Mammalian Oral Rhythms and Motor Control

Mastication is a derived mammalian trait, characterized by rhythmic jaw movements associated with intra-oral food handling, reduction and bolus formation. Hiiemae defined it as “a key feature of mammalian feeding that involves the coordination of complex movements and precise dental occlusion during a distinct power stroke of the chewing cycle (Hiiemae, 2000). Lund and Kolta refer to mastication as the time “during which the food is mechanically broken down and mixed with saliva to create a slurry of small particles or bolus that can be easily swallowed” (Lund & Kolta, 2006). There is a debate as to whether to define mastication in general or precise terms. The debate focuses on whether to include in its definition the requirements of precise post-canine occlusion, unilateral food bolus placement, and transverse motion of the mandible during the power stroke. Given that feeding in most mammalian and non-mammalian species has yet to be studied and characterized, we opt to use fewer qualifiers and to rely on a more general definition of mastication or chewing in this chapter. The variety of masticatory kinematics and dentoskeletal morphologies (Ungar, 2010) across mammals is almost as striking as plumage variation is among birds. The increased efficiency afforded by masticatory forms and functions may have been necessary to keep pace with another mammalian synapomorphy, the increased energy demands of endothermy. Alternatively, given that erupted enamel cannot be replaced, and that healthy teeth are requisite for longevity and fecundity, efficiency may be required to maximize the life of teeth. Whatever the case, mastication is only one of several distinct oral motor behaviors, which also include (a) suckling, a mammalian-specific trait involved in milk ingestion, (b) lapping or sucking which are used to ingest liquids, fruit juices or insects, (c) rumination or chewing of cud, (d) gnawing of bones or tough food items, (e) tongue rasping used by cats as a food softening behavior, (f) incising, chopping or cutting food, (g) tooth sharpening or thegosis, (h) speech, whistling and communication, (i) facial expressions such as smiling or gritting teeth aggressively, (j) protective behaviors such as sneezing, coughing, gagging or vomiting, (k) tool use such as blowing on, holding or catching objects, (l) respiratory behaviors such as breathing and panting (m) sensory pleasures such as tasting or kissing.