MCCARTHY, Robert C.; Florida Atlantic University: Supralaryngeal vocal tract morphology in the rhesus macaque (Macaca mulatta): A geometric morphometric analysis of growth, development and integration

Determining the size and shape of the non-human primate supralaryngeal vocal tract (SVT) is a necessary first step for reconstructing SVT shape, and inferring speech abilities, in fossil humans. To estimate minimum length of the SVT, the mandible�s inferior border is often used as a proxy for the superior-most position of the hyoid body. Justifications for this proxy include qualitative observations on hyoid position and the idea that the hyoid is tightly bound to the mandible by the suprahyoid muscles. To test these assumptions, 32 landmarks on the basicranium, face, and hyoid body were collected from 53 radiographs of 12 rhesus macaques x-rayed at several points during growth. Thin-plate splines and relative warps were used to analyze data treated cross-sectionally and longitudinally. In addition, hyoid movement relative to the skull base, face and mandible was assessed using Euclidean Distance Matrix Analysis (EDMA). Results indicate that the hyoid body tends to be positioned above the mandible�s inferior border, but below the attachment of m. mylohyoideus on the mandible�s inner border. Previous studies incorrectly position the hyoid lower in the neck because primates are scanned (via CT or MRI) in supine position, often with their heads extended. Relative warp and EDMA analyses indicate that the hyolaryngeal complex is tightly integrated with the gonial angle of the mandible. Natural variation in head position and posture acts to rotate the hyoid around this curve (like a train on its tracks), and not perpendicular to the mandibular border as suggested by previous studies. If this result holds true for other non-human primate species, then reconstructions of SVT size and shape in fossil humans need to be altered.