Mother-weanling interactions in Norway rats in the presence of a successive litter produced by postpartum mating.

When female Norway rats (Rattus norvegicus) become pregnant at the postpartum estrus they nurse this first litter concurrently with the gestation of a second litter. This second gestation is of variable length (23-31 days in this study) We investigated the behavior of mothers and their older litters around the time the second litter was born. Six female rats gave birth and were mated at the postpartum estrus. Continuous videotaped observation of each female and litter began approximately 3 days before, and continued for 3 days after the birth of the second litter. We found that suckling behavior and nest attendance by the older litter did not necessarily end when the second litter was born. When the gestation length of the second litter was short (23-25 days) older pups continued to spend time in the nest and to nurse along with their newborn siblings. These older pups also spent much time on the nest even in the absence of their mother. When gestation was long (27-31 days) older pups were weaned before the birth of the second litter and spent less time on the nest with the newborns. Stereotyped attacks by the female against her weanling age pups were seen both pre- and postpartum. This maternal aggression did not appear to deter suckling and nest attendance by the weanlings.