Production of grooming-associated sounds by chimpanzees (Pan troglodytes) at Ngogo: variation, social learning, and possible functions

Chimpanzees (Pan troglodytes) use some communicative signals flexibly and voluntarily, with use influenced by learning. These signals include some vocalizations and also sounds made using the lips, oral cavity, and/or teeth, but not the vocal tract, such as "attention-getting" sounds directed at humans by captive chimpanzees and lip smacking during social grooming. Chimpanzees at Ngogo, in Kibale National Park, Uganda, make four distinct sounds while grooming others. Here, I present data on two of these ("splutters" and "teeth chomps") and consider whether social learning contributes to variation in their production and whether they serve social functions. Higher congruence in the use of these two sounds between dyads of maternal relatives than dyads of non-relatives implies that social learning occurs and mostly involves vertical transmission, but the results are not conclusive and it is unclear which learning mechanisms may be involved. In grooming between adult males, tooth chomps and splutters were more likely in long than in short bouts; in bouts that were bidirectional rather than unidirectional; in grooming directed toward high-ranking males than toward low-ranking males; and in bouts between allies than in those between non-allies. Males were also more likely to make these sounds while they were grooming other males than while they were grooming females. These results are expected if the sounds promote social bonds and induce tolerance of proximity and of grooming by high-ranking males. However, the alternative hypothesis that the sounds are merely associated with motivation to groom, with no additional social function, cannot be ruled out. Limited data showing that bouts accompanied by teeth chomping or spluttering at their initiation were longer than bouts for which this was not the case point toward a social function, but more data are needed for a definitive test. Comparison to other research sites shows that the possible existence of grooming-specific sound dialects in chimpanzees deserves further investigation.