The Ontogeny of Mammalian Mastication

Movements of the oral apparatus begin during the fetal periodand develop in a consistent order. Jaw openingappears first,followed by active jaw closure and tongue movements, lip movements,sucking, and finally masticatory movements. The later developingmovements appear prenatally in precocious mammals such as guineapigs and sheep, but are postnatal in altricial mammals suchas rats, hamsters and rabbits. The orderly development of oralbehavior is probably related to the progressive maturation ofthe nervous system and neuromuscular connections. Most newbornmammals feed exclusively bysuckling, a combination of the tongueworking against the nipple and negative pressure at the backof the oral cavity. Thetransition from suckling to masticationis gradual and involves considerable learning. In at least onespecies, the domestic pig, infant animals chew using a somewhatdifferent muscular contraction pattern from that of adults.Age changes in muscle action lines are the most likely explanationfor this difference. After being established in infancy, theprocess of mastication undergoes only minor changes in rateand relative muscle activity during the juvenile period. Throughoutontogeny there is a reciprocal relation between morphology andbehavior. While masticatory performance depends on structureat any given stage, it also has profound effects on furthermusculoskeletal growth and differentiation.