Aggression is attenuated by ovariectomy in pregnant female rats given progesterone and estradiol replacement to maintain pregnancy.

Female rats were tested for aggression toward an unfamiliar female conspecific 13 days following the beginning of cohabitation with a male. Forty-eight hours later, half of the females that displayed aggression were ovariectomized. To maintain pregnancy, they were also given Silastic implants of estradiol (1-mm tube) and progesterone (6 tubes, 30-mm long; Dow-Corning tubing #602-305) sufficient to maintain pregnancy but at lower levels than those normally present. Control females were given a sham ovariectomy and implanted with empty tubes. Each animal received a second aggression test at 48 h following surgery and a third test, 3 days following the second. Hormone implants successfully maintained pregnancy and supported the normal development of the pups. However, Ovariectomized females with hormone implants did not spontaneously give birth. Ovariectomized females displayed less aggression than their sham-ovariectomized counterparts at the second and third aggression tests. The difference was due to an increasing level of aggression by the control but not the experimental animals. These results are consistent with other evidence that circulating gonadal hormones influence the onset of maternal aggression in the female rat.