Animal welfare committees have questioned whether mice in experiments enjoy running on wheels, or if wheel-running is a neurotic activity (like pacing in many species of zoo animals). Two researchers set out to answer this question, by placing an exercise wheel in the woods and just, well, seeing what happened!

Mice flocked to the wheel, spending between 1 and 18 minutes running (the former were likely the Usain Bolts of the mice world, the latter marathoners). Mice weren't the only species to choose to run on the wheel: frogs, and even slugs, also frequented the outdoor "gym!" (The slugs probably got there by accident, and the frogs mostly just hopped on and off).

Of course, this study only shows that wheel-running can be a voluntary activity. As noted by Marc Bekoff (professor at the University of Colorado), wheel-running in lab mice could still be stress-induced. But regardless of the animal welfare question, this study demonstrates that humans aren't the only species to run for fun! (It is also undoubtedly the first and only record of frog activities on an exercise wheel to be published in a peer-reviewed journal.)