Hamster wheels are exercise toys used by hamsters and other rodents. Most of these toys are composed of a runged wheel held on a stand by a pair of stub axles. Hamster wheels allow rodents to run even when their space is confined.
Most wheels are constructed of steel or plastic, both with advantages and problems. Solid plastic wheels are safer for some types of pets, such as hamsters, because the space between rungs is solid and the animal's feet or legs cannot get stuck between rungs, a injury risk in steel wheels. However, some rodents (such as gerbils) will quickly chew on and destroy plastic wheels, but not steel wheels.
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). The hamsters expressed no preference between a relatively uniform running surface made of plastic mesh and a surface made of rungs spaced 9 mm apart, though they did prefer the mesh over rungs spaced 12 mm apart, most likely because the wider space between the rungs let the legs slip through sometimes. 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.
Like other rodents, hamsters are highly motivated to run in wheels; it is not uncommon to record distances of 9 km (6 miles) ran in one night. Other 24-h records include 43 km (26 miles) for rats, 31 km (18.5 miles) for wild mice, 19 km (11.5 miles) for lemmings, 16 km (9.5 miles) for laboratory mice, and 8 km (5 miles) for gerbils. Hypotheses to explain such high levels of running in wheels include a need for activity, substitute for exploration, and stereotypic behaviour, but various experimental results strongly suggest that wheel running, like play or the runner's high, is rewarding in and of itself and highly valued by the animals. It is tempting to draw parallels with humans who run for fun. This makes running wheels a popular type of enrichment (even an essential one, in the view of some) to the captivity conditions of rodents.
The reliability with which hamsters and other rodents will use a running wheel has made wheel-running one of the most widely used markers of activity and wake time in circadian rhythms research.
A related toy, the hamster ball is a plastic ball into which a pet can be temporarily locked. The ball allows the pet to freely roll around on the floor to explore and exercise, while preventing escape. Care must be taken to block off stairways, and time in the ball should be limited as more than an hour without access to water could impose dehydration on the animal.
Hamster wheels have often been proposed as the basis of many speculative methods for power generation, though they are not feasible for large-scale applications; Make Magazine and other similar publications have included directions for rodent-powered nightlights, but such projects are explicitly small-scaled and meant mostly for humor value. Running hamsters are often said in jest to be the true power behind computer networks, especially slow-moving or unreliable ones. The joke is similarly applied to sub-compact automobiles and other small motor vehicles. Of course, energy can be produced more efficiently by burning food directly in a furnace rather than giving the food to a hamster and harnessing the power output of its running activity.