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Testicular hormones and intermale aggressive behaviour in the presence of a female rat.

A hypothesis was examined that increased levels of available testicular hormones are responsible for the increased aggressiveness of males in the presence of a female. In the first of two experiments intact male rats were injected with the androgen antagonist flutamide before exposure to oestrous, dioestrous or no females. Consistent with the hypothesis, flutamide-injected males were less aggressive than the respective control males which had been injected with oil alone. Yet the antiandrogen did not fully block the increased aggressiveness between males exposed to an oestrous female. In experiment 2 the aggressive behaviours of male rats in the presence of an oestrous female were observed after castration and restoration therapy with different amounts of testosterone propionate (TP). Neither a physiological amount (200 micrograms) nor an unusually large amount (800 micrograms) to TP could adequately mimic the effects of a gonadally intact male exposed to a sexually receptive female. These data suggest that differences in androgens available to target tissues influence aggressiveness among males exposed to a female. However, there are other factors involved, presumably the chronic morphological-physiological-behavioural changes that are provoked by copulatory experience.