Group behaviours and individual spatial sorting before departure predict the dynamic of collective movements in horses

International audience; Establishing how collective decisions emerge is central to our understanding of animal societies. A frequent approach in collective behaviour studies is to seek for leaders that will trigger and direct group movements. Relatively few studies on collective movements, notably in mammals, took into account the events preceding group departures. Success in recruitment can depend on the arousal state, the behaviour and the spatial distribution of animals before departure. We tested the effect of such factors on recruitment process in two groups of 9 domestic mares, Equus ferus caballus , at pasture. Predeparture periods were compared to control periods to identify their unique characteristics and to understand how they can predict the following collective outcome. We found that predeparture periods were characterized by the simultaneous increase in the number of active and goal-oriented individuals as well as in group cohesion. However, while the temporal patterns of activity and dispersion were unique to predeparture periods, those of orientation were not different from control periods. We also found that before departure individuals started to move towards the future goal area and to position themselves along the front-back axis. Moreover, the front-to-back position of group members was linked to the order in which they joined a collective movement. Finally, the duration of the joining process was positively correlated to the group dispersion. Our findings demonstrate that decision making is a continuous process and emphasize the necessity, in the study of collective decisions, to look beyond the event of initiation itself and the identity of the initiator as recruitment depends on group state before and at departure.