Some 'cage potatoes' don't get fat.

Some rodents stay fit even when they get no exercise at all, say scientists who analyzed the muscles of wild chipmunks, deer mice, and ground squirrels kept inactive for a month.

Melanie Thompson and Steven Wickler of California State Polytechnic University in Pomona brought these rodents into the lab and gave them plenty of food and water but no room in which to run. Like inactive people, deer mice (which do not hibernate) and ground squirrels (which fatten up to make it through their winter's sleep) became quite pudgy, Thompson and Wickler say. The animals' muscles shrank and lost some of the proteins involved in using oxygen.

But chipmunks, which hibernate but wake up every so often to eat their cache of food, stayed slim, says Wickler.

He sampled the muscles of 18 chipmunks before and after the experiment. Although the muscles got smaller, the concentrations of an enzyme called citrate synthase increased, indicating that these animals gained aerobic fitness during that period, he says.

Because chipmunks are so active when they emerge in spring, it is important that they stay fit over winter, Wickler adds. He hopes to identify the mechanism behind the chipmunks' ability to resist becoming "cage potatoes," as he calls them. Such knowledge may lead to ways for people to avoid turning into jelly during bed rest, space travel, or inactive periods, he says.