DIFFERENT FACIAL ACTION UNITS DEFINE THE INTENSITY OF MACAQUE FACIAL EXPRESSIONS

Abstract

Facial expressions are a fundamental, conserved component of communication across primate species, serving critical functions in emotional responses, behavioral coordination, and social interaction. Studying facial expressions in rhesus macaque monkeys, which not only share the ability to make different expressions but also share similar facial musculature with humans, allows for a greater understanding of how the brain controls and displays emotional states. A critical dimension of facial expressions that conveys information about affective magnitude and communicative intent is the intensity of the expression. While intensity coding has been an established component of the human Facial Action Coding System (FACS), it has not been studied as extensively in macaques. Using the macaque-based version of FACS (MaqFACS), we documented and compared intensities of four common facial expressions in rhesus monkeys. This extends intensity coding to nonhuman primates, enabling comparative investigations of how magnitude of facial expressions relates to internal affective states and social communication across primate species.