Commentary: is maternal stimulation the mediator of the handling effect in infancy?

In a recent report, a series of studies is described showing that individual differences in rat maternal licking and grooming are correlated with their offsprings' later adrenal response to a stressor (Liu et al., 1997). Pups that received more maternal stimulation in infancy had lower ACTH, corticosterone, and CRH mRNA, while they had greater amounts of GR mRNA. Liu et al. also compared maternal behavior in litters where rat pups were handled daily to maternal behavior of nonhandled litters. They found that mothers of handled pups licked and groomed their young significantly more often than did mothers of control litters. In their discussion, Liu et al. proposed that their data support Levine's (1975) thesis that handling effects in infancy are mediated via the mother. Levine had proposed that handling of pups will modify the mother--pup interaction, thereby causing a change in maternal behavior. The purpose of this commentary is to summarize data obtained with mice and rabbits which, in part, support and, in part, challenge the conclusions of Liu et al.