Intraspecific fighting during late pregnancy and lactation in rats and effects of litter removal

Female rats (Experiment 1A) were tested for aggression against an adult male intruder in a home-cage test situation at several-day intervals from Day 18 of gestation through Day 21 of lactation. The peak frequencies of attacks and bites and the lowest latency to the first attack occurred on Day 9 of lactation, with concomitant increases in subordinate behaviors by the intruders occurring at that time. Virgin females (Experiment 1B) isolated for comparable periods of time in the test cage showed no differences in fighting levels as a function of length of residence in the home cage. Removal of the litter 4 hr prior to aggression testing on either Day 9 or 10 of lactation (Experiment 2) resulted in significant decreases in fighting levels by lactating females and in significant decreases in subordinate behaviors by intruders. These studies demonstrate that high levels of intraspecific aggression are exhibited during lactation in the rat and that the aggression is dependent upon stimuli from the litter for its expression.