Aggression in the lactating female rat: the normal decline is not dependent on the physical development of the pups.

Female rats that had been assessed for aggression on two occasions during pregnancy were also assessed for aggressiveness toward an unfamiliar adult female on days 5, 12, and 19 of lactation. Each female had its litter of pups exchanged for another litter on days 10 and 17 following parturition. Half of the females received pups that were 2 days old. The other half received pups of the same age as those taken away. Aggression by females of the two groups declined at a similar rate during the period of lactation. Retrieval of a group of seven 6-day-old pups following the last aggression test was present in females whose litter had been replaced with 2-day-old pups but not in females whose litter had been replaced with same-age pups. Lactation was maintained in all females during the three weeks following parturition but declined to a level that was not sufficient to maintain growth following the normal 21 day lactation period. A single set of biological or stimulus variables does not maintain all aspects of maternal behavior. Physical and behavioral maturation of the pups does not appear responsible for the decline in maternal aggression during the final two weeks of lactation.