Group dominance increases territory size and reduces neighbour pressure in wild chimpanzees

Abstract

Territorial social species, including humans, compete between groups over key resources. This between-group competition has evolutionary implications on adaptations like in-group cooperation even with non-kin. An emergent property of between-group competition is group dominance. Mechanisms of group dominance in wild animal populations are difficult to study, as they require long-term data on several groups within a population. Here, using long-term data on four neighbouring groups of wild western chimpanzees, we test the hypothesis that group dominance impacts the costs and benefits of between-group competition, measured by territory size and the pressure exerted by neighbouring groups. Larger groups had larger territories and suffered less neighbour pressure compared with smaller groups. Within-group increase in the number of males led to territory increase, suggesting the role of males in territory acquisition. However, variation in territory sizes and neighbour pressure was better explained by group size. This suggests that the bisexually-bonded social system of western chimpanzees, where females participate in territorial behaviour, confers a competitive advantage to larger groups and that group dominance acts through group size in this population. Considering variation in social systems offers new insights on how group dominance acts in territorial species and its evolutionary implications on within-group cooperation.

Recent genetic connectivity and clinal variation in chimpanzees

“…Therefore, our observation of relatively high effective migration throughout P. t. verus may indicate higher levels of connectivity in western chimpanzees, but may alternatively or additionally be a signal of a relatively recent (likely northward) population expansion. Indeed, much of the P. t. verus range is savannah mosaic habitat with discontinuous gallery forests 53 , where chimpanzees tend to have both (1) larger home ranges (over 63 km 2 ) 8 than forest-dwelling populations, to seemingly compensate for lower fruit tree density and suitable habitat availability, and (2) a concomitant increase in dispersal distances 54 , 55 , thereby possibly driving a signal of increased migration. However, effective migration was uniform across most of the P. t. verus range, as similar rates were also detected among the forest-dwelling populations in southern Liberia and western Côte d’Ivoire, where home ranges have been reported to be between 6 and 37 km 2 in size 54 , suggesting that dynamics other than ecology are involved.…”

“…Indeed, much of the P. t. verus range is savannah mosaic habitat with discontinuous gallery forests 53 , where chimpanzees tend to have both (1) larger home ranges (over 63 km 2 ) 8 than forest-dwelling populations, to seemingly compensate for lower fruit tree density and suitable habitat availability, and (2) a concomitant increase in dispersal distances 54 , 55 , thereby possibly driving a signal of increased migration. However, effective migration was uniform across most of the P. t. verus range, as similar rates were also detected among the forest-dwelling populations in southern Liberia and western Côte d’Ivoire, where home ranges have been reported to be between 6 and 37 km 2 in size 54 , suggesting that dynamics other than ecology are involved. This and, especially, the low diversity in P. t. verus , suggest that the high effective migration we observed in this subspecies should be interpreted as evidence of a population bottleneck followed by a recent and fast range expansion.…”

Intercommunity interactions and killings in central chimpanzees (Pan troglodytes troglodytes) from Loango National Park, Gabon

“…However, studies on communities of western chimpanzees ( P. t. verus ) in the Taï National Park, Côte d'Ivoire, showed a different pattern. For instance, in this subspecies females are frequently involved in territorial activities and contribute significantly to displace individuals of other communities during encounters (Boesch and Boesch-Achermann 2000 ; Boesch et al 2008 ; Wilson et al 2014 ; Hashimoto et al 2020 ; Lemoine et al 2020 ). In addition, killings are less frequent than in eastern chimpanzees (Wilson et al 2014 ).…”

Home range size in central chimpanzees (Pan troglodytes troglodytes) from Loango National Park, Gabon

“…They contain a heavily used central area (i.e., core area) surrounded by a less frequently visited periphery that may overlap extensively with neighboring territories (Herbinger et al 2001;Wilson et al 2001). Home range size in chimpanzees is related to the size of the community, food availability, population density, intercommunity relationships, and interspecific competition (Nishida et al 1985;Goodall 1986;Boesch and Boesch-Achermann 2000;Herbinger et al 2001;Lehmann and Boesch 2003;Amsler 2009;Mitani et al 2010;Head et al 2012;Nakamura et al 2013;Lemoine et al 2020).…”