How Squirrels Fly
Fascinated by the graceful gliding of these mammals with “wings,” scientists take a close look.
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."
Squirrels also have a little flap on their patagium, a sort of winglet. The scientists noticed that the winglets curl upward, like the tips on many aircraft wings. One theory is that the slant reduces drag around the end of the wing. Another is that it acts to stabilize or control the glide. Or again, as in commercial planes, it may both increase flight efficiency and help control and stabilize the glide. This theory is quite likely, since the winglet is so far from the center of gravity that it has an exaggerated effect.
In 1999 Stafford spent about two months in Japan doing research with Takeo Kawamichi, a professor who has studied the Japanese giant flying squirrel for decades. Observing the squirrels in the wild, the scientists stayed up night after night making videos and measuring the animals’ speed and flight distances. Once, Stafford saw a giant squirrel at 2 a.m., or rather he saw the creature’s eyes glowing in the dark. Suddenly the eyes disappeared. "Did he just close his eyes, or did he move? They’re so quick and quiet you can’t tell."
Coasting low and slow, the Japanese giant flying squirrel was recorded gliding for nearly 160 feet. There have been some reports of 500-foot glides, "but that was on a downhill slope," Stafford said.
Flying squirrels vary anatomically, he explained. "All have a small membrane between the neck and forelimbs, and this seems related to how they glide. Larger ones have a membrane between the hind legs. The smaller animals don’t have this, but they do have featherlike tails. What is the function of that?"
These questions brought Stafford and Thorington to the wind tunnel and into collaboration with its director, Jewel Barlow, and research manager, Robert Ranzenbach. I got a tour of this remarkable device with Barlow. We entered a vast room with tilting walls. With no parallel sides or right angles, the facility makes for odd optical illusions. On one side, tucked at the end of a 40-foot-long pollywog-shaped structure, is the fan itself, which is 19 feet in diameter and has a 2,000-horsepower electric motor. It spins seven propeller blades modified from a B-29 bomber and can generate winds up to 230 miles per hour. Opposite the fan in a section of an enclosed tunnel circuit is the test area with an observation window.
To test wind impact, the powerful fan blasts a stream of air on objects such as aircraft, boats, cars, "anything that the wind blows on or anything that moves through water or air," Barlow told me. At this particular wind tunnel, a lot of the experiments are conducted to evaluate how various design concepts affect the aerodynamics of new cars. Using three-eighths scale models that are about six feet long, automobile manufacturers try to find out what the level of drag is for a particular design, or the degree of wind noise or debris distribution, even windshield wiper efficiency on a gusty day.
"We also conduct experiments on wind flow around buildings," Barlow added. "We measure pressure distribution on models, which helps the structural engineers design windows and glass walls."
Not long ago on TV, I saw a weatherman standing in a test chamber, chained to the steel floor, while he volubly described what it is like to stand in a hurricane. At 100 miles per hour his cheeks rippled, his ears flapped and he stopped talking.
Stafford has built life-size model squirrels of clay, fiberglass and steel rods, reproducing the exact wing shape and several levels of camber, or wing curvature.
"We’re now testing steady mid-flight patterns. We don’t have the data yet to study turns. The wing needs to be totally stable for this kind of testing, which is why we build the models out of steel."
Stafford is creating several variations of models that will mimic the different characteristics of flying squirrels. "There are many differences. For example, we will test one model with the winglets bent upward, and another with the winglets held flat. By comparing the results of these tests we will be able to determine the function of the winglets. We will know what they do. We are building 26 different models, designed to test our hypotheses about the function of different wing structures."
As I listen to all of this, a larger question occurs to me: Why glide at all? "Gliding may save energy getting from tree to tree," Stafford said. "Predator avoidance may also be a factor. Gliding may simply be the fastest way for these animals to get from one place to another, or get to widely scattered food sources."
Looking for answers, Stafford has been videotaping local gray squirrels—the non-gliders—in the wild to compare their behavior with that of gliders.
Being nocturnal, flying squirrels must have good eyesight, he said. "Even so, they often triangulate distance. You can see how their heads bob just before they take off."
The flying squirrel’s eyes are off to the sides of the head so the animal can spot attackers coming from any direction. But this fact, plus the small size of the head, does not make for great depth perception. That’s why the intended flight path has to be checked out from several angles to establish a workable parallax.
Sometimes a squirrel will drop like a stone for a few frightening yards after takeoff to gain speed. It turns by lowering one arm, just like a kid playing pilot. I thought it must be exciting to watch a creature seemingly in the middle of an evolutionary change, and I wanted to know where all this was leading: Would squirrels someday fill the sky like birds?
Stafford had to smile. "Evolution is not necessarily directional. There are all kinds of gliding animals—mammals, lizards, fish—but their development isn’t necessarily going anywhere. Gliding may be an end in itself."