Get the Popular Science daily newsletter💡

Breakthroughs, discoveries, and DIY tips sent six days a week.

By signing up, you confirm you are 16+, will receive newsletters and promotional content and agree to our Terms of Use and acknowledge the data practices in our Privacy Policy. You may unsubscribe at any time.

Everyone who has ever owned a hamster knows the sound: the small, relentless squeak of the exercise wheel, usually starting around two in the morning.

As you watch your cute furball running toward no destination whatsoever, you might wonder: What’s going on here? Is little Hammy acting out of restlessness or boredom? 

For decades, scientists assumed it was exactly that: a neurosis, an artifact of captivity, the hamster equivalent of doing push-ups in prison. 

But in 2014, researcher Johanna Meijer conducted a study that suggested a less depressing scenario. When wild mice came across a wheel in their natural habitat, they got on the wheel and ran—sometimes for up to 18 minutes at a stretch.

So if it’s not boredom or neurosis (wild mice surely have plenty of more important tasks than wheel running), what is it? 

Dr. Theodore Garland Jr., a professor of biology at UC Riverside, has spent more than 30 years trying to figure that out. 

“There’s still a lot of controversy about what, exactly, wheel running means to an organism,” Garland says. “What is it? What is the organism trying to do?”

Why wild mice run on wheels just like your hamster

In Meijer’s 2014 study, published in Proceedings of the Royal Society B, she and her colleagues placed exercise wheels in two different locations: a green urban area and a dune area not accessible to the public. For more than three years, they recorded wildlife activity at both locations.

They found that wild mice closely mirrored the behavior of their cage-dwelling counterparts. At both locations, the mice frequently ran on the wheels—often for lengths of time equal to the “workout” durations of captive mice.

Although food was initially used to attract animals to the wheel, the researchers found that wheel running continued even after the food was removed. This suggests that the animals not only ran voluntarily on the wheel, but did so without any external reward. 

The wheels attracted more than just mice, too. Shrews, frogs, and even slugs were recorded using the equipment (a few snails were excluded from the study due to “haphazard” movements on the wheel). But wild mice used the wheel far more than another animal, accounting for 88 percent of all wheel runners. 

Wild Animals Caught On Hamster Wheel

Hamsters aren’t the only creatures that like running on wheels. Video: Wild Animals Caught On Hamster Wheel, Live Science

So, why do rodents specifically enjoy a run to nowhere? Are slugs simply less committed to their cardio?

According to Garland, rodents are simply built for it—bigger home ranges, faster metabolisms, and the aerobic capacity to sustain speed over distance.

“A toad isn’t going to be running 10 kilometers in a day,” Garland says. “Whereas a chipmunk could be.”

Dopamine keeps mice and hamsters coming back for more

But that’s only part of the story. The more interesting question is why any animal would choose to do it at all.

According to Garland, the drive to run on wheels among free-ranging animals is not fully understood, but the behavior is likely tied to the reward centers of the brain. 

“Dopamine is viewed as the final common denominator,” Garland says, referencing the neurotransmitter that delivers a sense of pleasure to the brain’s reward system. Similar to a human working out at the gym, mice get a dopamine boost every time they run on their trusty wheel. 

In Garland’s own lab, mice placed in larger, rat-sized wheels will sometimes slow down mid-run and rather than jumping off as the wheel keeps spinning, complete a full 360, and keep going. It serves no obvious purpose. It looks, for all the world, like a bit of acrobatics, as if the little mouse is creating its very own roller coaster.

“I’m hesitant to use the ‘F-word’ about lower vertebrates,” he says, “but it’s hard to ignore the idea that they’re getting some sort of pleasure or enjoyment out of it.” 

Related ‘Ask Us Anything’ Stories

The reward system may explain the drive, but Garland sees something even more elemental at work—something similar to the “zoomies” dogs and other young animals get. 

A baby horse, Garland notes, will sometimes just tear around a field for no apparent reason—solo, unprompted, burning energy for the sheer joy of it. “We used to call it nip-norting,” he says, “just going crazy, even without another individual to egg it on.”

Exercising at a young age leads to lifelong habits, even for hamsters

Rodents’ love of running on wheels might even have implications for humans. Some of Garland’s work suggests that, when introduced at a young age, wheel running can become a lifelong habit.

In his study, Garland found that mice given access to a running wheel immediately after weaning, at just three weeks old, ran significantly more as adults.

“It’s got to be something up here,” Garland says, indicating the brain. “Their reward system has been permanently tweaked.”

Whatever it is keeping these little guys running, an early start seems to predict an ongoing practice. The implications, Garland believes, extend well beyond mice. For instance, cutting physical education from school curricula, he says, could be “a huge public policy disaster,” leading to adults who aren’t used to exercising.

“If you’re a kid who never gets to play basketball or tennis,” he says, “and then you get to college, and your friends are playing pickup games, it’s probably not even on your radar to do that kind of thing.”

Of course, none of this is on your hamster’s radar at all. They’re just galloping away, keeping you awake with the endless rotation of their squeaky wheel. But all that running can also lead to some good: Recently, a resourceful young YouTuber rigged his brother’s hamster wheel to charge his phone.  

But no need to worry—the clever teen isn’t exploiting the toil of a joyless captive. Hammy, it seems, is just doing what comes naturally. 

In Ask Us Anything, Popular Science answers your most outlandish, mind-burning questions, from the everyday things you’ve always wondered to the bizarre things you never thought to ask. Have something you’ve always wanted to know? Ask us.

 

2025 PopSci Best of What’s New

The 50 most important innovations of the year