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Ontogeny of maternal behavior in the laboratory rat: early origins in 18- to 27-day-old young.

The responses of unweaned juveniles, 18 to 27 days of age, in brief tests with pups younger than 10 days were observed during initial exposure and at 4-6-day intervals. They showed no aversive responses and actively sought contact with the pups until 24 days when contact-seeking declined sharply. Young exposed to pups before 24 days continued to seek contact with them after 24 days showing an effect of the prior experience. Juveniles given a choice between young pups and a warm bowl chose the pups; given a choice between young pups and age mates, they chose the young pups until day 23, but chose the age mates after then. When housed continuously with young pups at 22 days of age, 5 of 9 retrieved and grouped pups with a latency of 1 day. Our results indicate that timidity or fear of novelty develops in rats in this situation about Day 24 and thereafter is a factor inhibiting the expression of positive social behavior toward pups.