Maternal behavior in the rat: facilitation through gonadectomy

Prepartum and postpartum regulation of maternal behaviour in the rat

Maternal behaviour in the rat consists of four principal components: nursing or crouching over the young, retrieving pups to the nest, body and genital licking of pups, and nest-building. Normally the onset of maternal behaviour occurs at parturition but studies reveal that the true onset is somewhat earlier, around 24 hours pre partum. The onset of maternal behaviour is hormonally determined and it is most likely that the ovarian hormone oestradiol plays a major role under the specific conditions that exist pre partum. The onset of maternal behaviour requires the resolution of tendencies of fear-avoidance of pups, based on their olfactory characteristics, and attraction to them and response to the pup stimuli that elicit maternal behaviour. In most animals this resolution occurs almost immediately but in some animals several hours of contact with the pups is required for maternal behaviour to be firmly established. After parturition, maternal behaviour appears to be regulated chiefly by stimuli from the pups, and hormones do not play a role apart from their role in lactogenesis. Size of litter, age of pups, amount of externally induced stress and other factors can affect the mother-young interaction: there is a period of special vulnerability to disruption of the mother-young interrelationship shortly after parturition which corresponds, we believe, to the period of transition from hormonal to non-hormonal regulation of maternal behaviour. Weaning and the decline of maternal behaviour is a specific phase of maternal care. Maternal behaviours gradually decline as avoidance behaviours increase.

Aggression in the lactating female rat: the normal decline is not dependent on the physical development of the pups

Female rats that had been assessed for aggression on two occasions during pregnancy were also assessed for aggressiveness toward an unfamiliar adult female on days 5, 12, and 19 of lactation. Each female had its litter of pups exchanged for another litter on days 10 and 17 following parturition. Half of the females received pups that were 2 days old. The other half received pups of the same age as those taken away. Aggression by females of the two groups declined at a similar rate during the period of lactation. Retrieval of a group of seven 6-day-old pups following the last aggression test was present in females whose litter had been replaced with 2-day-old pups but not in females whose litter had been replaced with same-age pups. Lactation was maintained in all females during the three weeks following parturition but declined to a level that was not sufficient to maintain growth following the normal 21 day lactation period. A single set of biological or stimulus variables does not maintain all aspects of maternal behavior. Physical and behavioral maturation of the pups does not appear responsible for the decline in maternal aggression during the final two weeks of lactation.

Cohabitation with a sterile male facilitates the development of retrieval behavior in nulliparous female rats exposed to pups

Female rats were housed with a sterile male or another female. After 3 weeks, half of the females that had been housed with a female were rehoused with an intact male. At the end of 6 weeks female or sterile male cagemates were removed. Intact male cagemates and pups were removed 3 to 12 h following parturition. All females were tested for retrieval of three unfamiliar pups placed in their cage on the day following removal of their cagemate. Three unfamiliar pups were placed with each female and the female's behavior observed for 10 min. Observations were made in this way for 13 days or until the female retrieved all three pups within the 10-min interval. Pups were left with the female on days they were not retrieved. Females housed with a sterile male reached criterion for pup retrieval in 2.9 days, significantly fewer days than were required for females housed with another female (6.6 days) but significantly more than were required for a postpartum female (0.8 days). By demonstrating that cohabitation with a male fosters the development of retrieval, these results support evidence from the study of aggressive behavior that pseudopregnancy facilitates the development of behaviors associated with pregnancy and lactation.

Infanticide in the male rat: the role of the vomeronasal organ

The present study constitutes the first demonstration that vomeronasal chemoreception plays a role in the response of the virgin male rat to conspecific newborn. Specifically, it shows that virgin males, after removal of the vomeronasal organ, exhibit a decrease in the incidence of infanticidal behavior. This finding is discussed in terms of how responsiveness to young can be affected by changes in the accessory olfactory system.

Maternal behavior in cyclic and androgenized female rats: role of ovarian hormones

A comparative study on the induction of maternal behavior (MB) in response to pups was carried out in cyclic and androgenized rats. Moreover, the effects of exposure to ovarian hormones on the induction of MB in response to pups were studied in cyclic and androgenized rats. A high percentage of androgenized and proestrus rats displayed MB after 48 hr of exposure to pups (87.9% and 87.5% respectively). Diestrus day-1 rats were significantly less maternal (37.5%). Ovariectomy did not impair MB in androgenized rats. High serum progesterone levels induced in androgenized rats by treatment with 10 I.U. of HCG (40 hours before pup presentation) or with progesterone silastic implants prevented the onset of MB. Ovariectomy performed 8 hours before HCG administration restored MB. When behavioral testing began 72 hours after HCG treatment, 75% of the rats were maternal 12 hr after exposure to pups. This rapid onset of MB in proestrus and androgenized rats appeared to be estrogen dependent. The presence of progesterone inhibited the maternal response of androgenized rats.

Lordosis behavior and mounting behavior in male rats: effects of castration and treatment with estradiol benzoate or testosterone propionate

Male rats were selected for showing or not showing lordosis in response to manual stimulation. They were subsequently tested for mounting behavior with receptive females and for lordosis behavior in response to manual stimulation and to male mounting. Males showing lordosis as intacts displayed this behavior more readily following castration or castration and treatment with estradiol benzoate or testosterone propionate than males which did not show lordosis before castration. No group differences in mounting behavior could be detected under any of the endocrine conditions studied. It is suggested that the neural mechanisms mediated mounting and lordosis are dissociated and that individual differences in the occurrence of lordosis in male rats are due to differences in neural sensitivity to estrogen.

Induced parental care in male convict cichlid fish

Three parameters of parental care were recorded for pairs of cichlid fish (Cichlasoma nigrofasciatum) caring for their own young to determine the normal parental role of the male. Nonreproductive adult males were induced to exhibit parental behavior (e.g. herding, fin-digging, and defense of young) by daily presentation of free-swimming conspecific young. This process of sensitization to young appears to be species-specific in that males did not exhibit parental care toward alien (Hemichromis bimaculatus) young. Parental behavior in reproductive and nonreproductive males, as well as the implications and function of sensitization in these fish, is discussed.