Postpartum aggression in rats: II. Dependence on maternal sensitivity to young and effects of experience with pregnancy and parturition.

Two experiments were carried out to explore the relation between postpartum aggressive behavior in rats and other aspects of maternal behavior. In Experiment 1, nulliparous females experimentally induced to behave maternally; i.e., sensitized, failed to show elevations in aggressive responding above levels seen in untreated nulliparous controls; lactating females displayed characteristically high levels of aggression. In Experiment 2, animals in these groups were tested, along with parturient females allowed varying amounts (0 hr, 2 days, or 9 days) of postpartum exposure to young, spanning a period that is sensitive for the induction of maternal behavior. The three parturient groups responded with levels of aggression that were similar to one another and significantly higher than those seen in controls. Sensitized animals in Experiment 2 exhibited high levels of fighting; differences between these animals and those in Experiment 1 may be due to differences in both length of sensitization and ovarian cyclicity. These data indicate that the experiences of pregnancy and/or parturition prime the postparturient female to respond aggressively to an intruder later in lactation and that, unlike the initiation of pup-oriented maternal behaviors, establishment of postpartum aggression is not dependent upon pup exposure during the immediate postpartum period.