Objectives: Speech is unique to humans and characterized by facial actions of ∼5 Hz oscillations of lip, mouth or jaw movements. Lip-smacking, a facial display of primates characterized by oscillatory actions involving the vertical opening and closing of the jaw and lips, exhibits stable 5-Hz oscillation patterns, matching that of speech, suggesting that lip-smacking is a precursor of speech. We tested if facial or vocal actions exhibiting the same rate of oscillation are found in wide forms of facial or vocal displays in various social contexts, exhibiting diversity among species.
Materials and methods: We observed facial and vocal actions of wild stump-tailed macaques (Macaca arctoides), and selected video clips including facial displays (teeth chattering; TC), panting calls, and feeding. Ten open-to-open mouth durations during TC and feeding and five amplitude peak-to-peak durations in panting were analyzed.
Results: Facial display (TC) and vocalization (panting) oscillated within 5.74 ± 1.19 and 6.71 ± 2.91 Hz, respectively, similar to the reported lip-smacking of long-tailed macaques and the speech of humans.
Discussion: These results indicated a common mechanism for the central pattern generator underlying orofacial movements, which would evolve to speech. Similar oscillations in panting, which evolved from different muscular control than the orofacial action, suggested the sensory foundations for perceptual saliency particular to 5-Hz rhythms in macaques. This supports the pre-adaptation hypothesis of speech evolution, which states a central pattern generator for 5-Hz facial oscillation and perceptual background tuned to 5-Hz actions existed in common ancestors of macaques and humans, before the emergence of speech.
Keywords: central pattern generator; macaques; preadaptation; speech evolution; teeth chattering.
© 2017 Wiley Periodicals, Inc.