Reduced fearfulness in the lactating rat.

The freezing reaction in response to a brief auditory signal was assessed in lactating and virgin rats. Freezing was markedly reduced in lactating animals, but only when their litters were present in the testing cage. Pup exposure did not shorten the duration of freezing in virgin animals. The reduced fearfulness in lactating females, as measured by the freezing response, may complement maternal aggression in the protection of the offspring.