Aggression by ovariectomized female rats with testosterone implants: competitive experience activates aggression toward unfamiliar females.

Female hooded rats (250 to 325 g) were ovariectomized and bilaterally implanted with testosterone-filled or empty Silastic tubes. The testosterone-filled space in each tube was 10 mm long and this should produce a serum testosterone concentration 4 to 5 times that of an intact female, but well below that of a male. Three weeks following surgery, half of the animals with testosterone implants were housed with an animal with an empty implant and left for 6 weeks. The remaining animals were placed on a 23-hr food deprivation schedule, housed in testosterone implant/empty implant pairs, and then subjected to a series of food competition tests. Following the competition tests, all animals were individually tested in their living cage for aggression toward an unfamiliar female. In food competition, females with testosterone implants were more successful and more aggressive than their cagemates with empty implants. When tested for aggression toward an unfamiliar intruder, females with testosterone implants given competitive experience were more aggressive toward an intruder than were their cagemates with empty implants or females with testosterone implants not given the competitive experience. Females with testosterone implants but without competitive experience were not more aggressive toward an unfamiliar female than were their cagemates with empty implants. These results suggest that, in ovariectomized females with testosterone implants, hormone-dependent aggression fostered by a competitive situation is displayed toward unfamiliar females.