Morphological analyses and 3D modeling of the tongue musculature of the chimpanzee (Pan troglodytes).

Knowledge of the comparative anatomy of tongue musculature is crucial to the discussion of the origin and the evolution of speech because of the indispensable role played by this organ in speech. However, the tongue musculature of primates has rarely been studied. In a previous study, the author analyzed human tongue musculature and developed a 3D model of this organ [Takemoto, Journal of Speech, Language, and Hearing Research 44:95-107, 2001]. In this study, the tongue musculature of chimpanzees was examined using methods similar to those used for humans. Results showed that tongue musculature was topologically the same for both humans and chimpanzees. As in humans, the tongue musculature of chimpanzees consisted of inner and outer regions. The inner musculature was composed of serial "structural units," made up of two types of laminae whose fibers were perpendicular to the tongue surface. The outer musculature was a thin layer of fibers oriented parallel to the surface and superficial to the inner musculature. Although the tongue musculature of humans and chimpanzees is similar, the external shapes differ: the chimpanzee tongue is flat, whereas the human tongue is round. Applying the muscular hydrostat theory to the external shape of the tongue suggests that the primary actions of the chimpanzee tongue are protrusion and retrusion, whereas the human tongue can be deformed in the oral cavity with a high degree of freedom. It is hypothesized that the evolution of the external shape of the tongue is one of the factors that led to the development of human speech. The results of this study suggest that modeling based on muscular hydrostatic theory of the effects of changes in external tongue shape on articulatory movements should be included in discussions on the origin of speech.