblum et al, 2018; Wilson et al, 2014). The Waibira group in the Budongo Forest Reserve in Uganda also exhibits a sex ratio close to one (0.92; Badihi et al, 2022). However, this group is one of the largest recorded with around 120 individuals and it seems to accommodate many males because of a flexible core‐peripheral social structure that reduce male–male intragroup competition (Badihi et al, 2022).…”

“…The Waibira group in the Budongo Forest Reserve in Uganda also exhibits a sex ratio close to one (0.92; Badihi et al, 2022). However, this group is one of the largest recorded with around 120 individuals and it seems to accommodate many males because of a flexible core‐peripheral social structure that reduce male–male intragroup competition (Badihi et al, 2022). Finding a male‐biased sex ratio in the MBNP groups might suggest that it is common for chimpanzees inhabiting dryer environments.…”

“…Such male-biased sex ratios can occur in more forested areas but appeared to be contextual, for example resulting from group fission (Feldblum et al, 2018;Wilson et al, 2014). The Waibira group in the Budongo Forest Reserve in Uganda also exhibits a sex ratio close to one (0.92; Badihi et al, 2022).…”