Communicative complexity and development in chimpanzees (Pan troglodytes) and bonobos (Pan paniscus) in the wild

Abstract

In my doctoral thesis I examined the diversity and ontogeny of gestural communication in chimpanzees (Pan troglodytes) and bonobos (Pan paniscus) living in their natural environments. Great apes use gestures as intentional and flexible communicative strategies in a wide range of social contexts, which has led many researchers to emphasize the role of gestures in the evolution of language. However, little is known about the ontogeny of these communicative means, especially in wild ape communities that are not influenced by a human-modified environment. The developmental approach yet comprises an indispensable tool to gain insights into the cognitive complexity underlying gestural communication. Since we can only understand the communicative abilities of a species if within-species variability is considered, I conducted the first study on ape gestural communication in multiple social groups using a consistent methodology across study sites and subsequent years. I observed chimpanzees living in two communities of different subspecies (P. t. schweinfurthii at Kanyawara, Kibale National Park, Uganda, and P. t. verus at Taï South, Taï National Park, Côte d’Ivoire). Additionally, I included corresponding data on mother-infant dyads in two bonobo communities: LuiKotale at the fringe of Salonga National Park, and Wamba in the Luo Scientific Reserve, Democratic Republic of the Congo. I thus had the exceptional opportunity to compare social groups of different communities and sub-species, and for mother-infant interactions also different ape species.To better understand the communicative complexity of these species, I combined the comparative with the developmental approach, which included cross-sectional comparisons between individuals of different groups, as well as longitudinal comparisons of the same individual between subsequent years. To assess general developmental transitions of the infants and to ensure comparability across species, this thesis was built upon methods developed by researchers investigating communicative development of human and non-human primates. Moreover, I adopted a conversation-analytic, multimodal framework derived from linguistics to gain a deeper understanding of chimpanzee communication. I specifically focused on three communicative contexts—mother-infant joint travel, play solicitation and food sharing—to assure that the examined signals actually carried the ‘meaning’ perceived by the observer (i.e. ‘goal-outcome matching’). Thus, my thesis differed substantially from the majority of gestural research on great apes that included entire gestural repertoires of ape species across all age groups. The major results of my thesis are summarized below:First, based on my findings on the development and inter-individual variability of gesture production, my co-authors and I constructed a revised theory of gestural acquisition termed Social Negotiation. According to this theory gestures represent the output of social interactions, shared understanding and mutual construction in real time by both interactants. This study thus provided further support that apes acquire their gestures via learning mechanisms, as opposed to genetic predisposition (Chapter II). Second, by adopting a conversation-analytic framework for mother-infant interactions in chimpanzees and bonobos in Chapter III I demonstrated that both species are capable of engaging in cooperative, sequential turn-taking interactions to achieve a joint goal. Moreover, I found profound differences related to structural and temporal parameters of communicative interactions between the two species. In Chapter IV I demonstrated that chimpanzees possess the cognitive flexibility to adjust their gestural signalling to attributes of the recipient, and that age and relationships of the interactants strongly affected gestural usage. Lastly, I examined and compared gestural performance of infant chimpanzees in three social contexts and showed that communicative development crucially relies on interactive experience with social partners outside the mother-infant dyad (Chapter V).Overall, my thesis provides hitherto undocumented evidence to which extent communicative abilities in chimpanzees and bonobos are linked to input from their complex social environment. With my work I hope to have stimulated future studies to also adopt a more holistic view of ape communication in natural environments. Further in-depth studies of ape gestural production are necessary to test the Social Negotiation Hypothesis in detail and to examine the usage of gestural signals in relation to developmental phase, social context and interaction partner. Hence, future comparative research should implement a developmental approach and incorporate different signal modalities and social settings. This enables us to test how the ecology and social structure of a species shapes its communicative complexity. In turn, this might shed further light on the extent to which communicative and cognitive abilities of our primate ancestors informed the evolution of the unique human communication system that we termed ‘language’.