Multimodal communication development in semiwild chimpanzees

Human language is characterised by the integration of multiple signal modalities, including speech, facial and gestural signals. While language likely has deep evolutionary roots that are shared with some of our closest living relatives, to date studies of great ape communication have largely focused on each modality separately, thus hindering insights into the origins of its multimodal nature. Studying when multimodal signals emerge during great ape ontogeny can inform about both the proximate and ultimate mechanisms underlying their communication systems, shedding light on potential evolutionary continuity between humans and other apes. To this end, the current study investigated developmental patterns of multimodal signal production across a total of 28 semi-wild chimpanzees (Pan troglodytes) aged 1-11 years. We examined the production of facial expressions, gestures and vocalizations across a range of behavioural contexts, both when produced separately and as part of multimodal signal combinations (henceforth multimodal). Overall, we found that while unimodal signals were produced consistently more often than multimodal combinations across all ages and contexts, the frequency of multimodal combinations increased significantly in older individuals and most within the aggression and play contexts, where the costs of signalling ambiguity may be higher. Furthermore, older individuals were more likely to produce a multimodal signal rather than a unimodal signal and again, especially in aggressive contexts. Variation in production of individual signal modalities across ages and contexts are also presented and discussed. Overall, evidence that multimodality increases across age in chimpanzees is consistent with patterns of developing communicative complexity in human infancy, revealing apparent evolutionary continuity. Findings from this study contribute novel insights into the evolution and development of multimodality and highlight the importance of adopting a multimodal approach in the comparative study of primate communication.