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Properties of Facial Signaling in Captive Chimpanzees (Pan troglodytes)


Since the 4th century B.C., there has been great interest in facial signaling in both human and non-human animals (Fridlund 1994). Early naturalists believed that the face was linked to the soul, and facial signals were the product of emotional experience (Bell 1806; Descartes 1649). As a result, most facial signaling studies to date have focused on placing facial muscle movement into discrete categories of emotion (Ekman 1970). Facial signals are assumed to be spontaneously produced and inflexible due to their strong ties to emotion; these signals are contrasted to other forms of non-vocal communication such as manual gestures, which are intentionally and flexibly produced (Pollick and de Waal 2007). In recent years, the idea that facial signals can be categorized based on physical form or corresponding emotion has been contested (Fridlund 1994; Waller et al. 2017). Some studies have even found evidence for goal association in great ape facial signals types such as the ‘play face’, which is considered to be an important gesture property (Cartmill and Byrne 2010). But it is unclear as to whether great ape facial signals are capable of taking on all important gesture properties, and if so, how they compare to manual gestures. The goal of this dissertation project was to examine the physical form and social function of facial signals in our closest living relatives, the chimpanzee (Pan troglodytes). The history of facial signaling inspired two different research questions in this project: (1) can chimpanzee facial signals take on gesture properties, and if so, how do they compare to manual gestures; and (2) is there variation in the physical form of chimpanzee facial signals, and how does this compare to other ape species. To address these two questions, I collected data using a combination of two previously described sampling methods: the focal individual sampling method and the opportunistic sampling method. A comparison of these two methods can be found in Chapter 1. The results of this dissertation suggest that chimpanzee facial signals can be used as gestures (Chapter 2), and that there is extensive variation in the physical form of chimpanzee facial signals when compared to other ape species (such as gibbons; Chapter 3). These findings have important implications for the evolution of sociality, gestural communication, and human language, which is discussed at the end of each chapter.

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