How Squirrels Fly

Fascinated by the graceful gliding of these mammals with "wings," scientists take a close look.

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Frankly, it never would have occurred to me to test flying squirrels in a wind tunnel. I could just see it: little furry things buffeted by the artificial gale, their tiny eyes squeezed shut as they banked and dipped and climbed in the name of scientific research.

But it’s not that way at all. The cute little creatures couldn’t withstand the force. Besides, they don’t really fly, they glide. And it’s this long, graceful gliding that’s prompted a study by some scientists at the Smithsonian’s Museum of Natural History. 

"I was always interested in functional morphology, in the origin of primates, in how animals work," said Brian Stafford, a research associate at the museum who did his thesis on gliding mammals. "I got into flying lemurs, and that led me into research on all gliding mammals."

The purpose of using a wind tunnel to study flying squirrels—the test subjects, by the way, are models that Stafford constructs of steel and fiberglass—is to find out exactly how, in terms of physics, this gliding is done and how the squirrels’ bodies work to achieve it. How are the critters able to glide 10 to 30 miles per hour?

Stafford has been working with Dick Thorington, curator of mammals at Natural History, whose interest in flying squirrels goes back at least 20 years. The main purpose of the project is to learn more about the animals, Thorington said. "But wouldn’t it be fun if we discovered something useful about how to control flight or reduce drag on small objects, such as small flying robots that could be used in aerial photography?"

In addition to flying lemurs and flying squirrels, there are a variety of other flying mammals, including marsupials like the mouse-size feather-tail glider and the sugar glider, which you can find in pet stores, and the scaly-tail flying squirrel (an African rodent that looks like a squirrel but isn’t).

None of these mammals can actually fly. They develop no thrust. Nor are you going to see any of them catching a thermal and spiraling up into the sky. They’re arboreal, and they use their gliding skills for sailing from tree to tree.

Among the largest of the flying squirrels is the Japanese giant flying squirrel, which measures two feet from the top of its head to the tip of its tail, has a wingspan of more than a foot and a half and weighs up to five pounds. But some flying squirrels are no bigger than your hand. For instance, one of the two types found in North and Central America weighs a mere two to four ounces. Buff- and charcoal-colored fur, large eyes, a long flat tail and "wings" of loose skin that stretch from the forearms to the hind legs make the New World flying squirrels handsome though somewhat unusual-looking animals. They often nest in attics and eaves, though you might easily miss them because they are so tiny, nocturnal and fast.

The wing, or patagium, produces lift, enabling the squirrels to glide. When I visited Stafford at the Glenn L. Martin Wind Tunnel at the University of Maryland in College Park, he drew a series of rhombuses to show me how squirrel wings look when spread out. It’s the square shape that’s of particular interest to him and Thorington. Our modern aircraft design tends to be long and narrow, so they wondered how the square wings worked.

"Square wings for aircraft were investigated in the early days but didn’t progress," Thorington said. "They were not as efficient as narrow designs in terms of drag."


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