Facial muscle coordination in monkeys during rhythmic facial expressions and ingestive movements.

Evolutionary hypotheses regarding the origins of communication signals generally suggest, particularly for the case of primate orofacial signals, that they derive by ritualization of noncommunicative behaviors, notably including ingestive behaviors such as chewing and nursing. These theories are appealing in part because of the prominent periodicities in both types of behavior. Despite their intuitive appeal, however, there are little or no data with which to evaluate these theories because the coordination of muscles innervated by the facial nucleus has not been carefully compared between communicative and ingestive movements. Such data are especially crucial for reconciling neurophysiological assumptions regarding facial motor control in communication and ingestion. We here address this gap by contrasting the coordination of facial muscles during different types of rhythmic orofacial behavior in macaque monkeys, finding that the perioral muscles innervated by the facial nucleus are rhythmically coordinated during lipsmacks and that this coordination appears distinct from that observed during ingestion.