Hamster wheels are exercise devices used primarily by hamsters and other rodents, but also by other animals when given the opportunity. Most of these devices consist of a runged or ridged wheel held on a stand by a single or pair of stub axles. Hamster wheels allow rodents to run even when their space is confined. The earliest dated use of the term “hamster wheel”, located by the Oxford English Dictionary, is in a 1949 newspaper advertisement.
Most wheels are constructed of steel or plastic, both of which have advantages and disadvantages. Solid plastic wheels are safer for some types of animals, such as hamsters and hedgehogs, because the animal's feet or legs cannot get trapped and injured between rungs. However, some rodents (e.g. gerbils) will quickly chew and destroy plastic wheels but not steel versions.
Choice tests with Syrian hamsters (Mesocricetus auratus) have shown that they prefer larger wheels; the animals chose a wheel diameter of 35 cm (14 in) over 23 cm (9 in), which itself was preferred over 17.5 cm (7 in). An arched back is not a natural position for a running hamster, and if a wheel is so small that the animal must arch its back while using it, then the wheel needs to be replaced with a bigger one; however, younger animals can use small wheels while they are still growing.
Hamsters showed no preference between a relatively uniform running surface made of plastic mesh and a surface made of rungs spaced 9 mm apart, although they did prefer the mesh compared to rungs spaced 12 mm apart, most likely because the wider space between the rungs allowed their legs to slip through. The hamsters neither preferred nor avoided wheels that had small "speed bumps" installed along the running surface to provide environmental enrichment. 
Choice tests with mice have also shown a preference for larger wheels (17.5 cm over 13 cm in diameter) and a preference for plastic mesh over rungs and over solid plastic as a running surface. More acrobatic species, such as the canyon mouse (Peromyscus crinitus) and the deer mouse (Peromyscus maniculatus) can develop preferences for wheels that force the animals to jump, such as square wheels or wheels with hurdles along the running surface.
Use by animals
Like other rodents, hamsters are highly motivated to run in wheels; it is not uncommon to record distances of 9 km (5.6 mi) being run in one night. Other 24-h records include 43 km (27 mi) for rats, 31 km (19 mi) for wild mice, 19 km (12 mi) for lemmings, 16 km (9.9 mi) for laboratory mice, and 8 km (5.0 mi) for gerbils. Hypotheses to explain such high levels of running in wheels include a need for activity, substitute for exploration, and stereotypic behaviour. Alternatively, various experimental results strongly indicate that wheel-running, like play or the endorphin or endocannibinoid release associated with the 'runner's high', is self-rewarding. It is highly valued by several species as shown in consumer demand studies which require an animal to work for a resource, i.e. bar-press or lift weighted doors. This makes running wheels a popular type of enrichment to the captivity conditions of rodents.
Captive animals continue to use wheels even when provided with other types of enrichment. In one experiment, Syrian hamsters that could use tunnels to access five different cages each containing a toy, showed no more than a 25% reduction in running-wheel use compared to hamsters housed in a single cage without toys (except for the running wheel). In another study, female Syrian hamsters housed with a nestbox, bedding, hay, paper towels, cardboard tubes, and branches used a wheel regularly, and benefitted from it as indicated by showing less stereotypic bar-gnawing and producing larger litters of young compared to females kept under the same conditions but without a wheel. Laboratory mice were prepared to perform more switch presses to enter a cage containing a running wheel compared to several metres of Habitrail tubing or a torus of Habitrail tubing.
Running in wheels can be so intense in hamsters that it may result in foot lesions, which appear as small cuts on the paw pads or toes. Such paw wounds rapidly scab over and do not prevent hamsters from continuing to run in their wheel.
Use in science
The reliability which hamsters and other animals use running wheels has made voluntary wheel running one of the most widely used indicators of activity and wake-time in research on circadian rhythms and other aspects of chronobiology. In rodents, voluntary exercise is almost always measured by use of wheels.
The neurotransmitter systems involved in wheel-running behavior have received considerable study. Recent evidence suggests that changes in both dopaminergic and serotonergic tone alter running-wheel activity. For example, one study in mice has shown that several antidepressant medications (all of which directly or indirectly enhance serotonergic tone) suppress running-wheel activity without suppressing general locomotion. The endocannibinoid system also contributes to wheel running in a sex-specific manner in rodents.
Mice from lines that have been selectively bred for high levels of voluntary wheel running have altered responsiveness to drugs that alter dopamine and endocannibinoid signalling, and enlarged midbrains.
A related exercise device, the hamster ball, is a hollow plastic ball into which a pet can be temporarily placed. The ball allows the pet to freely roll around on the floor to explore and exercise, while preventing escape.
Some commercial plastic refuges for caged rodents have a shallow, concave, rotatable disc mounted on the roof at a slight angle. The rodents run on the rim of the disc in a similar way to running in hamster wheels.
- Los Angeles Times, 9 Oct. 1949, p. B20/4 (advt.), referenced in Oxford English Dictionary, Draft Additions, Sep. 2007, under entry “Hamster.”
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