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 wha t others do. This interpretation of meaning and function of signals ha s been applied primarily to ritualized competitive or courtship displa ys. Here we use the approach to analyse another context of signalling and a more subtle communicatory behaviour. Gorillas live in small cohe sive groups whose synchronized activities alternate between travel/fee ding periods and resting periods. We present data on gorillas' vocaliz ations - the grunts - prior to a coordinated departure from a rest per iod. We suggest that gorillas use these signals to indicate their read iness to depart and to assess that readiness in others. Vocal activity increased significantly towards the end of rest periods, due both to individuals calling al higher rates, and to a greater number of vocali zers. This increase in vocal activity was not associated with a greate r clumping of animals, nor with an increase in non-rest activities, an d therefore appeared to be related to subsequent departure. The freque nt 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.