The purpose of these experiments was to isolate some of the determinants of maternal behaviour in the rat by use of a fostering procedure. Four variables were investigated: the number of days the foster mother had been lactating prior to receiving foster young, the presence or absence of the placenta when young were fostered, the effects of administering oestradiol upon the behaviour of the foster mother, and the number of hours the young were with their own mother before being fostered. Body weights were obtained at 10, 15 and 21 days of age and the percentage alive at 21 days was recorded.
As the number of days the foster mother had been lactating increased, the mortality rate of the foster young also increased and the body weight of the survivors decreased. The absence of the placenta resulted in an increase in the mortality rate; a decrease in body weight occurred for young fostered without the placenta to 10-day-lactating mothers. The injection of oestradiol resulted in significant increases in survival percentages and body weights suggesting that the oestrogen content of the placenta might be significant in the maternal behaviour complex. All young fostered 12 hr after birth survived through weaning, thus suggesting that the mother's behaviour during the first few hours of life are critical both for the survival of the young and for the weight of the survivors.