How many for dinner? Recruitment and monitoring by glances in capuchins.

Group members present physical and physiological differences according to their age, sex or social status, which could generate motivation differences among individuals during travel. In spite of these divergences of interest among individuals, the group succeeds more often than not in making a collective decision about departure time and which direction to take. To reach a consensus decision, animals should exchange information relating to characteristics of group movement through different communication channels. The main purpose of this study is to understand the function of behaviour patterns displayed during movements of white-faced capuchins (Cebus capucinus). We designed experiments in which we provoked collective movements involving a binary choice. During experiments, a video camera recorded the behaviour of each capuchin, which enabled us to determine which individuals displayed a behavioural pattern during travel and how this behaviour influenced the other group members. We found that looking backwards seemed to permit the recruitment of group mates during collective movement. This behaviour also seemed to allow the quantification of the number of followers, since the emitter modified its locomotion speed according to this number. In this preliminary study, we showed that visual behaviour was used to recruit and monitor group mates during collective movements and provided information on mechanisms involved in maintaining cohesion and coordination among group members during travel.