Science

A Bunch of Mice Ran 11 Marathons in Six Weeks

What have you accomplished this pandemic?

A mouse on a hamster wheel.
GlobalP/iStock/Getty Images Plus

Over the course of six weeks, mice logged 300 miles each on hamster wheels in a lab in Boston. OK, that was not the main point of the study—that would be to look at the effect of a high-sugar, high-fat diet on training. (The effect is bad.) Actually, it’s pretty normal for mice to run that far. But as science writer Roxanne Khamsi pointed out when the study made the rounds earlier this week—wow, 300 miles in six weeks, that is a lot! “My pandemic self can’t help but be floored,” Khamsi tweeted.

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My pandemic self was floored, too, and interested in taking a quick break from *gestures broadly at everything* to focus on the wonders of these mice. The new paper’s first author, Tara MacDonald, a researcher at the Joslin Diabetes Center, graciously walked me through why mice make such good subjects for exercise studies—and in what conditions they managed to run more than 11 marathons in six weeks. Our conversation has been condensed and lightly edited for clarity.

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Shannon Palus: Do the mice have a preference as to when they run?

Tara MacDonald: They do the majority of their running in the dark. Almost all of their running occurs at night. For us it’s important to track how much they run to make sure any of our treatment groups in any experiment run the same amount, to make sure that the conclusions are solid. If you have one group running more than another, it’s important to know. We record with an odometer how many kilometers they run on their wheels.

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How do the mice break up their running? Are they going on a 7-mile run every night?

We didn’t measure them at that resolution. I do know from other studies that most of their bouts tend to be under three minutes, but they can be as long as 18—it depends on the experiment conditions. They kind of do bursts, with 15- to 30-second rests. Their average speed can be anywhere from 1.5 to 5 kilometers per hour—from other work, that’s the ballpark.

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For the most part, mice run between 4 and 20 kilometers per day [about 3 to 12 miles per day]. It’s a large range. It plateaus a little after a couple of weeks. They ramp up in the first week.

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Do you have to do anything to encourage them to run on the wheels?

They love running. We don’t have to encourage them very much at all. I would say we have 1 out of every 30 mice who doesn’t really like running too much. It doesn’t seem to be dependent on treatment. Our healthy control mice compared to our hyperglycemic ones, they have equal running behavior. But there’s a study from 2009 that showed that if you put a running wheel in the wild, mice will voluntarily run on it for hours a night. They love running wheels.

Mice in the wild without wheels, or in our houses, would they also be running around that much?

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They run a lot in the wild as well, which makes sense. They’re at the bottom of the food chain, fleeing predators. I’m not sure the head-to-head comparison [of how far a mouse runs in the wild versus if it has access to a wheel]. But I do know from the wheel-running study, when they put lab mice in versus wild mice, they did run for a similar amount of time. There’s some indication that the behavior would be similar.

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What does it sound like to be in the lab with all the mice running?

It’s actually silent. We’re really careful about this. The wheels that we have are metal. We order hamster wheels from Amazon. If you get them wet or anything, they can start to rust and creak. We use mineral oil just to make sure that it’s lubricated and not disrupting all the other mice that are in the room. I know if I were a mouse in a facility I probably wouldn’t want to listen to a squeaky wheel.

Did the mice finish up their running before the pandemic started, or were they logging all these miles while we humans struggled with the pandemic?

Thankfully, they finished before. We have made jokes about whether the pandemic influences mouse behavior—people are only in part time, there’s less traffic in the mouse room. So I don’t know. It could affect their behavior. It’s possible.

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