In 2014, a team of Dutch scientists set out to conduct an unusual experiment. They set out a hamster wheel in the wild to see whether mice in the wild would play with it like mice in captivity. Not only did the wild mice play with the wheel, but frogs, rats, shrews, and even slugs also interacted with the wheel -- and this could have important implications for animal welfare, and even our understanding of animal curiosity.
Wheel running in the wild
For pet and lab mice, running wheels are often used as a way to get the animals to increase their activity levels. But some have argued that mice use these running wheels simply because they live in cages, and they wouldn't do it in the wild.
But no one had actually tested this until 2014, when Johanna Meijer and Yuri Robbers decided to try it out.
"If wheel running is indeed caused by captive housing, wild mice are not expected to use a running wheel in nature. This, however, to our knowledge, has never been tested. Here, we show that when running wheels are placed in nature, they are frequently used by wild mice," the researchers explained in the study.
They set up a running wheel in their backyard and placed an infrared camera to film how animals in the wild would respond. The wheel was put inside an enclosure with a small entranceway that would keep the smaller animal safe from larger predators. They also cheated a bit and left some food near the wheel to attract animals to the area.
A lot of animals started using the wheel. It was mostly mice, but a few shrews, rats, frogs and slugs were also drawn to it. Granted, the frogs would only leap around the wheel (which triggered it), and the slugs appeared to be simply curious as to what the wheel was or just climbing a new surface -- but the other animals properly rolled the wheel.
The results were so encouraging that researchers set up another wheel and camera in a nearby dune area that was not available to the general public. This second wheel was left in place for a year and a half.
Remarkably, the animals kept gathering at the wheel. All in all, the team recorded over 200,000 animals using the two wheels over a three-year period. The main runners were mice, with many of them running on the wheel and then off and then on again -- one mouse ran for a whopping 18 minutes.
When the food was taken away from the wheel, the number of visits dropped significantly, but still didn't decrease to zero. Also, some mice using the wheel were observed to be too young to have known that there was ever a food reward; in other words, they were just using the wheel for the sake of it.
This begs the question of why animals run in wheels when there's no reward. Researchers propose one idea that could be linked to the metabolic reward system: it's a motor response to hunger or other stimuli related to foraging. It could also satisfy the animals' curiosity or desire to play. This idea is backed by the fact that when the food was taken away from the wheel, fewer animals showed up -- but those that did show up were more likely to use it.
"Our results indicate that while the number of visits to the recording site decreased when no food was present, the fraction of visits including wheel running increased. This implies that wheel running can be experienced as rewarding even without an associated food reward," the researchers write.
It's likely that there's more than just one motivation at play, but whatever the case may be, one thing is for sure: mice don't just use the wheel in captivity, they also use it in the wild. So this is feeding a natural desire for them, not creating an artificial need. This could have implications both for people keeping mice as pets, and for researchers using lab mice.
Journal Reference: Wheel running in the wild, Published 21 May 2014 DOI: 10.1098/rspb.2014.0210