Exploring the causation of a group split in semi-free-ranging Japanese macaques (Macaca fuscata)

Primate social groups can split into multiple, socially independent factions through fission events. Fission events are common among social primates and have long-lasting effects on a group’s social structure, well-being, and habitat use. The immediate cause of a fission often remains unclear, although earlier studies identified multiple, not necessarily mutually exclusive, ecological, demographic, and social factors. In the present study, we examined which of these factors caused a fission in a semi-free-ranging population of Japanese macaques ( Macaca fuscata ). First, we provide an overview of how the fission developed through repeated network visualisations. Then, we reanalysed a longitudinal dataset containing 23 years of ecological and demographic data and six years of behavioural data. We found that the population was socially united until high-ranking individuals began to retreat to the peripheral areas of the large enclosure, leading to increasing separation. The group fully divided after high-ranking individuals, including the alpha-female, died, and the alpha-male fully retreated from the group with closely affiliated members. These results suggest that a growing instability in the dominance hierarchy was the primary cause of the fission. Our study thereby increases our understanding of the behavioural and demographic characteristics of Japanese macaques and provides a rare chronological description of the origins and consequences of a fission event.