Postpartum aggression in rats: I. Effects of hypophysectomy.

For the purpose of testing the hypothesis that the high level of aggressive behavior exhibited by postparturient rats is mediated by lactogenic hormones, hypophysectomy was performed on Day 5 postpartum, and fighting against an adult male rat was measured 4 days later. Pups were exchanged daily between hypophysectomized and sham-operated or unoperated controls animals. In Experiment 1, no differences in aggressive behavior were seen between hypophysectomized, sham-hypophysectomized, and untreated lactating animals on Day 9 postpartum. In Experiment 2, the hormonal dependence of the characteristic sensitivity to the presence of the litter in the elicitation of postpartum aggression was investigated. All animals were tested three times, on Day 9, 12, and 15 postpartum; litters were removed 4 hr prior to the second aggression test. For all groups, aggression levels were high on Day 9 with the litter present, dropped on Day 12 after removal of the litter, and showed some increment or stabilization on Day 15 with the litter again present. These experiments demonstrate that neither the expression of postpartum aggression nor the sensitivity to the presence of stimuli from the litter is directly dependent upon the hormonal milieu characteristic of lactation.