Monkey Facial Dynamics in ‘Minimal Interactions’

ABSTRACT Increasingly, researchers are interested in studying the neural substrates of monkey social interactions within laboratory settings. However, semi-naturalistic monkey interactions within lab settings have not been well characterized. We here analyze “minimal interactions” between monkeys in the laboratory. Monkeys, like humans, produce a variety of facial expressions when they encounter one another. It has been unclear what specific information they use to guide their behavior. I recorded the facial signals of captive long-tailed macaques ( Macaca fascicularis ) while they visually interacted while seated in primate chairs. I found that the most consistently-evoked expressions were affiliative in character, most notably in the form of reciprocal lipsmacking. Consistent with prior experiments, lipsmacks were most evident when situational and social ambiguity was maximal: that is, in the first sessions, in the first moments of each session, and when interacting with unfamiliar individuals. Unexpectedly, head and eye orientation played dissociable roles in interactive behaviors. Most intriguingly, monkeys’ facial behaviors reflected both received and previously sent signals, suggesting they interpret others’ current signals in light of their own past communications.