Sexual dimorphism in the hormonal control of aggressive behavior of rats

Male and female rats housed together in small groups both became aggressive toward strange rats placed into their cages. However, male rats mainly attack male intruders and female rats mainly attack female intruders. The hormonal requirements for this aggression are also sexually dimorphic. Castration of the male residents causes a decrease in aggressive behavior, but ovariectomy of the female residents has little effect on their aggression. Replacement therapy to castrated males with 500 microgram/day of testosterone propionate (TP) restores all components of aggressive behavior within 2-3 weeks. Replacement therapy with 5 microgram/day of estradiol benzoate for 3 weeks had no effect on castrated males' aggressive behavior, nor did it cause the males to start attacking female intruders. Replacement therapy to ovariectomized females had a quite different effect. Treatment with EB or EB and progesterone reduced or blocked attack toward intruder females but did not alter the low level of attack toward males. Treatment with TP for 3 weeks also slightly reduced attack toward female intruders but increased the probability of attack toward male intruders. The sex-specificity of aggression and the differential influence of hormones suggests that different mechanisms may be responsible for aggression in males and in females.