Le vine and others suggested that the effects of handling are actually mediated by changes in maternal care.35-37 Indeed, handling increases the licking/grooming (LG) of pups by the mother.38,39 Subsequent studies strongly support the maternal-mediation hypothesis. One approach was to examine the consequences of naturally occurring variations in maternal LG. These studies indicate that the adult offspring of high-LG mothers resembled postnatally handled animals on measures of behavioral and endocrine responses to stress, while those of low-LG mothers were comparable to nonhandled animals. Cross-fostering studies, where pups born to high-LG mothers are fostered at birth to low-LG mothers (and vice versa), suggest a direct relationship between maternal care and the postnatal development of individual differences in behavioral and HPA responses to stress.40,41 Finally, these studies suggest that variations within a normal range of parental care can dramatically alter development. As in humans, parental care need not include forms of overt abuse or extreme neglect in order to influence the development of the offspring. In large measure, this is most likely due to the fact that natural selection shaped offspring to
