Hormone-dependent aggression in female rats: testosterone implants attenuate the decline in aggression following...

Female rats were individually housed with a sterile male for a 4- to 5-week period. Each female was then tested for aggression toward an unfamiliar female intruder at weekly intervals. Those females that displayed a high level of aggression on each of three weekly tests were ovariectomized and given subcutaneous implants of testosterone-filled tubes, ovariectomized and given subcutaneous implants of empty tubes, or sham-ovariectomized and implanted with empty tubes. These implants should produce a serum testosterone concentration of about 0.6 ng/ml, compared to 0.17 ng/ml in intact females. Beginning 1 week postoperatively, the aggression of each female was tested weekly for 4 weeks. Ovariectomized females with testosterone implants displayed a level of aggression significantly higher than that of ovariectomized females with empty implants on 3 of 4 weekly tests. The level of aggression by females with testosterone implants was not significantly different from that of sham-ovariectomized females on the first postoperative test. Additional observations showed that testosterone implants did not produce an increase in aggression in females whose preoperative level of aggression was low. Further, Silastic implants containing estrogen (1 to 2 mm long) sufficient to maintain a serum estrogen level of 20 to 30 pg/ml also attenuated the decline of aggression following ovariectomy. These results suggest that testosterone and estrogen may both contribute to the biological substrate of hormone-dependent aggression in female rats.