Suppression of infanticide in mother rats.

In order to test the hypothesis that infanticidal tendencies are suppressed when rats become mothers, very young newborn pups, either naturally born or cesarean-delivered, were presented to virgin females and to newly delivered mothers. Provided that the pups were lively, uncleaned of fetal fluids or membranes, and presented without placentas, nearly all virgins killed and nearly all mothers did not. Newborns were also presented to Day 22 pregnant rats and to rats whose pregnancies had recently been surgically terminated. Large proportions of both groups either were nonkillers or were actively maternally responsive (and a smaller proportion were both) despite the fact that none of these rats had undergone parturition or cared for pups. These results indicate that, independent of its effect on maternal caretaking, pregnancy suppresses infanticide in previously infanticidal nulliparae even before they become mothers. Hence, infants are protected from their own parents. In addition, evidence was obtained in support of the idea that freshly delivered pups have stimulus properties that make them specially suited for promoting the onset of maternal caretaking.