General discussion: Suckling stimulus and lactation

Abstract

Lactation in the rat can be prolonged indefinitely by the provision of a succession of young foster fitters to continue the stimulus of suckling. Under these conditions lactation becomes a steady state. In three rats nursing their third or fourth litters, lactation was so maintained for periods of 9, 10 and 12 months, respectively, and failure occurred only after the experimental conditions were deliberately altered. To attain steady lactation, foster litters were nursed only for the last few days of the period during which they depend wholly on the mother, i.e. shortly before their eyes open, and thus furnish a strong suckling stimulus not varying too greatly at each exchange of foster litter. In these circumstances sufficient milk was being secreted continuously to increase litter weight by 9 to 10 g per day (figure 49). The oestrous cycle was prolonged, the mean cycle length for the three females during prolonged lactation being 17.8 days. This condition has been described by Selye (1934) as ‘suckling pseudo-pregnancy'. In other experiments with prolonged lactation mating was allowed to take place after periods varying from 50 to 106 days of continuous lactation. There was no diminution of milk production as judged by litter growth over the night of mating if the male was introduced into the female’s cage and the young allowed to remain so that suckling continued. Neither was there a delay in implantation such as occurs after a mating at post-partum oestrus in the presence of a suckling litter, parturition occurring after the normal gestation period of 22 to 23 days (five out of five females).