Fission-Fusion Dynamics of Large Ateline Species

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Enhanced sociality has long been recognized as one of the traits distinguishing primates from other mammals. Most day-active primates live in cohesive social groups in which members range in close proximity most of the time. However, data emerging from long-term field studies are revealing much greater variation in cohesiveness in primate social groups over time. We now recognize a spectrum of cohesiveness, termed “fission-fusion dynamics”, in which subgroups form and change in size, composition, and spatial cohesion. The long-term objective of my research program is to uncover the causes and consequences of variation in fission-fusion sociality through the study of two closely related primate species that vary dramatically in their levels of cohesion. Spider and howler monkeys, members of the large bodied Neotropical subfamily Atelinae, are often found living in the same forests, yet they show striking contrasts in ecology and social organization. Howlers are leaf eaters that live in consistently small and tightly cohesive groups (low fission-fusion dynamics) with small ranges and close spatial proximity. Spider monkeys are ripe fruit specialists that live in large, highly fluid communities (high fission-fusion dynamics) with large home ranges over which subgroups with fluid (often changing) membership are widely dispersed. Spiders and howlers, both found at the Runaway Creek Nature Reserve (RCNR) study site in Belize, are thus excellent subjects for the study of variation in primate sociality and fission-fusion dynamics. This research program has been active since 1999. Over the next five years I will investigate: how ecology and relatedness determine variation in fission-fusion dynamics; the relationship between sexual segregation, sexual selection, and fission-fusion dynamics; and the consequences of such flexible sociality in terms of stress and disease. This program will provide a comprehensive account of the manner in which relatedness, ecology, sexual selection, stress, and disease shape and constrain mammalian sociality. By investigating the environmental and social conditions that affect group cohesiveness and association patterns, I aim to better understand the evolution of mammalian sociality itself.