Spider monkeys (Ateles sp.) are characterized by high fission-fusion dynamics, meaning their social grouping pattern is fluid and consists of subgroups that vary in size, composition, and spatial cohesion over time. In this study, we quantify the fission-fusion dynamics of a group of spider monkeys at Runaway Creek Nature Reserve in Belize by measuring subgroup size, spatial cohesion, and stability using data spanning 5 years. We then test whether variation in these three subgroup measures differ according to season, subgroup sex composition, and the reproductive status of female subgroup members. Our results show that subgroups were larger in size and less stable in membership during the wet season compared to the dry season. All-female subgroups were less spatially cohesive but more stable in membership than all-male subgroups. Finally, we report that subgroups with one or more non-lactating females (i.e., without nursing young) were smaller on average than subgroups containing lactating females with nursing young. These data contribute to a growing body of research documenting the ecological and social dimensions along which grouping patterns might vary.
Keywords: Ateles; Belize; Fission–fusion dynamics; Seasonality; Sex differences; Subgroups.