Fission-fusion dynamics as a temporally and spatially flexible behavioral strategy in spider monkeys

Abstract: Fission-fusion dynamics (FFD) encompass a behavioral strategy present in many animal species that reduces the costs and increases the benefits of group living. In this case study, we investigated how group characteristics (size and composition) and fission rates in spider monkeys varied in space and time with rainfall, fruit availability, and fruit variability in two sites, each presenting different characteristics regarding the distribution and size of food patches and rainfall. Habitat characteristics strongly influenced FFD in spider monkeys, particularly subgroup size and fission rate. Subgroup size varied with fruit availability and its variability, while fission rates varied with rainfall and fruit variability. However, both subgroup size and fission rate varied in opposite ways, depending upon habitat type. Subgroups tended to present stable mixed-sex composition regardless of fruit availability. We conclude that for spider monkeys, FFD are part of a flexible behavioral strategy to cope with a locally fluctuating environment and with different environments within the geographic range of the species. Significance statement: Fission-fusion dynamics (FFD) constitute a form of social organization that allows some species to take advantage of living in groups under different environmental conditions. The relationship between social organization and environmental variables has been well studied, but inconsistencies remain. One potential reason for these inconsistencies may be the focus of most studies on a single habitat type, with few formal comparisons of FFD in the various habitats occupied by a species. We evaluated how habitat characteristics (e.g., food availability and rainfall) affect FFD (assessed through subgroup size, subgroup composition, and fission rate) in spider monkeys (Ateles geoffroyi) in two different habitat types. We found that within a single habitat type, food availability, variability, and rainfall affected subgroup size and fission rate. Crucially, these relationships contrasted sharply, depending on habitat type. Our study shows that FFD are flexible within and across habitats, indicating the importance of considering habitat characteristics when conducting socio-ecological observations. We caution against generalizations based upon single-habitat studies.