How the neonatal rat gets to the nipple. II. Changes during development.

In the course of postnatal development, the motor sequence executed by pups in order to attach to the dam's nipple undergoes extensive changes. During the 1st postnatal week, the pup performs a rotation along the longitudinal axis of its trunk to achieve a supine posture under the mother. The pup then crawls on the maternal ventrum while in the supine posture, searching for, finding, and attaching to a nipple. During the 2nd postnatal week, this sequence is modified and the pup first searches and establishes contact with a nipple before rotating to the supine posture. This sequence of movements is then truncated. By postnatal Day 11, pups may attach to a nipple while in a prone posture. Developmental changes in supination before attaching to the nipple are reminiscent of changes in righting during a similar period of development. These observations support the idea that both righting and postural adjustments involved in attachment to the nipple derive from common motor modules, with righting executed in the direction of gravity and rotation to the nipple executed against the force of gravity. The parallel structure of these behaviors is consistent with a common origin and similar control mechanisms for these distinct motor behaviors that are expressed early in postnatal development.