Do caged mice run on exercise wheels because their captivity has made them crazy or do these animals just find wheel-running fun by nature? Exercise wheels are often associated with caged animals but even wild animals including mice, rats, slugs and frogs hop onto running wheels apparently because they find this rewarding.

A new study that looked at the natural behavior of mice towards exercise wheels, two researchers from The Netherlands placed exercise wheels outdoors and monitored the subjects using motion-activated video cameras which would capture videos once the wheel starts moving. The experiment was conducted to determine if mice really enjoy wheel-running and whether or not they would still run on wheels if they were not caged and free.

Study researchers Johanna Meijer and Yuri Robbers, from the Department of Molecular Cell Biology at the Leiden University Medical Centre in The Netherlands found that wild mice as well as shrews, snails and other animals are drawn to the exercise wheels. Within 24 months, the exercise wheel placed in a green urban area had 1,011 of wheel running while the exercise wheel placed in a sandy dune area had 254 instances of wheel running within 20 months.

Of the 1,011 observations of wheel running in the urban area, 734 were by mice and 232 of the 254 instances of wheel running in the dunes were also by mice. Mice also run on wheels from one to 18 minutes. Gene Block, from the University of California, Los Angeles, said that these suggest that wheel running among mice may not have been driven by stress and may even be a form of a rewarding behavior.

Marc Bekoff, from the University of Colorado, however, said that while mice may run on wheels voluntarily, mice that are confined in laboratories may do it more because of the stress associated with captivity.

Mice are popularly used for experiments and tests in laboratories. New drugs, for instance, are often tested on laboratory mice to assess safety and efficacy. In the U.S., it is estimated that 15 to 20 million rats and mice are used for animal testing per year.

"Here, we show that when running wheels are placed in nature, they are frequently used by wild mice, also when no extrinsic reward is provided," Meijer and Robbers reported on the study 'Wheel running in the wild' which was published in the journal Proceedings of the Royal Society B on May 21. "This finding falsifies one criterion for stereotypic behaviour, and suggests that running wheel activity is an elective behaviour."