Maintenance and decline of the suppression of infanticide in mother rats

Virgin female rats kill foster neonates, whereas newly parturient mothers do not. We demonstrated previously that this tendency to kill is suppressed shortly prepartum, presumably by physiological factors. In this study, we show that a) suppression of infanticide is maintained through the first two weeks of lactation; b) the mothers that do not kill foster neonates are not necessarily the same mothers that respond maternally toward older foster pups, and those that kill neonates are not necessarily the same ones that are nonmaternal to older pups, the two behaviors being somewhat independent; and c) some virgins can be induced to be noninfanticides by prolonged exposure to young, but only under special testing conditions not required by actual mothers, which are nonkillers of foster young. This suggests that the maintenance of the suppression of infanticide in mothers owes something to the special circumstances of lactation other than continued exposure to young.