Social Network Influences Decision Making During Collective Movements in Brown Lemurs (Eulemur fulvus fulvus)

Many animal species live as a group and must therefore move as such. Several authors have suggested that the mechanisms underlying collective movements in primate species appear to rely on complex cognitive skills, given their high level of cognitive abilities. However, recent studies have highlighted the fact that complex patterns do not necessarily imply complex mechanisms. We used a modeling approach to investigate the patterns of collective movement in a semi-free-ranging group of brown lemurs. We recorded via digital video cameras the order and joining latencies of the 11 individuals of the group during the departure time of spontaneous group movements. We then assessed whether mimetic mechanisms or the existence of a leader were underlying conditions for the joining process by testing 5 computer models relying respectively on 5 hypotheses: the independence of individuals, an anonymous mimetism, a mimetism according to kinship, a mimetism according to affiliation, and eventually the existence of a leader. We found that departure latencies, associations, and the order of individuals at departure time could all be explained by the mimetism according to affiliation model. Thus, an individual's decision to join the collective movement or not depended on the decision taken by its preferred social partners. These results show the importance of social parameters in primate decision making and that the high cohesion displayed by the group members might not be constrained merely by ecological factors such as predation or foraging consideration.