These studies investigated the role that social facilitation, availability of the dam, and milk play in the maintenance of suckling behavior. Beginning at Day 20, rat pups' suckling experiences were restricted to testing with an anesthetized dam. In the first experiment, nipple attachment was abandoned by about Day 25 in rats that were tested alone with an anesthetized dam for 1 hr per day. When tested in groups of four, nipple attachment was maintained until about Day 27. Increasing exposure to an anesthetized dam to 2 hr per day prolonged nipple attachment about another 4 days. In the second experiment, pups were given either one or two daily 1-hr attachment tests and tested with either an anesthetized dam or an anesthetized dam in which the milk letdown reflex was reinstated.
Both increasing the daily exposure to an anesthetized dam and reinstating milk letdown significantly prolonged suckling. Pups given two daily exposures to an anesthetized, milk-laden dam attached until about Day 47, long past the normal age of weaning.
Suckling is the predominant ingestive behavior of virtually all newborn mammals. As the young grow older, their mother's milk is insufficient to meet their increasing nutritional demands, and they begin to obtain their nutritional requirements through feeding. The offspring gradually spend less time suckling and eventually abandon it entirely. The notion that suckling is an ontogenetic adaptation (Oppenheim, 1981) and not merely a precursor for adult ingestive behavior is well supported (Hall, 1975;1979). However, the mechanisms that underlie this transition from complete dependence on the mother to independent feeding are not well understood.