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

Nous avons étudié la latéralité gestuelle intraspécifique de chimpanzés et de gorilles captifs dans des contextes socio-écologiques proches des conditions naturelles de vie. Nous avons montré que les chimpanzés et les gorilles étudiés présentaient un biais populationnel pour la main droite pour la majorité des gestes les plus fréquents de leur répertoire. Par la mise en oeuvre d’une approche multifactorielle, nous avons montré pour la première fois que la latéralité gestuelle intraspécifique de ces primates était influencée par plusieurs facteurs et par leurs interactions: contexte de l’interaction (champs visuels de l’émetteur et du récepteur et contexte émotionnel), caractéristique du geste (modalité sensorielle, utilisation d'un outil de communication, degré de partage et durée du geste) et par certaines composantes sociodémographiques, particulièrement le rang hiérarchique de l’émetteur et son âge dans une moindre mesure. De plus, nous avons comparé la latéralité manuelle des chimpanzés lors de l'utilisation d'outil pour des actions non-communicatives et des gestes intraspécifiques. Notre analyse multifactorielle suggère que l’utilisation d’outil dans les gestes serait plus contrôlée par l'hémisphère cérébral gauche que l’utilisation d’outil dans des actions non-communicatives. Globalement, nos résultats vérifient le modèle de Ghirlanda et collègues (2009) selon lequel les biais de latéralité au niveau populationnel pourraient être expliqués par une stratégie évolutive stable basée sur les interactions intraspécifiques. Nos résultats sont également en accord avec les études mettant en évidence l'utilisation préférentielle de la main droite pour la communication gestuelle des primates non humains et suggérant que la latéralité gestuelle serait un précurseur de la spécialisation hémisphérique gauche pour le langage. En outre, nos résultats confortent l'hypothèse que certaines espèces de primates pourraient avoir un traitement spécifique de l’hémisphère gauche pour les gestes communicatifs distinct de celui des actions manuelles non-communicatives. Du point de l’évolution, nos résultats soulignent l’importance d’étudier en détail la latéralité intraspécifique en considérant des espèces de différents degrés de socialité et en prenant en compte des contextes socioécologiques proches des conditions naturelles ainsi que de multiples facteurs potentiellement influents. / We studied intraspecific gestural laterality of captive chimpanzees and gorillas in real-life social-ecological relevant contexts. We evidenced that chimpanzees (respectively gorillas) exhibited a right-hand bias at the population level for the majority of the most frequent gestures of their specific natural communication repertoire. By designing and applying a multifactorial approach, we showed for the first time that intraspecific gestural laterality of primates was influenced by several factors and their mutual intertwinement: interactional context (visual fields of both signaller and recipient as well as emotional context), gesture characteristic (sensory modality, use of a communication tool, sharing degree, and gesture duration) and by some socio-demographic components in particular signaller’s hierarchical rank, and to a lesser extent signaller’s age. Similarities but also some discrepancies between chimpanzees and gorillas may be related to the lateralization of emotional processing, to communication strategies, and to social selection pressures related to the social structure and dynamics of the study species. Moreover, we compared manual laterality of tool use by chimpanzees in both non-communication actions and intraspecific gestures. Our multifactorial analysis showed that tool-use in gestures appear to be governed more by the left cerebral hemisphere than tool-use in non-communication actions. Our findings support Ghirlanda and colleagues’ (2009) model postulating that population-level bias could be explained by an evolutionary stable strategy based on intraspecific interactions. Our results also agree with previous reports evidencing predominant right-hand use by nonhuman primates for gestural communication and suggesting that gestural laterality would be a precursor of the left-brain specialization for language. Furthermore, our results support the hypothesis that some primate species may have a specific left-cerebral system processing gestures distinct from the cerebral system processing non-communication manual actions. From an evolutionary point of view, our findings emphasize the importance to study intraspecific laterality in detail by considering species varying in their degree of sociality and taking into account real-life social-ecological contexts and multiple potentially influential factors.

Bien que l’utilisation précoce de gestes communicatifs par de jeunes enfants soit reconnue comme étant étroitement liée au développement du langage (e.g., Colonnesi et al., 2010), la nature des liens gestes–langage doit encore être clarifiée. Cette thèse a pour but d’étudier la production de gestes de pointage au cours du développement afin de déterminer si la relation prédictive et facilitatrice entre les gestes et l’acquisition du langage implique des fonctions spécifiques du pointage, en association avec des caractéristiques spécifiques en terme de forme de mains, regard et vocalisations. De plus, une attention particulière a été apportée à l’étude des préférences manuelles dans le but de mieux comprendre le développement de la spécialisation hémisphérique gauche pour les comportements communicatifs. Nos résultats ont révélé des relations complexes entre le langage, les gestes communicatifs et les activités de manipulation, qui dépendent de la fonction des gestes (i.e., pointage impératif versus déclaratif) et des étapes spécifiques de l’acquisition du langage. Les gestes déclaratifs sont plus étroitement associés au développement de la parole que les gestes impératifs, au-moins avant la période d’explosion lexicale. De plus, la comparaison des patterns de préférence manuelle chez l’enfant et l’adulte a montré une plus grande proximité pour les gestes que pour la manipulation d’objet. L’asymétrie manuelle droite pour les gestes communicatifs est ainsi établie à des stades précoces, ce qui suggère un rôle primordial des gestes dans la spécialisation hémisphérique.Finalement, nos résultats ont mis en évidence l’existence d’un système de communication dans l’hémisphère cérébral gauche contrôlant à la fois la communication gestuelle et verbale, qui pourrait avoir une origine phylogénétique ancienne (e.g., Corballis, 2010). Par conséquent, le présent travail peut améliorer notre compréhension des origines du langage, y compris des mécanismes de la spécialisation cérébrale pour les comportements communicatifs. / Even though children’s early use of communicative gestures is recognized as being closely related to language development (e.g., Colonnesi et al., 2010), the nature of speech–gestures links still needs to be clarified. This dissertation aims to investigate the production of pointing gestures during development to determine whether the predictive and facilitative relationship between gestures and language acquisition involves specific functions of pointing, in association with specific features in terms of hand shape, gaze and accompanying vocalizations. Moreover, special attention was paid to the study of hand preferences in order to better understand the development of left hemisphere specialization for communicative behaviors. Our results revealed complex relationships between language, communicative gestures and manipulative activities depending on the function of gestures (i.e., imperative versus declarative pointing) as well as on specific stages of language acquisition. Declarative gestures were found to be more closely associated with speech development than imperative gestures, at least before the lexical spurt period. In addition, the comparison of hand-preference patterns in adults and infants showed stronger similarity for gestures than for object manipulation. The right-sided asymmetry for communicative gestures is thus established in early stages, which suggests a primary role of gestures in hemispheric specialization.Finally, our findings have highlighted the existence of a left-lateralized communication system controlling both gestural and vocal communication, which has been suggested to have a deep phylogenetic origin (e.g., Corballis, 2010). Therefore, the present work may improve current understanding of the evolutionary roots of language, including the mechanisms of cerebral specialization for communicative behaviors.