Changes in Grooming Networks Among Male Chimpanzees Preceding a Permanent Community Fission at Ngogo

Chimpanzees form social communities with memberships that change only with births, deaths, and female emigrations or immigrations and that are characterized by high fission-fusion dynamics. Relations between neighboring communities are intensely hostile, and males in the same community cooperate in potentially lethal intercommunity aggression. Permanent community fissions sometimes occur, but genetic data indicate that these are rare, and only a single case, from Gombe, had been known until recently. The unusually large Ngogo chimpanzee community in Kibale National Park, Uganda, observed continuously since 1995, underwent a gradual permanent fission during the 2010s that culminated in the inception of lethal intercommunity aggression in 2018. Males at Ngogo could be assigned to different socio-spatial neighborhoods based on association data before the permanent fission, but data on characteristics of grooming networks show that all belonged to a single community until around 2011. Subsequently, grooming networks showed clear differentiation between what became the Ngogo Central and Ngogo West communities. This preceded the first obvious behavioral signs of permanent fission. In principle, a model developed by Sueur et al. (2011; cf. Sueur and Maire 2014) to explain how increases in group size in papionin primates could lead to fracturing of female grooming networks and facilitate permanent fissions should apply to male chimpanzees, given the importance of male-male grooming for maintaining social bonds and cooperation in intergroup aggression. However, analysis of long-term data on male-male grooming indicates that increases in the number of females at Ngogo and variation in their residence decisions apparently had a greater effect on the dissolution of a community-wide male grooming network than did increases in the number of males. These results support the hypothesis that the main drover of the permanent fission was male reproductive competition, not constraints on the ability of males to maintain grooming networks.