Exercise wheels are popular among laboratory mice, but it seems that wild mice - and even frogs and shrews - also use them to go for a jog. Some have argued that wheel-running is not a natural behaviour and could even be a result of neurosis. Johanna Meijer and Yuri Robbers at Leiden University Medical Centre in the Netherlands set up exercise wheels at two sites where wild mice live, and collected video for more than three years. They found that the median running speed of wild mice was lower than that of their captive counterparts, but wild mice reached higher maximum speeds. The animals were initially attracted to the equipment by food rewards, but continued to use the wheels when food was absent.
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