Mice: the initiation and maintenance of pregnancy-induced aggression following thelectomy

Two experiments were conducted in order to assess the effects of thelectomy (i.e., nipple removal) on the display of pregnancy-induced aggression in Rockland-Swiss (R-S) albino mice. Pregnant animals, thelectomized on Gestation Day (GD) 12, exhibit a high incidence and intensity of aggression toward adult R-S male intruders during tests conducted on GDs 14, 16, and 18 (i.e., during the last third of gestation in this species). Since thelectomized dams display levels of aggression (i.e., 63% incidence) equivalent to those of sham-operated and nonoperated animals (53 and 67% incidence, respectively), it would appear that nipple presence is not a critical factor for the maintenance of pregnancy-induced aggression in mice (Experiment 1). To examine the effects of nipple deprivation on the initiation of pregnancy-induced aggression, virgin animals were thelectomized prior to mating, then repeatedly tested for aggression at 2-day intervals of gestation. Unlike nipple-intact dams, pregnant mice, deprived of nipples prior to conception, rarely exhibit (less than 17% incidence) agonistic behaviors toward intruder males (Experiment 2). These findings suggest that the development and self-stimulation of nipples early in pregnancy may be important conditions for the display of heightened aggression with advancing pregnancy in mice.