Grunt to go-Vocal coordination of group movements in redfronted lemurs

To remain cohesive as a group, individuals must coordinate their movements between resources. In many species, vocalisations are used in this context. While some species have specific movement calls, others use calls which are also employed in different contexts. The use of such multicontextual calls has rarely been studied quantitatively, especially during both the pre-departure and departure period associated with collective decisions. We thus investigated the use of close calls (“grunts”) for the coordination of collective movements in four groups of wild redfronted lemurs (Eulemur rufifrons) in Kirindy Forest, Western Madagascar. Group movements are started by an initiator, who moves away from the group and is joined by followers setting out in the same direction. We observed collective movements and recorded vocalisations from 18 focal individuals (54 movements recorded for followers, 21 for initiators). The grunt rate of both initiators and followers was higher in the pre-departure period than in a control context (i.e., during foraging). Initiators of collective movements grunted more often than followers in the pre-departure period as well as at individual departure. The latter difference was due to the initiators’ grunt rates increasing earlier than the followers’ and remaining at an elevated level for longer. These observations suggest that grunts serve to coordinate the departure by indicating the individual's readiness to move. The pre-departure period, in which both initiators and followers showed an elevated grunt rate, may provide the basis for a shared decision on departure time. The difference in initiator and follower call rates suggests that grunts may have a recruitment function, but playback experiments are required to test this potential function. Overall, our study describes how multicontextual close calls can function as movement calls, with changes in call rate providing a potential feedback mechanism for the timing of group movements. This study thus contributes to a more detailed understanding of the mechanisms of group coordination and collective decision-making.