Cineradiography of Monkey Lip-Smacking Reveals Putative Precursors of Speech Dynamics

• Published in: Current biology Vol. 22; no. 13; pp. 1176 - 1182
• Main Authors: Ghazanfar, Asif A., Takahashi, Daniel Y., Mathur, Neil, Fitch, W. Tecumseh
• Format: Journal Article
• Language: English
• Published: England Elsevier Inc 10.07.2012
• Subjects: Animals; Facial Expression; humans; Hyoid Bone; Lip - physiology; lips; Macaca fascicularis - physiology; Male; Mastication; monkeys; Periodicity; Primates; radiography; Singing; Speech; Tongue; Vocalization, Animal - physiology; X-radiation
• ISSN: 0960-9822; 1879-0445
• DOI: 10.1016/j.cub.2012.04.055

A key feature of speech is its stereotypical 5 Hz rhythm [1, 2]. One theory posits that this rhythm evolved through the modification of rhythmic facial movements in ancestral primates [3, 4]. If the hypothesis has any validity, then a comparative approach may shed some light. We tested this idea by using cineradiography (X-ray movies) to characterize and quantify the internal dynamics of the macaque monkey vocal tract during lip-smacking (a rhythmic facial expression) versus chewing. Previous human studies showed that speech movements are faster than chewing movements, and the functional coordination between vocal tract structures is different between the two behaviors [5–9]. If rhythmic speech evolved through a rhythmic ancestral facial movement, then one hypothesis is that monkey lip-smacking versus chewing should also exhibit these differences. We found that the lips, tongue, and hyoid move with a speech-like 5 Hz rhythm during lip-smacking, but not during chewing. Most importantly, the functional coordination between these structures was distinct for each behavior. These data provide empirical support for the idea that the human speech rhythm evolved from the rhythmic facial expressions of ancestral primates. ► Lips, tongue, and hyoid move during lip-smacking with a speech-like 5 Hz rhythm ► Movements during chewing were significantly slower for each of these structures ► Coordination between vocal tract structures during lip-smacks was like speech