Joseph T Feldblum1, Sofia Manfredi1, Ian C Gilby2, Anne E Pusey1. 1. Department of Evolutionary Anthropology, Duke University, Durham, North Carolina, 27708. 2. School of Human Evolution and Social Change, and Institute of Human Origins, Arizona State University, Tempe, Arizona, 85287.
Abstract
OBJECTIVES: While permanent group fissions are documented in humans and other primate species, they are relatively rare in male philopatric primates. One of the few apparent cases occurred in 1973 in Gombe National Park, Tanzania, when a community of chimpanzees split into two separate groups, preceding the famous "Four-Year War." We tested the hypothesis that the original group was a single cohesive community that experienced permanent fission, and investigated several potential catalysts. MATERIALS AND METHODS: We calculated association, grooming, and ranging metrics from historical data and used community detection algorithms and matrix permutation tests to determine the timing, dynamics, and causes of changes in social network subgrouping structure. RESULTS: We found that the two communities indeed split from one cohesive community, albeit one with incipient subgrouping. The degree of subgrouping in grooming and association networks increased sharply in 1971 and 1972, a period characterized by a dominance struggle between three high-ranking males and unusually high male:female sex ratios. Finally, we found a relationship between post-split community membership and previous association, grooming and ranging patterns in most periods of analysis, one that became more pronounced as the fission approached. DISCUSSION: Our analysis suggests that the community began to split during a time of an unusually male-biased sex ratio and a protracted dominance struggle, and that adult males remained with those with whom they had preferentially associated prior to the split. We discuss the costs and benefits of group membership in chimpanzees and contrast these results with group fissions in humans and other taxa.
OBJECTIVES: While permanent group fissions are documented in humans and other primate species, they are relatively rare in male philopatric primates. One of the few apparent cases occurred in 1973 in Gombe National Park, Tanzania, when a community of chimpanzees split into two separate groups, preceding the famous "Four-Year War." We tested the hypothesis that the original group was a single cohesive community that experienced permanent fission, and investigated several potential catalysts. MATERIALS AND METHODS: We calculated association, grooming, and ranging metrics from historical data and used community detection algorithms and matrix permutation tests to determine the timing, dynamics, and causes of changes in social network subgrouping structure. RESULTS: We found that the two communities indeed split from one cohesive community, albeit one with incipient subgrouping. The degree of subgrouping in grooming and association networks increased sharply in 1971 and 1972, a period characterized by a dominance struggle between three high-ranking males and unusually high male:female sex ratios. Finally, we found a relationship between post-split community membership and previous association, grooming and ranging patterns in most periods of analysis, one that became more pronounced as the fission approached. DISCUSSION: Our analysis suggests that the community began to split during a time of an unusually male-biased sex ratio and a protracted dominance struggle, and that adult males remained with those with whom they had preferentially associated prior to the split. We discuss the costs and benefits of group membership in chimpanzees and contrast these results with group fissions in humans and other taxa.
Authors: Sylvain R T Lemoine; Liran Samuni; Catherine Crockford; Roman M Wittig Journal: Philos Trans R Soc Lond B Biol Sci Date: 2022-04-04 Impact factor: 6.237