Well-groomed rodents stay cool, calm.

Some young rats take a maternal licking that keeps them ticking by developing an ingrained sense of curiosity and physiological calm under duress. Others, a new study suggests, are literally groomed to react with heightened sensitivity and alarm to new and potentially threatening situations.

These findings emphasize the power of the early social environment to mold individual responses to stress, at least in animals capable of inhabiting surroundings that pose an array of survival challenges. For the rats under study, a mother's natural style of caring for her offspring may tailor their stress reactions to suit the challenges they will most likely encounter as adults, contends a team of neuroscientists headed by Michael J. Meaney of McGill University in Montreal.

Meaney's group drew inspiration from a 40-year-old study in which newborn Norway rats, taken from their mothers by a scientist for 15 minutes daily during the first few weeks of life, excreted low amounts of certain stress hormones when under threat and explored new surroundings with gusto, compared to nonhandled pups. Mothers fervently licked and groomed the handled pups, but the significance of that behavior eluded the scientists.

The new experiment, described in the Sept. 12 Science, finds that rat moms spend about twice as much time licking and grooming offspring that have been removed daily by a handler during the 3 weeks after birth. Moreover, about one in three mothers of nonhandled pups lick and groom at that higher rate, Meaney and his coworkers report.

As adults, the recipients of the exuberant licking and grooming display signs of a reduced responsiveness to stress, the scientists hold. For instance, these animals exhibit low concentrations of adrenocorticotropic hormone and corticosterone--two stress hormones--after being restrained briefly while researchers collected blood samples. In addition, cells in a crucial part of the animals' brain, known as the hippocampus, show responses associated with efficient regulation of another stress hormone, glucocorticoid.

Moreover, adult rats that had experienced vigorous licking and grooming as pups explored open spaces with particular ease and possessed an abundance of brain receptors for a class of anxiety-reducing substances called benzodiazepines.

Norway rats inhabit many ecological niches, each of which may help to shape mothering practices and, as a result, the stress responsiveness of succeeding generations, according to the researchers. One possibility is that mothers that must leave their pups briefly to store and retrieve food tend to lick and groom them with particular vigor, rendering the offspring better able to cope with short separations.

The findings may apply to humans, since we also live in a vast spectrum of environments, the scientists add.

"This current study must spur on work examining how early experience alters the trajectory of [human] development," writes Stanford University biologist Robert M. Sapolsky in an accompanying comment.

Indeed, exposure to touch provides crucial sensory support in the development of human infants. Researchers have noted significant gains in weight and physical health in premature babies given gentle massages while in the hospital.

Further rodent research needs to examine whether even subtler individual differences in mothering style exist, Sapolsky says. Investigators can then explore how these differences trigger neurotransmitter and receptor changes in pups' brains.