Abstract
Three-dimensional (3D) tongue movements are central to performance of feeding functions by mammals and other tetrapods, but 3D tongue kinematics during feeding are poorly understood. Tongue kinematics were recorded during grape chewing by macaque primates using biplanar videoradiography. Complex shape changes in the tongue during chewing are dominated by a combination of flexion in the tongue's sagittal planes and roll about its long axis. As hypothesized for humans, in macaques during tongue retraction, the middle (molar region) of the tongue rolls to the chewing (working) side simultaneous with sagittal flexion, while the tongue tip flexes to the other (balancing) side. Twisting and flexion reach their maxima early in the fast close phase of chewing cycles, positioning the food bolus between the approaching teeth prior to the power stroke. Although 3D tongue kinematics undoubtedly vary with food type, the mechanical role of this movement—placing the food bolus on the post-canine teeth for breakdown—is likely to be a powerful constraint on tongue kinematics during this phase of the chewing cycle. The muscular drivers of these movements are likely to include a combination of intrinsic and extrinsic tongue muscles.
Highlights
Mammal tongues can assume a wide range of shapes during vocalization, grooming and feeding
More data on the activation of longitudinal intrinsic muscles are necessary to test this hypothesis. This is the first quantification of 3D twisting and flexion of the tongue during chewing: the tongue posture illustrated in sketches by Ab-el-Malek is shown to occur during the fast open (FO) and fast close (FC) phases of the gape cycle and to be produced by sagittal flexion and middle tongue roll, accompanied by balancing side flexion of the tongue tip
Despite the behavioural flexibility of the tongue during vocalization, drinking and food ingestion, the movements during chewing seem remarkably stereotyped: they were very similar in all four animals and resemble those illustrated for humans eating a wide range of foods, including nuts, candies and gum
Summary
Mammal tongues can assume a wide range of shapes during vocalization, grooming and feeding. Because the tongue is largely hidden within the mouth, it has been difficult to make detailed measurements of these movements and shape changes during feeding [1,2] and to relate these tongue movements to simultaneous mandible movements. Such information is important context for studies of tongue movement during feeding, as rehabilitation of tongue movements, if and where possible, must occur in coordination with the mandible for feeding to be successful. Abd-el-Malek’s model of tongue kinematics during chewing is often referred to (a)
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