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Summary.

The following factors were found to influence the superovulatory response in immature mice and rats: (1) the dose of pregnant mares' serum (pms), (2) the dose of human chorionic gonadotropin (hcg), (3) the time interval between pms and hcg injections, and (4) the time interval between hcg and recovery of maximum number of ova. Dose-response curves for pms were bell-shaped in both species whereas increasing doses of hcg resulted in a plateau. Histological examination of the ovaries indicated that high doses of pms resulted in cystic follicles in the rat and preluteinized follicles in the mouse. Both conditions led to a failure to induce superovulation.

Summary.

Litter size in mice influences the body growth of the young under normal conditions, on inadequate diet and under propylthiouracil (ptu) treatment. Invariably the larger litter size of seven showed poorer growth curves when compared with the litter size of four. A significant increase in the relative ovarian weight was noted in the litter size of four when treated with ptu, but this was not seen in the litter size of seven. No significant differences were noted in the number of ova released in the mice under various conditions of diet, goitrogen administration and litter size. Increasing dosages of goitrogen caused both a decrease in body weight and in ova release in the rat. The number of ova fell from a count of thirty-five in normal rats to eighteen in rats on 0·01% ptu and to no ova in rats on 0 ·05 and 0·1% ptu in the diet.

Summary.

The Dutch-belted and grey chinchilla strains of rabbits, Oryctolagus cuniculus, were compared for incidence of maternal nestbuilding after normal pregnancy, pseudo-pregnancy induced by human chorionic gonadotrophin (hcg), castration during pregnancy, oestradiol treatment during pregnancy and treatment of the castrated doe with oestradiol and progesterone. In both strains all does built nests at the end of normal pregnancy, but only 85% of the Dutch-belted does and 31% of the grey chinchilla does at the end of pseudo-pregnancy. All other treatments induced maternal nest-building in both strains; but the grey chinchilla rabbits required longer treatment for the induction of nestbuilding after castration and more oestradiol for the induction of nestbuilding during pregnancy. The critical period for the induction of maternal nest-building after castration in the pregnant rabbit is before Day 20 of gestation in the Dutch-belted and about Day 25 for the grey chinchilla strain.

Summary.

Rat offspring were delivered by Caesarian section at term and fostered to lactating mothers which had had their own young removed. The major variables studied were the presence or absence of the placenta at the time of fostering, the rate of presentation of the foster young to the mother, and the length of time a mother was without any young. The percentage of animals surviving until weaning (21 days) and the body weight of the survivors were recorded.

The presence of the placenta at fostering resulted in a decrement in survival probability. The time that the foster mother was without pups and the rate at which foster pups were presented to the mother had an interactive effect: the highest survival rate was obtained when pups were presented 5 min apart to mothers which had had their own young removed 60 min before fostering.

Summary.

Three strains of mice (SWR/J, C57BL/6J and BALB/cJ) were examined for pheromonal facilitation of both PMSG- and HCG-induced ovulation. Immature females were injected with sufficient levels of the hormones to induce minimal egg release. Exposure to adult males involved the use of both the homologous and heterologous strains. The SWR/J and BALB/cJ females showed significant facilitation of ovulation with either gonadotrophin and exposure to adult males of the same or heterologous strain. Exposure of the three strains of females to C57BL/6J males, however, failed to result in any facilitation; but exposure of the C57BL/6J females to males of the SWR/J and BALB/cJ strains did result in facilitation. It may be concluded that the failure of pheromonal facilitation in the C57BL/6J strain is due to an inability of the males of this strain to produce the pheromone. The C57BL/6J females, however, still possess the ability to respond and, consequently, have not lost the receptors for the pheromone.

Summary.

Delivery by Caesarean Section (CS) of pregnant rats at 09.00 hours on the 23rd day of gestation reduced the range of times of ovulation after parturition by 18 hr. Ovulation had not occurred by 15 hr after CS but was completed by 18 hr, an interval which is similar to that of ovulation for females of this colony littering at 09.00 hours. Operation on Day 21 or 22 post coitum did not change the day of ovulation (Day 24) but CS on Days 19 or 20 advanced ovulation to Days 22 or 23, respectively. Prevention of normal delivery by ligation of the uterine horns on Day 22 resulted in no females ovulating on Day 24, and few females ovulating on Days 25 and 26. In such females, CS did not increase the number of rats ovulating.

Removal of the olfactory bulbs in pregnant and non-pregnant mice has been shown to lead to a loss of maternal behaviour (Gandelman, Zarrow, Denenberg & Myers, 1971). Cannibalism of the young occurs in practically all cases, and in the few instances when cannibalism does not take place, the young invariably die from neglect. In mice, therefore, the olfactory system must be functional for maternal behaviour to occur. By contrast, data obtained from the rat indicate that no one sensory system is necessary for the exhibition of maternal behaviour (Beach & Jaynes, 1956).

In our original experiment (Gandelman, Zarrow, Denenberg & Myers, 1971), multiparous mice were bulbectomized on Day 12 of gestation, allowed to litter, and were killed immediately after the death of their young in order to verify the brain lesions.

During the course of studies on pmsg- and hcg-induced pheromonal facilitation of ovulation (Zarrow, Christenson & Eleftheriou, 1971; Zarrow, Eleftheriou & Denenberg, 1972), strain differences were noted in the response of the immature female mouse to both pmsg and hcg. Since strain differences have been reported previously for pheromonal block of pregnancy (Bruce, 1968), and it was postulated that this system existed as a single gene (Chapman & Whitten, 1968), the following experiments were conducted to determine the heritability of the male pheromonal system involved in pmsg-induced facilitation of ovulation.

Female mice of the C57BL/6J and SWR/J strains were housed, one per cage, following weaning at 21 days of age. Lighting was kept on a 12 hr on/12 hr

Summary.

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.

Summary.

Social isolation of immature female rats produced an inhibitory effect on pmsg-induced ovulation. No effect on ovulation was observed following grouping of animals in numbers from six to twenty-four.