And you thought the term “gym rat” was just a figure of speech.

After scientists set up little exercise wheels in the great outdoors, wild animals from mice to rats to frogs gave the workout equipment a spin, with no personal trainer in sight. One marathon mouse ran for 18 minutes on one of the wheels, which were similar to those used by caged hamsters and other small pets. Even slugs used the wheels, though it’s not clear they succeeded in raising their heart rates.

“Even if (animals) are in nature, even if they have lots of space around them and can … do whatever they want, they also run on the wheel,” says study co-author Johanna Meijer of Leiden University Medical Center in the Netherlands. “And they do that quite a bit.”

Meijer and her colleague, Yuri Robbers, also of the Leiden medical center, rigged up video cameras and running wheels in a seaside nature preserve and in a habitat closer to home: an overgrown section of Meijer’s yard next to a city park. For part of the experiment, the researchers left dishes of mouse chow and, in the city, bits of chocolate near the wheels, allowing critters to binge, then conveniently run off the calories.

Animals ran on the wheel whether the treats were there or not. Nearly 90 percent of the exercise sessions were by mice, but the free gym also drew rats and shrews, the scientists report in this week’s Proceedings of the Royal Society B: Biological Sciences. All three were filmed leaving the wheel, then hopping back onto it a few minutes later, as if they wanted a longer workout. The scientists say the slugs didn’t actually mean to feel the burn.

“They happened to end up in the wheel – poor slugs, maybe – and they were moving and moving and didn’t get any further,” Meijer says, laughing. “It is very funny because (the wheel) goes very slowly, but it goes.”

Behind the funny video lies a serious purpose. Every year, scientific journals publish hundreds of studies of lab mice running on wheels, making them a convenient stand-in for human exercisers. There have been worries that such wheel use is a pathological behavior akin to the pacing of zoo animals. Meijer says her study doesn’t support the idea that wheel running is just a “neurotic” activity seen only in captivity.

The new results are “extremely helpful,” says Theodore Garland, an evolutionary physiologist at the University of California-Riverside, who was not associated with the study.

“If (wheel running) were considered abnormal or pathological, that would really call into question many of the studies that use it,” Garland says. The study shows the wheel is “clearly pleasurable,” he says.

“Presumably, it feels as good” to the wild animals, says Georgia Mason of Canada’s University of Guelph via email. But “what does this new finding say about wheel-running by caged lab rodents? Not that much, I’m afraid.”

“It’s difficult to judge whether (the animals) really understand what’s going on,” says Hanno Würbel of Switzerland’s University of Bern. “I’m not sure they’re not being tricked into something by this artificial device.”

Meijer argues that the mice definitely were exercising free choice. Maybe they were driven by a need for activity, she says, or maybe they were playing. Whether the mice were having a good time or not, the scientists certainly were.

The research was fun, Meijer says. “This is a study I enjoyed doing very much.”

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