Comparisons between animal and human communication are invaluable for understanding the evolution of language and, as our closest living relatives, chimpanzees can provide particularly important insights into this. Here I examined unimodal (UM) and multimodal (MM) communication in wild chimpanzees, in an integrated manner. I found that although MM signals were produced infrequently, and at lower rates compared to captivity, the vast majority of adult and sub-adult individuals did freely combine vocal, gestural and facial signals to produce MM signals. A total of 48 free MM signal combinations were observed, incorporating a wide range of different signal types from different modalities. Focusing on one specific vocalgestural MM signal, I found that MM combinations and UM gestural signals were more successful in eliciting responses compared to UM vocal signals. To investigate signal function more systematically I focused on one common grooming gesture, the big loud scratch (BLS), and tested several competing functional hypotheses. I found little evidence to support the hypotheses that this signal operates as an attention-getter, or as a referential signal. In contrast, my data suggested that in this community of chimpanzees, the BLS facilitates the negotiation of roles within a grooming bout. Groomers used BLSs to request grooming during grooming bouts and the BLS seemed to show willingness to groom, both to initiate a grooming bout, and potentially during a bout when groomees intend to start grooming their partner. Finally, to explore the theoretical link between the evolution of communication and cooperation I tested whether, on an individual level, there was a positive relationship between communicativeness and cooperativeness in chimpanzees. In contrast to theoretical predictions, I found a significantly negative relationship between these two domains, indicating that more communicative chimpanzees were less cooperative. I explore several potential explanations for this highly unexpected finding.

Vocal interactions, such as call exchanges or choruses, are common behaviours in animals and their function has been often linked to social bonding. In this study, I examined the relationship between two types of vocal behaviour common in chimpanzees, pant hoot chorusing and food calling, and social bonds between males. The results of this study, which was conducted on the Kanyawara community of chimpanzees (Pan troglodytes schweinfurthii) in the Kibale National Park, Uganda, show that both pant hoot choruses and food-associated calls reliably reflect social bonds between males. For example, males were more likely to join in another male’s pant hoot when a well affiliated male was calling. Males, irrespective of the long-term affiliation status between them, were also more likely to be involved in grooming or form coalitions on days when they chorused together, suggesting that chorusing is also a flexible bonding behaviour operating on a short-term basis. Males were also considerably more likely to produce food-associated calls when feeding in close proximity to well affiliated males than to less affiliated ones or females. Importantly, a male feeding partner was more likely to remain with the focal until the end of a feeding bout if the focal food called at the onset of this bout, suggesting that these vocalisations mediate feeding decisions between affiliated males. The results of my study suggest that these two types of vocalisations play important functions in chimpanzee fission-fusion societies. Pant hoot chorusing, for example, might facilitate the occurrence of other affiliative interactions between individuals who are not necessarily long-term preferred social partners. Food-associated calls, on the other hand, might facilitate anticipation of each feeding decision which in turn facilitates individuals remaining in proximity or in the same party. Both these types of vocalisations, therefore, might mitigate the costs of living in unstable societies formed by these primates.

An important element in understanding the evolutionary origin of human language is to explore homologous traits in cognition and communication between primates and humans (Burling, 1993, Hewes, 1973). One proposed modality of language evolution is that of gestural communication, defined as communicative movements of hands without using or touching objects (de Waal, 2003). While homologies between primate calls and language have been relatively well explored, we still have a limited understanding of how cognitive abilities may have shaped the characteristics of primate gestures (Corballis, 2003). Chimpanzees (Pan troglodytes) are our closest living relatives and display some complex cognitive skills in various aspects of their gestural behaviour in captivity (de Waal, 2003, Pollick and de Waal, 2007). However, it is not yet currently clear to what extent these abilities seen in captive apes are typical of chimpanzees in general and to what extent cognitive capacities observed in captive chimpanzees have been enhanced by the socio-cultural environment of captivity such as language training. In this Ph.D. research, I investigated the cognitive skills underlying gestural communication in both wild and language trained chimpanzees, with a special focus on the repertoire and the intentionality of production and comprehension. The study of cognitive skills underlying the production of the repertoire and the role of intentionality is important because these skills are cognitively demanding and are a prerequisite in human infants for their ability to acquire language (Baldwin, 1995, Olson, 1993). My research suggests that chimpanzee gestural communication is cognitively complex and may be homologous with the cognitive skills evident in pre-verbal infants on the cusp of language acquisition. Chimpanzees display a multifaceted and complex signal repertoire of manual gestures. These gestures are the prototypes, within which there is variation, and between which the boundaries are not clear-cut, but there is gradation apparent along several morphological components. Both wild and language trained chimpanzees communicate intentionally about their perceived desires and the actions that they want the recipients to undertake. They do not just express their emotions, but they communicate flexibly by adjusting their communicative tactics in response to the comprehension states of the recipient. Whilst chimpanzees communicate their intentions flexibly, the messages conveyed are specific. However, recipients comprehend gestures flexibly in light of the signaller’s overall intentions. Whilst wild and language trained chimpanzee gestural communication revealed similar cognitive characteristics, language trained chimpanzees outperformed wild apes in that they had ability to use signals which made distinctions that human deictic words can make. Whilst these differences between wild and language trained chimpanzees may be due to the different methodological approaches used, it is conceivable that language training may have influenced captive ape cognitive skills in the representational domain. These results from wild and language trained chimpanzees indicate that chimpanzees possess some form of cognitive skills necessary for language development and that cognitive skills underlying repertoire and use in chimpanzees are a shared capacity between humans, other apes and a common ancestor. These findings render theories of the gestural origins of language more plausible. Related publications: 1. Roberts, A. I., Vick, S.-J., Roberts, S. G. B., Buchanan-Smith, H. M. & Zuberbühler, K. 2012. A structure-based repertoire of manual gestures in wild chimpanzees: Statistical analyses of a graded communication system. Evolution and Human Behavior, Published online: http://dx.doi.org/ 10.1016/j.evolhumbehav.2012.05.006 2. Roberts, A. I., Vick, S.-J. & Buchanan-Smith, H. 2012. Usage and comprehension of manual gestures in wild chimpanzees. Animal Behaviour, Published online: http://dx.doi.org/10.1016/j.anbehav.2012.05.022