Persistent effects on maternal aggression of pregnancy but not of estrogen/progesterone treatment of nonpregnant ovariectomized rats revealed when initiation of maternal behavior is delayed.

Two experiments explored the effects on subsequent maternal aggression of interspersing 5-24 days between pregnancy termination and pup exposure to elicit maternal behavior. Pregnancy-terminated females displaying maternal behavior were compared to nonpregnant females also stimulated by pups to behave maternally and to pregnancy-terminated groups that were not exposed to pups. Pregnancies were terminated by hysterectomy-ovariectomy on Gestation Day 19 (G19), with pup exposure commencing after 6 days (Expt. 1), and by Cesarean section + ovariectomy (OVX), after delays of 5, 10, or 24 days (Expt. 2). In each study pregnancy-terminated, maternal groups were more aggressive than both nonpregnant, maternal groups and pregnancy-terminated, nonmaternal groups regardless of the imposed delays. In Expt. 2, postsurgical interval also was a significant factor, suggesting that surgical stress may elevate maternal aggression for several days. Prolonged treatment of nonpregnant OVX females (Expt. 3) with estrogen and progesterone elevated maternal aggression when pup exposure commenced 2 but not 7 days after termination of hormone treatment (Expt. 3). Therefore the physiological changes initiated during pregnancy that support maternal aggression persist for at least 4 weeks and may involve factors in addition to ovarian hormones that stimulate maternal behavior and maternal aggression. An additional unexpected finding was that nonpregnant (OVX) females had significantly higher Aggression scores after initiating maternal behavior.