Unravelling the Complexity of Positive Communication in Great Apes: a FACS-based Analysis of Laugh Face Variations and their Level of Dependence on the Age and Sex of their Social Partners, their Social Relationship, and their Attentional State in Orangutans and Chimpanzees during Spontaneous Playful Interactions

Abstract

Human smiles and laugh faces play a crucial role in everyday social interactions as pervasive tools of nonverbal communication and indicators of positive emotions. They can serve different functions and convey different meanings, for instance by varying the degree of mouth opening and teeth exposure. In humans, these morphological variants are highly context-dependent and influenced by the social partner’s characteristics. A considerable amount of literature indicates evolutionary continuity in facial expressions from nonhuman great apes (hereafter great apes) to humans. Human laugh faces and great ape open-mouth faces of play are likely to be homologous so the term “laugh face” can be applied to great apes too. Previous research has shown that great apes also produce laugh faces with varying degrees of teeth exposure and mouth opening. However, despite significant knowledge about ape laugh faces, very little is known about their function with regard to the social partner’s characteristics (i.e., their sex, age, social relationship, and attentiveness/facial orientation). My thesis aims, thus, to fill this gap by analysing with FACS the laugh faces of orangutans and chimpanzees during spontaneously occurring social play. We have investigated the occurrences of single facial muscles involved in laugh face production in three empirical studies. Firstly, the orangutans of our study modified their facial expressions with dependence on the age difference between the players and the sex of the playmate. We found that our subjects exposed their upper teeth more often towards younger playmates (and therefore weaker), suggesting an affiliative function. Secondly, we found that chimpanzees also modify their laugh faces depending on the age gap between the individuals. In this case, however, our subjects expose both their upper and lower teeth more often with older (and therefore stronger) playmates, suggesting a submissive function. As results of this study seemed also to suggest that chimpanzees might expose their lower teeth more often while facing the playmate, thirdly, we specifically investigated the impact of facial orientation on laugh face morphology in chimpanzees, by expanding our sample of individuals and facial expressions. We found that our subjects, when facing the social partner, exposed their lower teeth more often, and opened their mouth more widely with older playmates and when their playmates were also producing a laugh face. Our findings suggest that laugh face variants may serve different functions within great apes and that playmate characteristics can impact laugh face morphology in nonhuman apes too. These
results contribute to our understanding of great ape's laugh faces, in accordance with the Complexity and Continuity Hypothesis, suggesting that such complexity has been present at least since the last common ancestor of today’s human and great ape species.

Date of Award	30 Jun 2025
Original language	English
Awarding Institution	University of Portsmouth
Supervisor	Marina Davila Ross (Supervisor), Leanne Proops (Supervisor) & Guillaume Dezecache (Supervisor)