Modulation of intra-oral processing in mammals and lepidosaurs.

The mammalian masticatory apparatus is distinguished from the intra-oral processing systems of other amniotes by a number of morphological and functional features, including transverse movements of the teeth during the power stroke, precise occlusion, suspension of the teeth in the socket by a periodontal ligament, diphyodonty (reduction to two generations of teeth), a hard palate, and the presence of a single bone (the dentary) in the lower jaw which articulates with the skull at the temporomandibular jaw joint. The evolution of these features is commonly argued to have improved the efficiency of food processing in the oral cavity. The present aricle highlights the existence in mammals of the fusimotor system and afferent fibers from the periodontal ligament through which the CNS modulates the responses by the muscle spindles. Published data suggest that the fusimotor system and the periodontal afferents are important components in feed-forward (or anticipatory) control of chewing behavior. We hypothesize that this feed-forward control is used to maintain relatively constant cycle lengths in mammals in the face of intra-sequence and inter-sequence variation in material properties of the food, and that this enables them to maintain a higher average chewing frequency than that of lizards. These predictions were evaluated using data on mean cycle length and its variance from the literature and from our own files. On average, mammals have less variable cycle lengths than do lizards and shorter cycle lengths than do lizards of similar size. We hypothesize that by decreasing variance in cycle length, presumably close to the natural frequency of their feeding systems, mammals minimize energy expenditure during chewing, allowing them to chew for longer, thereby maintaining the high rates of food intake required for their high metabolic rates.