Weaning in the Norway rat: relation between suckling and milk, and suckling and independent ingestion

Abstract

We examined in rat pups the relation between the decline of suckling and the emergence of independent ingestion, and the role of milk delivery conditional on nipple attachment in the maintenance of suckling. Fifteen-day-old litters were reared for either 5 or 10 days, and 20-day-old litters for 5 days by either a thelectomized (no nipples), ligated (nipples but no milk), or intact (nipples and milk) dam. Pups' food and water intakes were monitored daily, and their suckling, feeding, and drinking behaviors were videorecorded for 24 hr in the presence of their foster dam (Day 19 or 24) and for 24 hr in the presence of an intact, lactating dam (Day 20 or 25). There were no differences between treatment conditions with respect to either the onset or rate of increase of independent feeding or drinking. Pups reared by a thelectomized dam for 10 days displayed a pronounced, lasting depression of suckling. Twenty-five-day-old pups reared by a ligated dam displayed suckling levels comparable to those of control pups; in the presence of the ligated dam, however, their tendency to attach to a nipple was notably reduced. The implications of the findings for our understanding of the weaning process are discussed.