In all the team recorded over 200,000 animals using one or the other of the wheels over a three year period. The main runners were mice, some of which jumped on, ran for a while, jumped off, then jumped back on and ran some more. One mouse ran for an incredible 18 minutes.
So it's settled: they loves them some wheels. This puts the issue to bed forever, I assume. One thing pissed me off, though. The writer apparently felt compelled to include this statement:
The researchers claim the animals ran on the wheels because they enjoyed it, which could be a little bit of anthropomorphizing, as no one has been able to prove that animals other than humans experience emotions.
Duh. Whenever I see a statement like this, I wonder if the writer has actually met a real, living animal. Of course they have emotions. People who think otherwise are privileged gits with a huge blind spot.