Mice: progesterone stimulates aggression in pregnancy-terminated females

Three experiments were conducted in order to assess the role of progesterone (P) in the aggressive behavior displayed by late pregnant Rockland-Swiss mice toward adult male intruders. In Experiment 1, hysterectomy on the 15th day of gestation reduced the aggressive behavior normally displayed by pregnant mice toward male intruders. In Experiment 2, Silastic implants of P stimulated aggression in hysterectomized mice but did not fully restore the behavior to the level of fighting normally observed in pregnant animals. In Experiment 3, aggressive behavior in P-treated hysterectomized animals was inhibited by simultaneous exposure to estradiol (E). Also, treatment with E alone did not stimulate aggression in hysterectomized mice. While pregnancy-induced aggression is promoted by P, other neuroendocrine factors may act in concert with the steroid to fully stimulate aggression in gravid mice.