The term ‘fission-fusion’ describes social systems in which individuals belonging to the same community are rarely all together, but spend most of the time in subgroups that frequently merge and split again with similar or different membership depending on the species. Although such flexible social organisation is rare, it has been described in some primate and non-primate species. Fission-fusion patterns are also typical of humans, although it is not often explicitly recognised. The sharing of such flexible social nature with our closest living relatives, the chimpanzee and bonobo, suggests that fission-fusion dynamics may have influenced important adaptations during the course of human evolution.
Little systematic research on the factors affecting moment-to-moment changes in subgroup membership and the behavioural and cognitive implications of fission-fusion dynamics has been carried out. The need for such research was emphasised by leading experts at the recent symposium ‘Fission-Fusion Societies’ (held at the 20th Congress of the International Primatological Society, Turin, Italy, 23-28 August 2004) and the post-congress workshop ‘Fission-Fusion Societies and Cognitive Evolution’, both of which were organised by us. One of the outcomes of the symposium and workshop was the consensus for a broader use of the term ‘fission-fusion’, not just as a label of a particular social organisation, but as a characteristic of social organisation that varies along a complex continuum from highly cohesive to highly fluid with stable or flexible membership. There is therefore the need for comparative investigations on the degree of spatial and social fluidity across various social organisations. Another related outcome was the emphasis on the importance of research on the regulation of social relationships, which can be particularly challenging in highly fluid groups and impose specific cognitive demands.
During the past five years we have studied the social dynamics of spider monkeys at two zoological gardens and two field sites in Costa Rica and Mexico. The spider monkey is a critical species on which to investigate the dynamics related to spatial and social fluidity because the degree of flexibility in its social organisation is similar to that of chimpanzees and may shed indirect light on the evolution of human sociality and cognitive abilities.