Directedness and engagement in chimpanzee vocal ontogeny

Directedness and engagement during pre-verbal vocal communication play a major role in language development. What was their role in the evolution of language? This question invites us to examine these behaviours in chimpanzee vocal ontogeny. We collected observational data on infant (N = 15) and juvenile (N = 13) chimpanzees at Chimfunshi Wildlife Orphanage, Zambia. We examined the impact of age and vocalization type (grunts, whimpers, laughs and screams) on directed cues (gaze directedness and face directedness) and engagement (mutual face directedness) during vocal communication. We also assessed the impact of directed cues and engagement on social interactions by coding the behaviour of social partners before, during and after a vocalisation, and examining whether they contingently changed their behaviour in response to the vocalisation if it was directed or if engagement occurred. We found that face directed vocalisations showed a general increase during ontogeny and we observed call-type dependent effects of age for mutual face directedness. Only face directed vocalisations were significantly predictive of behavioural responses in social partners. We conclude that like young humans, young chimpanzees routinely exhibit directed behaviours and engagement during vocal communication. This social competency improves during ontogeny and benefits individuals by increasing the chances of eliciting behavioural responses from social partners. Directedness and engagement likely provide a foundation for language phylogenetically, as well as ontogenetically. RESEARCH HIGHLIGHTS: We show that directedness and engagement routinely occur during early chimpanzee vocalisations. Directedness increases throughout chimpanzee vocal ontogeny, similar to human infants. Directedness enhances social partner responsiveness, demonstrating a direct benefit to this style of communication. Directedness and engagement could provide a route towards language phylogenetically as well as ontogenetically.