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Friday, July 12, 2013

Catching Great Air

Catching Great Air (1902)

A scientist documents the remarkable aerodynamic adaptations of northern flying squirrels.

Article and photograph by Alexander V. Badyaev
Alex Badyaev kicks off a series on Michigan squirrels and squirrel-like animals that will be featured in the next several issues of The Wildlife Volunteer.  Michigan is home to both flying squirrel species found in North America [the northern and southern flying squirrels.]  Although Badyaev researches the northern flying squirrel, the southern species is very similar in characteristics and ecological niche.

[Alex Badyaev, a professor of evolutionary biology at the University of Arizona, conducts long-term field research projects throughout Montana and Arizona. Also a professional photographer, Badyaev is a multiple winner of the BBC Wildlife Photographer of the Year, National Wildlife, and Nature Best Photography competitions. Most recently his photos are featured in a new book Mammals of Montana.  To view Alex Badyaev’s photography and order prints, please visit www.tenbestphotos.com.

I spend the day in my research cabin north of Ovando in the Blackfoot River Valley poring through photographic equipment manuals to determine the lowest temperature of operation. Meanwhile, the radio is broadcasting severe winter weather warnings, with dangerously low overnight temperatures. Finally, I decide on a plan and head into the forest.
A few hours later, after snowshoeing six or seven miles into the backcountry, I stop and begin working in the diminishing February twilight. As quickly as possible in the freezing cold, I string a rope of strobe lights along the branches of several trees. The lights are connected to a high-speed camera set on the ground and aimed at a gap in the tree canopy framing a tiny half-acre forest pond on the southern boundary of the Bob Marshall Wilderness. From previous field research by me and my graduate students, I knew that local female northern flying squirrels regularly travel along the lakeshore. In winter, the squirrels emerge from roosting cavities shortly after midnight and range throughout the forest, traveling to their under-snow food caches by remarkably consistent routes.  My goal was to photograph squirrels in flight in a natural context, something that has rarely been done before.
Based on my observations from previous nights, I expected the female squirrel I’d targeted to fly over the lake sometime between 2:20 a.m. and 2:50 a.m. Unfortunately, the overnight temperature was predicted to plummet to -40 degrees F, greatly increasing the chances of camera failure. But the risks were worth it. In Montana, February is the middle of the northern flying squirrel’s mating season. During that time, even in severe cold, each female is typically escorted through the forest by a squabbling squadron of ardent males. I was hoping to also photograph those males and their dizzying aerial mating chases.
AN AIRBORNE KEYSTONE SPECIES
The northern flying squirrel is one of two flying squirrel species found in North America. The other is the smaller but almost identical southern flying squirrel. The species we see in Montana ranges across Canada, Alaska, and the northern, Great Lakes, and Appalachian states, often in cool mountain zones, as far south as North Carolina. The southern flying squirrel ranges across much of the eastern third of the United States, from Florida north to the Great Lakes.
Flying squirrels feed on plant material, including seeds, nuts, and flowers, and also eat insects, bird eggs, and, occasionally, meat scavenged from dead animals. Their passion for eating lichen, truffles, and other mushrooms helps spread the mycorrhizae, essential for plant root growth, through forest ecosystems. What’s more, when excavating fungi on the ground in the middle of the night, flying squirrels get so preoccupied with finding food they become highly vulnerable to great horned owls and great gray owls, their primary predators. The squirrel’s role as a central link in the forest food chain makes it a “keystone species,” one essential to maintaining the ecological integrity of its habitat.
The flying squirrel is well known for its amazing ability to glide among tree trunks on its outstretched patagium, the expandable furred flap of skin on either side of its body that spans uninterrupted from the animal’s neck to its ankles. For years, scientists assumed that flying squirrels were passive gliders that use their patagium simply to prolong jumps across canopy gaps and lessen the impact of landing.
These assumptions became suspect, however, when recent laboratory studies uncovered several exceptional features of squirrel aerodynamics that strongly hinted that the species might be capable of more than passive gliding. Time-lapse lab photos indicated that flying squirrels were capable of airborne feats that aerodynamic theory suggested should be impossible for a species that was simply gliding through the air. In particular, airborne squirrels have an unusually high “angle of attack”--the angle between the gliding membrane and the direction of oncoming airflow. While greater angles generate greater lift, valuable for gaining midair height and distance, the angles observed in flying squirrels far exceed those that could be sustained even by advanced military jets. In theory, the high angle should cause the soaring squirrel to stall midair and crash.
The laboratory studies also suggested that in order to sustain such high angles of attack, the squirrels need to eliminate the destabilizing forces of unequal air pressure above and below the patagium. These “mini-tornadoes” on either side of the jet wings are the cause of turbulence, and the forces increase as the plane angles upward. Such turbulence should greatly reduce the squirrels’ gliding distance and speed. But that doesn’t seem the case. How do they do it?
