A comparative developmental approach to multimodal communication in chimpanzees (Pan troglodytes)

Abstract

Studying how communication of our closest relatives, the great-apes, develops can inform our understanding of the socio-ecological drivers shaping language evolution. However, despite a now recognized ability of great apes to produce multimodal signal combinations, a key feature of human language, we lack knowledge about when or how this ability manifests throughout ontogeny. In this thesis, I aimed to address this issue by examining the development of multimodal signal combinations (also referred to as multimodal combinations) in chimpanzees. To establish an ontogenetic trajectory of combinatorial signalling, my first empirical study examined age and context related variation in the production of multimodal combinations in relation to unimodal signals. Results showed that older individuals used multimodal combinations at significantly higher frequencies than younger individuals although the unimodal signalling remained dominant. In addition, I found a strong influence of playful and aggressive contexts on multimodal communication, supporting previous suggestions that combinations function to disambiguate messages in high-stakes interactions. Subsequently, I looked at influences in the social environment which may contribute to patterns of communication development. I turned first to the mother-infant relationship which characterises early infancy before moving onto interactive behaviour in the wider social environment and the role of multimodal combinations in communicative interactions. Results indicate that mothers support the development of communicative signalling in their infants, transitioning from more action-based to signalling behaviours with infant age. Furthermore, mothers responded more to communicative signals than physical actions overall, which may help young chimpanzees develop effective communication skills. Within the wider community, I found that interacting with a wider number of individuals positively influenced multimodal combination production. Moreover, in contrast to the literature surrounding unimodal signals, these multimodal signals appeared highly contextually specific. Finally, I found that within communicative interactions, young chimpanzees showed increasing awareness of recipient visual orientation with age, producing multimodal combinations most often when the holistic signal could be received. Moreover, multimodal combinations were more effective in soliciting recipient responses and satisfactory interactional outcomes irrespective of age. Overall, these findings highlight the relevance of studying ape communication development from a multimodal perspective and provide new evidence of developmental patterns that echo those seen in humans, while simultaneously highlighting important species differences. Multimodal communication development appears to be influenced by varying socio-environmental factors including the context and patterns of communicative interaction.