Communicating the motivation to play: gestures and facial expressions in bonobos

The correct production and interpretation of communicative signals is of primary importance for social animals. Free brachio-manual gestures, so far only reported for humans and other apes, are considered as intentional signals. Facial expressions, on the other hand, although used during intentional communication, are influenced by emotional states. Play is one of the most sophisticated forms of social communication and it has also been reported to be the context in which gestures occur more frequently in two Pan species. Through a video-analysis of playful sessions, we investigated the use of a) facial expressions and b) gestures in Pan paniscus. Both of them were more frequent during the most risky form of play, that is play fighting. The redundancy of signals is effective in reducing the probability of escalation into overt aggression. The difference between the use of gestures and facial expressions emerges with the variation of players' number. Gestures tend to decrease as the number of players increases; whereas facial expressions are independent from the number of players involved. Both communicative patterns are performed mainly when the receiver can perceive them, thus suggesting the intentional nature of these displays. The importance of deeply understanding the way apes and other primates communicate through gestures and how this capacity develops becomes central when considering that our ancestors' first linguistic expressions were probably in the gestural more than in the vocal domain.