The role of emotions in the communication system and behaviour of female crested macaques (Macaca nigra): A new approach using infrared thermography

Animal vocal communication was long considered as being mainly driven by emotional reactions to environmental, social and/or internal stimuli, making these signals more similar to human vocalisations like laughter and crying than to spoken language. But these last decades, a growing number of ethological studies provided evidence in favour of the existence of a certain level of intentionality in call production by the signallers and of intentional control and flexibility in the way the receivers respond to certain vocal signals, blurring the lines between human language and the communication systems of other primates. However, due to methodological limitations, the role (or lack thereof) of short-term emotions in animal communication was often assumed, but rarely directly assessed. It has thus been difficult to clearly assess the level of involvement of emotions in the proximate mechanisms of communication and to make comparisons in this regard across different primate species. Recent technological advances, particularly in the field of infrared thermography, provide new tools that overcome such difficulties, making possible the exploration of the role of emotions in the different aspects of communication in our closest relatives. In this thesis, I made use of this state-of-the-art technology ‒ recently validated to quantify the strength of emotional arousal in wild primates. Coupled with field experiments (playbacks and predator model presentations), I provide empirical evidence regarding the role of emotions in call production, signal responsiveness and anti-predator behaviour of wild female crested macaques (Macaca nigra) as a model species.

To understand how a communication system has been shaped by evolution, it is necessary to consider both signallers and receivers. Thus, firstly, I have investigated the level of involvement of emotional arousal in the production of the receiver's response to three different signals: aggressive screams, affiliative grunts and alarm calls. The results show that the behavioural response of the receivers of aggressive screams and affiliative grunts ‒ two vocal signals frequently used by crested macaques to navigate their daily social interactions ‒ is closely tied to their emotional response. However, the social attributes of the caller, and particularly the quality of its socio-positive relationship with the receiver, is the main predictor of the behavioural response to alarm calls - i.e., a rarer signal uttered during life-threatening situations.

Secondly, I have investigated the role of context-specific emotion in the production of alarm calls of signallers detecting a fake python. The results show that, while a change of emotional arousal is necessary to trigger an anti-predator response and particularly the production of alarm calls, variation in the strength of emotional arousal is not sufficient to explain the observed variations into alarm calling behaviour. Other factors, particularly the social attributes of the audience, are taken into account during the process of elaboration of the anti-predator response.

The findings resulting from this work contribute to better understand underlying affective, perceptual and, indirectly, cognitive mechanisms involved in animal communication and social behaviour in my model species. My work also demonstrates that the use of infrared thermography offers a unique and innovative approach to address long-lasting questions, allowing direct empirical evidence regarding the importance of emotional states in the processes involved in animal communication rather than making assumptions about the function of emotions based on intuitions and indirect evidence.