Effects of prolonged estrogen-progesterone treatment and hypophysectomy on the stimulation of short-latency maternal behavior and aggression in female rats

Two experiments were undertaken to examine the stimulation of home-cage and/or maternal aggressiveness by a hormonal treatment stimulating short-latency maternal behavior. Nonpregnant ovariectomized rats were treated with a 16-day regimen providing pregnancy levels of estrogen (E, 5-mm Silastic capsule) and progesterone (P, daily injection of 4 mg) followed by E and P withdrawal, with or without a terminal injection of estradiol benzoate (EB, 5 micrograms/kg). In Experiment 1, hormonally treated and control females were exposed continuously to pups and tested for aggression toward male intruders on the fifth day of pup exposure. Females receiving E/P/Oil and E/P/EB were highly aggressive whether or not they had yet shown maternal behavior, whereas vehicle-treated females were nonaggressive. In Experiment 2, hypophysectomized (HYPX) and Sham-HYPX females received either E/P/EB or a control treatment and were tested with male intruders (a) immediately preceding and (b) on the fifth day of continuous pup exposure. HYPX and Sham-HYPX females treated with E/P/EB were almost equally aggressive both preceding and following pup exposure (during which they initiated maternal care), whereas HYPX and Sham-HYPX vehicle-treated females were nonaggressive at both tests. In contrast, maternal behavior latencies were reduced by E/P/EB only among Sham-HYPX females. The results establish that an E/P/EB-treatment which elicits short-latency maternal responses also increases aggressiveness toward intruders. Pituitary products, although involved in the mediation of maternal responsiveness, do not contribute significantly to the stimulation of female aggressiveness by ovarian hormones.