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 ol dcl to attach to the clam's nipple undergoes Extensive changes. Duri ng the Ist postnatal week, the pup performs a rotation along the longitudin al 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, thi s sequence is modified and the pup first searches and establishes contact w ith a nipple before rotating to the supine posture. This sequence of moveme nts is then truncated. By postnatal Day 11, pups may attach to a nipple whi le in a prone posture. Developmental changes ira supination before attachin g to the nipple are reminiscent of changes ill righting during a similar pe riod 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 an d rotation to the nipple executed against the force of gravity. The paralle l structure of these behaviors is consistent with a com,non origin and simi lar control mechanisms for these distinct motor behaviors that are expresse d early in postnatal development. (C) 1999 John Wiley & Sons, Inc.