Animals, such as pet hamsters, really do enjoy exercise wheels, suggests a new study that found most small wild creatures voluntarily use the wheels when they encounter them.
The study, published in the latest issue of Proceedings of the Royal Society B, is the first to look at wheel running in the wild.
"Locomotion can be inherently rewarding for animals," Johanna Meijer, a professor in the Department of Molecular Cell Biology at Leiden University Medical Center, told Discovery News. She co-authored the study with colleague Yuri Robbers.
Exercise wheels are ubiquitous for small captive animals. Larger animals may use them too, as this cat shows, if the wheel is big enough and the animal is not afraid of the device.
Some animal activists have questioned the behavior, concerned that it is a product of being cooped up in a cage. The new study, though, found otherwise.
"Our results indicate that running wheel activity is not necessarily an aberrant behavior initiated as a consequence of captivity and perhaps stress, but rather an elective behavior that can be observed in a natural environment," Meijer said.
Among the animals proving Meijer and Robbers' pro exercise wheel stance was this wild mouse. For the study, the researchers anchored an exercise wheel in both a "spacious green urban area and a dune area not accessible to the public." They then set up a video camera with night vision and a motion detector. A food tray was placed nearby, just to attract wild beasties to the spot.
Wild mice went for the food, and the wheel too.
"Mice run in the wheel; they never just walk," Meijer said. "They are frequently observed leaving the wheel, and immediately going back in, suggesting it is a voluntary act."
She added, "Sometimes two mice run in the wheel at the same time."
Surprisingly, the second most common visitors to the wheel placed in the wild were slugs.
"They can slide along for hours," Meijer said.
Slugs aren't exactly high on the food chain, so no one has ever studied whether or not they play. Meijer, however, said they could be exhibiting play behavior, although she thinks that slugs and other organisms "have an intrinsic motivation to be active."
Perhaps, like a couch potato person getting the sensation of activity while watching television, the slug has the sensation of exciting activity as it slides and the wheel creates additional movement.
Snails might be cleverer than we think.
"Snails in the wheel cheated," Meijer said. "They activated the wheel without running. They climbed up vertically until center of mass was moved above the central axis of the wheel, making it turn."
Frogs don't walk or run, and yet wild frogs seemed to enjoy the exercise wheel too.
"Frogs sit in the wheel and jump, making the wheel move back and forth, then jumping again," Meijer said. PHOTOS: Superhero Frogs, Giant Fish
In the future, larger exercise wheels might become more commonplace for larger pets, such as dogs. It is likely that younger pets will be more likely to use the equipment.
Throughout the study, the researchers noticed that juveniles of all species used the wheel more often than adults did. This was true for mice, rats, shrews, frogs, snails and slugs.
"Shrews occasionally enter the wheel, run for a moment, and then leave again," Meijer said. "Rats enter the wheel, but do not use it for very long at all, possibly because the wheel is a bit small for them."
Playgrounds are starting to include more equipment modeled after exercise wheels. Even adults often can't resist hopping on for a spin. Our usage of the equipment isn't clear. Do we have an urge to hop on because we want to exercise, because we enjoy playtime, because we inherently enjoy unexpected movement, all of the above, or for some other unknown reasons?
The researchers hope to better understand animals' attraction to exercise equipment. As it stands, there is just one animal that seems not to gravitate to exercise wheels: birds. But even they might get their kicks in another, creative way.
Meijer said that one bird seemed to like jumping up and down on top of the wheel, making it move.