Development of locomotion toward home nesting material in neonatal rats.

Neonatal rats were individually placed in a narrow choice zone between home and fresh nesting materials. At Day 2 postpartum the pups made significantly more home-choosing responses (vs responses for fresh nesting material). At a mean of 5.6 days (range, 4 to 9 days) the pups reached a criterion of 2 consecutive days of responses all for home nesting material. The number of head contacts increased abruptly upon reaching the criterion. In a 2nd experiment we observed that the pups which were repeatedly tested performed no better than those which were tested for the 1st or the 2nd time. Thus, (a) home-seeking responses appear to emerge substantially earlier than previously reported by other investigators, (b) head-contact responses appear to be related to successful home-chossing, and (c) some maturational rather than experiential factors are important for the development of both head-contact and home-choosing responses.