Wild animals run for fun
Do captive mice run in their wheel because they are neurotic and stressed? Or is it a natural mouse behavior? Well, it turns out that if you put a running wheel out in the wild, the mice will come. And they will run. Turns out frogs and other animals will join in too. Pretty cool. Read more about the study here, or download the open access paper here.
Note: this does not "prove" that mice in captivity aren't running in a wheel due to stress!
Best video from this story: a slug in a running wheel. Please watch.