The use of infra-red thermography to study emotions of wild crested macaques (Macaca nigra) in their natural habitat

Abstract

Infra-red thermography (IRT) has been validated across a range of taxa as a non-invasive method to quantify physiological changes (variation in skin temperature) that serve as a proxy for emotional arousal in humans and other animals. While its effectiveness has been demonstrated in captivity, its application in wild animals under natural conditions remains underexplored. This study validates IRT in wild crested macaques (Macaca nigra), observed in their natural habitat, to assess its potential for studying short-term emotions as proximate mechanisms of behavior of wild animals. Seventy-five playback experiments and predator model presentations were conducted on 14 adult females, exposing them to stimuli including affiliative grunts, contact calls, aggressive screams, alarm calls, and python models. The objectives were to: (1) evaluate whether environmental and physical factors, such as atmospheric conditions and camera-subject distance, impact thermal measurements; (2) determine whether facial skin temperature variations reliably indicate emotional arousal and valence in response to ecologically relevant stimuli; and (3) explore correlations between temperature changes and behavioral indicators of arousal, such as physical activity and self-directed behaviors. Results showed that: (1) environmental and physical factors did not significantly affect thermal measurements when following our protocol; (2) facial skin temperature changes reliably reflected the intensity of emotional arousal elicited by different stimuli; and (3) temperature variation was associated with behavioral indicators of arousal and anxiety. These findings establish IRT as a reliable tool for investigating emotion-driven skin temperature changes in wild primates, providing valuable insights into the role of short-term emotions in shaping behavior