Adaptive Party Choice of Low-Ranking Males in Fission-Fusion Dynamics of Chimpanzees in Kalinzu Forest Reserve, Uganda

Simple Summary Various mammalian species, including primates, exhibit fission-fusion dynamics (FFD), which alters the size of subgroups through splitting and merging. Among primates, chimpanzees (Pan troglodytes) have been particularly studied as a species that exhibits a high degree of FFD, wherein members of the same group form temporary subgroups (parties) that vary in both size and composition. Most studies on the FFD of chimpanzees have focused on its role in reducing food competition among individuals. In this study, we focused on the association patterns of each adult male chimpanzee and examined whether the social factors, including male dominance rank and aggression from other males, affect their party attendance behavior. Low-ranking males spent more time alone than other males when mating opportunities were absent. When females were available for mating, males of all ranks showed a similar party attendance behavior. The aggression increased with the number of males in the party, and low-ranking males received more aggression than higher-ranking males. These results suggest that low-ranking males frequently traveled alone to avoid aggression from other males unless they attended parties to seek mating opportunities. The FFD seemed to offer alternative tactics for low-ranking males to balance the costs and benefits of attending parties. Several studies have examined factors that regulate fission-fusion dynamics (FFD) in chimpanzee communities, such as receptive females, predation risks, and food availability. However, the effects of these factors vary between populations. In this study, we conducted focal animal observations of adult males in the M group in Kalinzu to examine the influence of male dominance rank, aggression from other males, the presence of females exhibiting maximum sexual swelling (MS), and fruit abundance on male tendencies of party attendance. We found that low-ranking males spent more time alone than other males when females with MS were absent. In contrast, when females with MS were present, males of all ranks showed a similar tendency of party attendance. We also found that the aggressive interactions increased with the number of males irrespective of the presence or absence of females with MS, and low-ranking males attracted aggression more frequently than higher-ranking males. These results suggest that low-ranking males frequently ranged alone to avoid aggression from other males unless they attended parties to seek mating opportunities. We conclude that low-ranking males have alternative tactics to balance the costs and benefits incurred or gained when attending parties.