Progesterone inhibition of aggressive behaviors in hamsters.

Effects of progesterone on aggressive behaviors were tested in male-male or female-female pairs of hamsters, after both members of the pair had received the same experimental treatment. In castrated males, progesterone increased the latency to attack, and decreased the frequency of attack and on-back fighting postures. Similarly, in ovariectomized females, progesterone decreased the frequencies of upright and attack fighting responses, as well as the frequency of rolling fights. Estradiol treatment did not produce these effects on aggressive behaviors, and was not required to prime the nervous system for the progesterone effects. These results suggest that progesterone may act by means other than estrogen-inducible progestin receptors to inhibit aggressive behaviors.