Run for fun

Even wild mice enjoy exercising in treadwheels

by Bart Braun

Leiden researchers have discovered that wild mice use treadwheels voluntarily, an important fact to remember when studying their tame cousins in the lab. “It seems as if they have an urge to move.”

The mice in Joke Meijer’s back garden made global headlines, starring in The New York Times, The Guardian, Der Spiegel, New Scientist and numerous other national and international media recently although, oddly enough, Meijer’s main job is brain research. “We study the body’s biological clock, specifically the network of nerve cells in the brain that control the rhythm of our 24-hour cycle. We use mice to measure that rhythm and to measure the activity of the mice, we hook up a computer to the treadwheels in their cages.”

The professor of neurophysiology is not the only scientist to work with mice. All sorts of research into the effects of something on activity or the effects of activity on something is based, at least partly, on measurements of lab animals running in treadwheels.

“That of course always raises the question of whether we’re measuring pathological behaviour”, continues Meijer. If you keep animals or humans in captivity, they might start displaying what’s known as stereotypical behaviour: compulsive, repeated behaviour that’s quite clearly related to stress: walking to and fro, shaking their heads, licking the bars of the cage, etc. If a mouse only uses the treadwheel because it’s deeply troubled, you’re not only measuring the results of exercise, but the results of its unhappiness too.

Meijer adds: “If that were true, our research would be in trouble. But I never once thought it was stereotypical behaviour: the mice use the treadwheel on their very first day in the cage and use it less frequently as they get older, while animals display more and more stereotypical behaviour the longer they are kept in captivity. Even so, I decided to study them in the wild because I was truly curious about the results.”

The only other observation of treadwheel use by animals outside captivity was recorded by Konrad Lorenz, the founding father of research into animal behaviour. A few of his rats escaped and he managed to recapture them by putting out a treadwheel. But would animals that had never seen such a device use them?

Meijer teamed up with the Leiden University Medical Centre’s delicate-engineering department to design special open cages containing treadwheels. The cages were equipped with infrared cameras and their wires were wide enough to allow mice and rats to enter but close enough to keep out foxes and other animals that might damage test set-up. “Many university medical centres have closed their engineering departments to economise although those departments can build things that allow you to measure things no one else can. You certainly can’t walk into a shop and just buy the equipment.”

PhD student Yuri Robbers, who teaches biology at the Stedelijk Gymnasium School in Leiden, wrote the necessary programs and did the data analysis. Special funds were set aside some years ago for teachers who wanted to do PhD research on top of their teaching job and Robbers was one of the first to take advantage.
“His plans suited our study to a T.” One of the cages was placed in Meijer’s back garden and the other in a closed-off area of the nature conservation area Meijendal near Wassenaar. “The bunkers there still have live sockets so we could use them to power the measuring apparatus with the help of long cords.”

In late May, the results were published in the biology journal Proceedings of the Royal Society B. And what do they reveal? Mice, shrews, rats and even frogs climb into the cages of their own accord for a run, even when there’s no food for them there. Infrared footage of a slug on the wheel has already been viewed some hundred and fifty thousand times on YouTube, although Meijer won’t make any claims on the slug’s conscious decision to exercise. “I have the impression that the mice do have the intention to run. You can see them jump off and then jump back on again to continue their run.” The first emails from enthusiastic readers who set up cameras and treadwheels in their gardens – and who have managed to capture running mice on films – have started to arrive.

The running behaviour displayed by rodents is apparently not something they do because they are nervous wrecks in captivity. And that’s good news for thousands of researchers across the globe who work with running mice. So why do they run? “It seems as if they have an urge to move. Biologists are quick to assume that all behaviour has a purpose; after all, it takes time and energy. But maybe they’re just playing. We noticed that younger animals use the treadwheel more often than the older creatures.”

One detail fits in nicely with Meijer’s lab work: the treadwheel in the dunes was used far more often at night than during the day, though this difference was less noticeable in the cages in Meijer’s back garden. She suggests that perhaps the light pollution in Leiden has disturbed the circadian rhythm of mice.

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Run for fun

Leiden researchers have discovered that wild mice use treadwheels voluntarily, an …