Maternal and peer contact during the postnatal period participate in the normal development of maternal aggression, maternal behavior, and the behavioral response to novelty.

In the rat, social isolation during the early postnatal period disrupts the adult function of certain neuroendocrine and neurobehavioral systems. In the present study, we assessed the effects of peer and maternal contact during this period on the adult expression of aggression, maternal behavior, and the behavioral response to novelty. Female rat pups of the Wistar strain were reared across postnatal days 3-19 in one of the following conditions: (1) by their mother and with littermates (MR, control); (2) artificially reared in complete social isolation (AR); or (3) artificially reared in the presence of two same-age conspecifics (AR-Soc). As adults, all females were administered a resident-intruder aggression test, first in a non-maternal context ("territorial" aggression) and then in the presence of their pups ("maternal" aggression). Additionally, their maternal behavior and response to a novel object placed in a familiar open field arena were quantified. We found that maternal isolation impaired maternal behavior, increased maternal aggression, and increased the olfactory investigation of a novel object. The presence of peers prevented the effect of artificial rearing on aggression, but not its effects on maternal behavior or response to novelty. In the maternal aggression test, AR-Soc females engaged in significantly more sniffing of the intruder compared to the other two groups. The present results confirm and extend those of previous studies in a different strain (Sprague-Dawley), and indicate that peer-derived stimuli impact on the development of neurobehavioral systems underlying aggressive and non-aggressive social interactions, whereas the normal development of maternal behavior and response to novelty requires maternally derived stimuli.