The wheels then were unlocked and for nine days, the mice could run at will, while also eating and moving around off the wheels as much as they chose.

The mice, which seem to enjoy running, hopped readily on the wheels and ran, off and on, for hours.

They showed a subsequent spike in their daily energy expenditure, according to the metabolic measures, which makes sense, since they had added exercise to their lives.

But they did not change their eating habits. Although they were burning more calories, they did not gorge on more chow.

They did, however, alter how they moved. Almost immediately after they started using the wheels, they stopped roaming around their cages as they had before the wheels were unlocked.

In particular, they stopped engaging in the kind of lengthy meanders that had been common before they began to run. Instead, they now usually jogged on their wheels for a few minutes, hopped off, rested or roamed in short spurts, and then climbed back on the wheels, ran, rested, briefly roamed, and repeated.

These changes in how they spent their time neatly managed to almost counteract the extra calorie costs from running, says Daniel Lark, a research fellow in molecular physiology at the Vanderbilt University School of Medicine, who led the new study.

In general, the running mice showed a slightly negative energy balance, meaning that they were burning a few more calories over the course of the day than they were taking in by chowing down.