Wheel running in the wild
Johanna H. Meijer and Yuri Robbers
The importance of exercise for health and neurogenesis is becoming increasingly clear. Wheel running is often used in the laboratory for triggering enhanced activity levels, despite the common objection that this behaviour is an artefact of captivity and merely signifies neurosis or stereotypy. 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, also when no extrinsic reward is provided. Bout lengths of running wheel behaviour in the wild match those for captive mice. This finding falsifies one criterion for stereotypic behaviour, and suggests that running wheel activity is an elective behaviour. In a time when lifestyle in general and lack of exercise in particular are a major cause of disease in the modern world, research into physical activity is of utmost importance. Our findings may help alleviate the main concern regarding the use of running wheels in research on exercise. …
2. Observations in nature
We observed wheel running both in the urban area (1011 observations in the first 24 months, of which 734 were of mice) and in the dunes (254 observations in 20 months, of which 232 were of mice). Wheel movement not caused by mice was caused by shrews, rats, snails, slugs or frogs (figure 2 and the electronic supplementary material, movie clips). Of these, only the snails caused haphazard rather than directional movement of the wheel and were therefore excluded from the analysis. Cases where animals set the wheel in motion from the outside were also not considered proper wheel running and were therefore excluded.
The observations showed that feral mice ran in the wheels year-round, steadily increasing in late spring and peaking in summer in the green urban area, while increasing in mid-to-late summer in the dunes, reaching a peak late in autumn (see the electronic supplementary material, figure S1a,b). Some animals seem to use the wheel unintentionally, but mice and some shrews, rats and frogs were seen to leave the wheel and then enter it again within minutes in order to continue wheel running. This observation indicates that wheel running may well be intentional rather than unintentional for these animals. Video recordings show that the wheel running mice were primarily juveniles, possibly explaining the higher incidence of wheel running around the summer.