In 2009, neurophysiologist Johanna Meijer set up an unusual experiment in her backyard. In an ivy-tangled corner of her garden, she and her colleagues at Leiden University in the Netherlands placed a rodent running wheel inside an open cage and trained a motion-detecting infrared camera on the scene. Then they put out a dish of food pellets and chocolate crumbs to attract animals to the wheel and waited.

Wild house mice discovered the food in short order, then scampered into the wheel and started to run. Rats, shrews, and even frogs found their way to the wheel—more than 12,000 animals over 3 years. The creatures seemed to relish the feeling of running without going anywhere.

The study "puts a nail in the coffin" of the debate over whether mice and rats will run on wheels in a natural setting, says Ted Garland, an evolutionary physiologist at the University of California, Riverside, who was not involved in the work. More importantly, he says, the findings suggest that like (some) humans, mice and other animals may simply exercise because they like to. Figuring out why certain strains of mice are more sedentary than others could help shed light on genetic differences between more active and sedentary people, he adds.

Even before Meijer got creative in her yard, researchers knew that captive mice are exercise maniacs. In laboratories and bedrooms, they frequently log more than 5 km per night on stationary running wheels. But scientists didn’t know why the animals did it.

One thing was clear: They seem to enjoy it. Mice find exercise rewarding; just as they can be trained to press a lever dozens of times to release a pellet of food or a dose of cocaine, the rodents will go to great lengths to unlock a running wheel when it has a brake on, and get back to spinning, Garland says. But is the drive to run normal, or is it an aberrant, obsessive behavior triggered by living in a shoebox-sized cage?

Meijer’s work seems to have answered that question. On average, the backyard mice she and colleagues observed ran in 1 to 2 minute stints, roughly the same duration as that seen in lab mice, they report online today in the Proceedings of the Royal Society B. The team also set up a second wheel in a nearby nature preserve of grassy dunes and attracted a similar crowd of enthusiasts. The animals kept running even when Meijer removed the food from the garden site, although they came in smaller numbers, she notes. Sometimes the rodents were so eager to run that they couldn’t wait to take turns, she says: At one point, a large mouse sent a smaller mouse flying when it climbed on to the wheel and started running in the opposite direction.

The fact that the wild mice and other animals were bold enough to enter the cage and use the wheel is "very weird," but perhaps not as surprising when one considers that many domesticated animals also like to run on wheels, including dogs and chickens, says Justin Rhodes, a neuroscientist at the University of Illinois, Urbana-Champaign.

Although the common house mice observed in the study tend to be more leery of novel structures than other species—an evolutionary adaptation to the human penchant for building mousetraps—Garland suggests that the wheel may provide a more secure way for the animals to run than darting across an open field. "There's something attractive about being able to get in a wheel and run unfettered.”

*Correction, 26 December 2019, 10 a.m.: More than 12,000—not 200,000—animals were caught running on wheels. This story has been updated.