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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.

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.

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.

Following removal of their olfactory bulbs, virgin female rats consistently attack and kill rat young whereas non-lactating anosmic primiparous animals do not. The present studies were undertaken to compare the effects of bulbectomy on maternal behaviour in the parous and virgin female rat, and to analyse those experiences responsible for the marked behavioural differences towards young seen in these two groups.

Adult female Wistar rats, maintained by random breeding within a closed colony in the laboratory of M. X. Zarrow since 1949 (W/Z strain), were individually housed in 15 × 10 × 7 in. stainless steel cages under a 13-hr light/11-hr dark regimen (lights on at 07.00 hours), and at an ambient temperature of 70 to 74° F. Food and water were freely supplied. The virgin and non-lactating

Summary.

Facilitation of pmsg- or hcg-induced ovulation occurs in the immature female mouse exposed to the presence of, but not in contact with, an adult male. The effect is not seen if an immature or castrated adult male is used. Removal of the olfactory bulbs in the female also prevents facilitation due to the presence of the male. It is suggested that a number of different effects seen in the female due to a pheromone released by the male may be mediated through a single environmental stimulus.

Summary.

Groups of pregnant, Purdue, Dutch-belted rabbits were injected with three levels of testosterone propionate from Days 17 to 29 of gestation. Treatment periods and the daily amount of testosterone injected were: Days 17 to 21, 5 mg; Days 22 to 25, 2·5 mg; Days 27 to 29, 2·5 or 1 mg. Testosterone treatment for the complete schedule (i.e. Days 17 to 29) resulted in significant suppression of maternal-nest-building behaviour, failure to rear litters, increased incidence of abortion or resorption and presence of scattering, cannibalism or failure to nurse. Only the complete treatment schedule significantly inhibited maternal nest building while lack of litter care could be produced by treatment during Days 17 to 25 or Days 27 to 29. In addition, treatment during Days 17 to 25 produced a significant increase in abortion and/or resorption. Results are discussed in terms of the interaction between testosterone and normal endocrine requirements for maternal behaviour.

Summary.

Experiments have been conducted on maternal-nest building and factors affecting its occurrence in the rabbit. Females deprived of nesting material built a nest out of their own body hair and were as successful in rearing young as does which used both body hair and straw; females deprived of body hair (by shaving) and nesting material showed defective maternal behaviour. Interrupting pregnancy by spaying or foetectomy resulted in maternal-nest building. Nest building was induced in spayed females by the withdrawal of progesterone after injections of progesterone and oestradiol for several weeks. Similar regimes did not induce nest building by castrated males. Further experiments strongly suggest that the onset of nest building is governed by the change in the ratio of progesterone to oestrogen.

Summary.

Two types of nest building are found in the rabbit. One type of nest is made of straw or similar elements available to the animal, and a second type of nest is composed of straw into which hair is incorporated after being plucked from the body. The second type of nest is called a maternal nest and is considered to be part of the maternal behaviour complex. Maternal-nest building was induced in 209 out of 250 (84 %) rabbits following complete removal of the entire conceptus mass on Days 20 to 27 of gestation, as compared with a 96 % maternal-nest-building rate in 941 control rabbits from the same colony. The maternal-nest-building behaviour was also induced in twelve out of sixteen (75 %) rabbits following ovariectomy on Days 20 to 24 of gestation and in five out of eleven rabbits made pseudopregnant with human chorionic gonadotrophin (hcg). Comparable results were obtained in a separate study that used the Dutch-belted rabbit, although a higher percentage of animals in this race built maternal nests following pseudopregnancy or ovariectomy during gestation. Treatment with stilboestrol, progesterone and prolactin induced maternal-nest building in four out of seventeen rabbits and lactation in all the animals. Possible endocrine mechanisms involved are discussed.