Uncovering the decision rules behind collective foraging in spider monkeys

Unlike animals living in cohesive groups, those in groups with high fission–fusion dynamics can separate temporarily from other group members. While mechanisms underlying collective decisions are not well understood, it is possible that these separations occur by a divergence of interests between individuals. We evaluated the collective foraging decisions made by a group of Geoffroy's spider monkeys, Ateles geoffroyi, in a tropical forest in the Yucatan peninsula, Mexico. We assessed whether each individual's centrality in the social network, years living in the group, age, sex and social relationships affected the collective decisions. We examined two collective dynamics: foraging movements and arrivals to novel food patches, and in the latter, identified knowledgeable and naïve individuals. In both dynamics, spider monkeys exhibited partially shared leadership influenced by individual attributes, particularly the centrality in the social network, age and time in the group. These attributes have been associated with ecological knowledge. In arrivals to novel food patches, individuals most likely to have information about food resources (i.e. central individuals and males) had a greater likelihood to be followed, suggesting an influence of social information on the tendency to follow others. Also, we show evidence that social information could accelerate the arrival of naïve individuals to these patches. Collective foraging dynamics in our study group seem to arise from local rules that each individual follows when foraging: when a naïve individual needs social information to forage, it will follow the individual that is more likely to have this information, i.e. a central individual or a male; otherwise, when it is not possible or necessary to use social information, naïve individuals will follow a partner with which they have a strong social relationship. Thus, the interaction between local decision rules and the structure of the social network could drive collective coordination in fission–fusion dynamics.