Gorillas' Vocalizations During Rest Periods: Signals of Impending Departure?

Animal signals have been interpreted as indicating something about the signaller's internal state and hence its subsequent behaviour, while at the same time eliciting a response from the receiver. Such signals are often given when the costs and benefits of an action depend on what others do. This interpretation of meaning and function of signals has been applied primarily to ritualized competitive or courtship displays. Here we use the approach to analyse another context of signalling and a more subtle communicatory behaviour. Gorillas live in small cohesive groups whose synchronized activities alternate between travel/ feeding periods and resting periods. We present data on gorillas' vocalizations - the grunts - prior to a coordinated departure from a rest period. We suggest that gorillas use these signals to indicate their readiness to depart and to assess that readiness in others. Vocal activity increased significantly towards the end of rest periods, due both to individuals calling at higher rates, and to a greater number of vocalizers. This increase in vocal activity was not associated with a greater clumping of animals, nor with an increase in non-rest activities, and therefore appeared to be related to subsequent departure. The frequent exchange of grunts supports the notion that gorillas might use the calls of others to assess their readiness to depart. We speculate that such signalling could function to synchronize the behaviour of group members and lead to coordinated group movement.