Expanding the Horizons of Facial Mimicry in Chimpanzees ( <i>Pan troglodytes</i> ): New Insights into Diverse Signal Types and Contexts

Abstract Facial Mimicry (FM) is an evolved mechanism in social mammals that potentially fosters emotional and physical connections through the production of congruent facial signals. Most research on FM has primarily examined relaxed open-mouth facial displays in the context of play. However, if the primary function of FM is to enhance the accuracy of predicting others’ behaviors, it stands to reason that FM would extend to various contexts and signal types. The aim of our current study was to determine whether rapid FM also occurred in different contexts and types of facial signals in a captive troop of chimpanzees. We hypothesized that rapid FM would be more prevalent in social contexts where evidence of FM has been reported across multiple species (hypothesis 1), as well as in the facial signal types associated with these contexts (hypothesis 2). Additionally, we expected that rapid FM would occur more frequently with simpler facial signals composed of fewer distinct muscle movements (hypothesis 3). We tested our three hypotheses across ten different contexts and with nine types of facial signals. We employed new computational methods that measure the number and proportion of facial muscle movements mimicked, which we refer to as RMsize and RMratio, respectively. These methods help us differentiate between types of rapid FM (exact matching versus type matching). We found no evidence to support our three hypotheses, suggesting that FM may be a more widespread mechanism than previously believed.