Overlapping litters in rats: effects on maternal behavior and offspring emotionality

Female rats have a fertile postpartum estrus, which can result in a simultaneous gestation and lactation and later in the overlapping of two different-age litters. These different physiological and contextual situations may affect the maternal behavior of lactating rats and provoke long-lasting changes in the affective behavior of the litter. Therefore, we aimed to assess the effect of pregnancy and of litter overlapping on the maternal behavior of lactating rats and to describe the maternal- and anxiety-like behaviors of the juveniles that remained in contact with their younger siblings. Results showed that pregnant lactating rats spent more time outside the nest and less time nursing than non-pregnant mothers. On the other side, mothers with overlapping litters licked less the newborn pups than mothers with single litters. These deficits in maternal licking received by neonates were overridden by the juveniles' licking behavior to their younger siblings. Adult male and diestrous female rats reared with younger siblings showed a reduced anxiety-like behavior as compared to age-weaning matched animals without this experience. Thus, natural changes in the reproductive conditions and in the early experience, affect the maternal behavior and long-term modulate affective behavior of the individuals.