Do Barbary macaques 'comment' on what they see? A first report on vocalizations accompanying interactions of third parties.

Primates acquire knowledge about relationships of third parties and group structure by monitoring their conspecifics. We show that Barbary macaques (Macaca sylvanus) utter specific vocalizations while monitoring interactions of other group members. As they did not direct other behaviours to the interacting group members, we provisionally termed these vocalizations 'vocal comments'. We investigated the acoustic properties of these comments and the social contexts in which they occurred. Most adult males and females of two studied groups produced low-amplitude calls when observing close contact interactions of other group members. The acoustic features of these calls varied with characteristics of the commented situation. Our results suggest that such calls might not be directed towards the agents of the commented situation, but towards other group members. The vocal comments may signal the caller's awareness of the observed interaction and possibly attract the attention of others to the situation.