when placed in a box. Evidently something happens during their infancy that extinguishes their normal fighting tendencies. That "something" is not the difference in mouse and rat milk, for mouse pups raised by rat "aunts" whose nipples had been surgically removed were also likely to refrain from fighting, even when they had been raised with other mouse "siblings."
This effect apparently applies only to inbred laboratory mice and does not obtain with Swiss albino mice, whose fighting tendencies are undiminished by experience with rat foster mothers or aunts. Another mouse tendency, however, was affected by foster care. Mice show a great deal more open-field activity, such as running around and exploring, than do laboratory rats. When the amount of open-field activity was taken as a dependent variable, both inbred and Swiss albino mice who had been reared by rat mothers or aunts showed less activity. The lower activity level was also positively correlated with lower levels of corticosterone in the blood plasma of the experimental animals. Corticosterone is released by the cortex of the adrenal gland its level in the blood may be taken as an index of emotional reactivity. In other words, both the overt behavior and the blood chemistry of the experimental animals were evidently modified by their experience as pups.
Denenberg confessed that he was surprised at the results he had obtained in his research and commented:
Even though I am a firm believer in motherhood, I must admit that when we started this set of experiments I did not expect to find that the mother's behavior during the nursing period would have such a powerful effect upon so many different biobehavioral systems of the animal. Clearly, if these results have any degree of generality to other mammals, the subtle and not so subtle behavior patterns of the mother during the early stages of the neonate's development have very profound and far- reaching effects.
Infants are influenced by caregivers (mothers, fathers, siblings, parent substitutes) and influence them in return. Although fathers have significant affects on infant development, most research deals with the infant's interaction with his mother. Much of what caregivers provide for infants can be classified as stimulation. Research with animals indicates that early stimulation has generally positive effects on infant development and can promote significant growth of the cerebral cortex--the "thinking area" of the brain. The theory that such stimulation must occur during a critical period in development is not well supported, however, for research suggests that deficiencies resulting from early understimulation can be made up later.