Infrared thermal imaging: Positive and negative emotions modify the skin temperatures of monkey and ape faces.

Facial thermography has enabled researchers to noninvasively and continuously measure the changes of a range of emotional states in humans. The present work used this novel technology to study the effect of positive and negative emotions in nonhuman primates by focusing on four facial areas (the peri-orbital area, the nose bridge, the nose tip, and the upper lip). Monkeys and apes were examined for positive emotions (during interactions with toys and during tickling) and for negative emotions (during food delay and teasing). For the combined toy and tickling condition, the results indicated a drop in the nose tip temperature and a tendency of an increase in the peri-orbital temperature. For the combined food delay and teasing condition, the results also revealed a rise in the upper lip temperature of the subjects. These different effects on the facial temperatures in monkeys and apes most likely reflect distinctive physiological reactions of a primordial primate emotion system. We conclude that facial thermal imaging represents a promising physiologically grounded technology to noninvasively and continuously obtain reliable data on emotional states in nonhuman primates, which may help modernize research on emotions in nonhuman primates and enhance our understanding of the evolution of human emotions.