Scientists have also long been puzzled that flying squirrels don’t crash. Simple calculations show that a squirrel landing from a routine 40-foot glide would hit a tree with an impact of more than 30 times its body weight unless it actively stalls well in advance of the landing. Yet such a stall would further decrease flight stability and duration. Based on what what’s known about aerodynamics, flying squirrels should be confined to slow, short, and steady glides or risk constant crashes, stalls, and falls. Yet they have been documented soaring great distances. How was that possible? I hoped to learn the answers as I knelt in the snow that frigid February night.
EYEWITNESS
Shortly after 2:30 a.m., under a nearly full moon, I was treated to a remarkable air show. It began with a cloud of snow kicked up by two males chasing each other on the upper branches of a spruce tree high over my head. One lost his grip then dove into a long glide over the lake, followed immediately by the second male in a rapidly accelerating glide.
Both landed in the upper canopy across the lake--seemingly without much loss of elevation, despite a glide of at least 60 feet--and resumed their squabble. Then I spotted a female sitting quietly on a snow-covered branch against a tree trunk, inspecting a large fir cone probably left by a red squirrel during the day. A few seconds later, another male parachuted down from a nearby tree, somehow steering the end of his near vertical descent to land on the trunk right below the female.
The female crouched, and in an exceptionally powerful jump with a fully extended body and outstretched hind- and forelimbs, launched herself at a 40 degree angle high into the air. She kept her patagium completely folded until reaching a height of about 10 feet above her perch  She then spread the membranes wide open, and lighted by a series of high-speed strobes triggered by my camera, seemed to freeze in midair for a moment before gracefully gliding out of view across the snow-covered lake. After engaging in a few barely audible squabbles from across the frozen expanse, occasionally kicking up more snow dust, the squirrel group disappeared into the dark and the night’s silence was restored.
I was amazed. What I had witnessed and documented with my camera that night and subsequent ones were a series of astonishing aerial accomplishments:  150-foot-long flights across open fields; mid-air turns to evade attacking owls; vertical leaps so high the squirrels could then soar from midair to a nearby tree trunk. It was obvious this species is capable of much more than just simple static gliding.
I spent the rest of that night walking around to keep warm, watching an occasional owl for entertainment. At first light, I dismantled the by-then solidly frozen equipment with its long-dead batteries and started my way back to the cabin. I would spend many days afterward replaying and analyzing, frame-by-frame, the footage of these stunning performances, learning more about how the species elegantly solves major aerodynamic problems.
Foremost among these solutions is the squirrel’s “wing tip”—a short rod of cartilage outside the wrist that the animal moves at various angles to enable exceptional flight control and precision landings. This anatomical novelty, sort of like a large sixth digit though not attached to the others, is controlled by a powerful muscle. By adjusting the angle of the wing tip, the squirrel can generate a substantial lift modifying the speed, distance, and trajectory of its glides in mid-flight. This anatomical innovation precedes the static endplates (“winglets”) that NASA began installing on the wings of modern jets in the mid-1970s by at least 20 million years.
            A flying squirrel’s second novel physiological adaptation is the extensive musculature that crisscrosses the thin gliding membrane. These independently controlled muscles, combined with limb movements during flight, allow a squirrel to actively modify the billowing of its “wings” and the orientation of fur on their surface. This produces, in a typical aerial chase, wing shapes such as completely folded patagium during powerful takeoffs; thin, fully extended membranes in the middle of long-distance glides; and fully inflated furry parachutes for slowing nearly vertical descents.
Finally, unlike most other gliding mammals, flying squirrels have an additional fur-covered membrane between their neck and wrists they can curve down during flight. This “mini-patagium” guides air flow away from the larger membrane to lessen turbulence, while generating significant forward acceleration and lift.
In short, flying squirrels flawlessly combine, in a small furry package, features of heavy transport planes, agile military jets, and flexible-wing parachute gliders, making them one of the world’s most sophisticated mammalian gliders.
Scientists recently started to realize that flying squirrel’s seemed to be loaded with excess anatomical features. What purpose did they serve? Flying squirrels seemed overbuilt for simply gliding from one tree to another. My contribution from the nights spent in western Montana’s frigid woods was to document in the wild just how the squirrels use those remarkable features in flight. It indeed turned out that flying squirrels are more than just passive gliders. For instance, I saw them leap into the air from a tree trunk and then, as if forgetting something, turn 180 degrees in midair, independently adjusting wing tips on left and right membrane, and return to the same trunk.  And I documented the ways they accomplish both rapid acceleration at the onset of prolonged glide and remarkably effective deceleration before landing so they don’t smash into the destination tree.
Over millions of years, flying squirrels have come up with elegant solutions to the same aerodynamic problems that face modern aircraft engineers. Undoubtedly we have much to learn from these small, furry mammals. I have to wonder: What other marvels in these and Montana’s many other mammal species are still out there waiting to be discovered? 
(reprinted with permission of Montana Outdoors)


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