Influence of combined estradiol and testosterone implants on the aggressiveness of nonaggressive female rats.

Female rats that had been cohabiting with a sterile male or with another female for 6 weeks were tested for aggression toward an unfamiliar female once each week for 3 weeks. Females that were not aggressive as a result of cohabitation with a sterile male were ovariectomized. Half were implanted with a Silastic tube containing estradiol (1 mm long hormone-filled space) and a tube containing testosterone (5 mm long hormone-filled space). The other half were implanted with empty tubes. All females that had been cohabiting with another female were ovariectomized and implanted with an estradiol- and a testosterone-filled tube. Three additional weekly tests of aggression were given beginning 1 week postoperatively. Females given hormone replacement displayed only a slight increase in aggression postoperatively. Females not given hormone replacement declined in aggressiveness. These results indicate that hormone replacement levels sufficient to maintain aggression in highly aggressive females following ovariectomy are not sufficient to produce a high level of aggression in females that have not become aggressive following cohabitation with a sterile male or that have been cohabiting with another female.