OBSERVATIONS ON THE SUCKLING STIMULUS AND LACTATION IN THE RAT

Abstract

Summary. Experiments are described that illustrate the close control over mammary-gland activity exercised by the stimulus of suckling in pregnant and non-pregnant rats. By selection of suitable foster litters, chosen to reduce to a minimum the change in the demand for milk at each exchange, lactation was maintained for 10 to 12 months in three females. Sufficient milk was produced regularly to support a uniform rate of growth of the young, giving a total of 2 to 4 kg milk per female during the course of a single lactation. Oestrus recurred regularly during prolonged lactation at intervals of about 18 days. When mating was permitted, pregnancy was normal, parturition taking place as expected in the non-lactating animal. Lactation was not affected by the presence of the male or by the initiation of pregnancy, provided the young were left with the mother. Towards the end of pregnancy, however, lactation always failed despite continued suckling. It was thus not possible to maintain a continuous lactation from the birth of one litter to the birth of the next. This was true also of lactations concurrent with post-partum pregnancies. At various intervals up to 3 months since the previous lactation, mammary-gland activity sufficient to support normal litter growth was induced in parous females after some weeks by suckling alone. Virgin females could not be brought into lactation by this means.