Movements of the oral apparatus begin during the fetal period and 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 developing movements appear prenatally in precocious mammals such as guinea pigs and sheep, but are postnatal in altricial mammals such as rats, hamsters and rabbits. The orderly development of oral behavior is probably related to the progressive maturation of the nervous system and neuromuscular connections. Most newborn mammals feed exclusively bysuckling, a combination of the tongue working against the nipple and negative pressure at the back of the oral cavity. Thetransition from suckling to mastication is gradual and involves considerable learning. In at least one species, the domestic pig, infant animals chew using a somewhat different muscular contraction pattern from that of adults. Age changes in muscle action lines are the most likely explanation for this difference. After being established in infancy, the process of mastication undergoes only minor changes in rate and relative muscle activity during the juvenile period. Throughout ontogeny there is a reciprocal relation between morphology and behavior. While masticatory performance depends on structure at any given stage, it also has profound effects on further musculoskeletal growth and differentiation.