Flying Squirrels & Moonlight Gliding

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Alexander V. Badyaev

We had not known of bioGraphic until just now, and want to shout out to the source before anything else. Our thanks to the California Academy of Sciences, who we look forward to hearing from more in the next few years, for the service that bioGraphic provides to all of us. Vigilance, informed by science, will be more important than ever. You know what we mean.

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This recent story in bioGraphic seems like as good an option as any to link you to. We realize now that we have not posted any stories on the flying squirrels of the Malabar coastal region where we have been based since mid-2010, so glancing at this creature in the western USA habitat first seems a fine reminder of a pending task. Thanks for this story and photographs by Alexander V. Badyaev:

After listening all day to relentless warnings of “severe winter weather” and poring over equipment manuals to determine the lowest operating temperature for various pieces of photographic gear, I decided to stick with the plan. A few hours and several miles of snowshoeing later, I was hard at work in the diminishing February twilight, setting up lines of strobes and high-speed cameras along gaps in the tree canopy that framed a forest lake at the edge of Montana’s Bob Marshall Wilderness. I knew this lakeshore to be a primary movement corridor for a resident female northern flying squirrel (Glaucomys sabrinus), and based on observations from previous nights, I expected my nocturnal subject to launch herself across the lake sometime between 2:20 and 2:50 a.m.
By that time, the temperature was expected to be in the neighborhood of minus 30 degrees Fahrenheit, greatly increasing the chances of camera failure. But it was a risk I was willing to take, since I knew how spectacular that night’s acrobatics were likely to be. February marks the start of the northern flying squirrel’s mating season in Montana. On a typical night during this period, each female will be escorted through the forest by a squabbling squadron of ardent males. It was those energetic males and their dizzying aerial mating chases that I sought to film.

Until recently, flying squirrels were assumed to be passive gliders, using their expansive patagium—the furry wing membrane that spans from the squirrel’s neck to its forelimbs and back to its hind limbs—to simply prolong jumps across canopy gaps, and to lessen impacts upon landing. During passive gliding, travel occurs along a declining linear path. This is what paper airplanes do, trading height for horizontal distance. Although gliding like this is the cheapest form of locomotion, it is also the least stable, because any change in posture, wing symmetry, or weight distribution has the potential to disrupt the glide and result in an uncontrollable fall. Imagine a paper airplane with the sudden addition of a heavy weight on one side, or with one wing that suddenly changes size or shape.

Once scientists began studying flying squirrels in the lab, it didn’t take long to discover that there is nothing passive or constant about the species’ flight. Researchers would ultimately document in flying squirrels a wider variety of aerodynamic modifications and flight types than had been described in any other species of animal glider. In a single flight episode, a flying squirrel might use a dozen separate flight-control techniques and—frustratingly for the graduate students and research assistants tasked with documenting the patterns—different squirrels would use different combinations of these techniques. Ironically, the one type of flight that has never been observed in this species is passive gliding.

As more and more squirrels flew through wind tunnels and along blocked-off biology department corridors, it became clear that flying squirrels have a marked disregard for basic aerodynamic constraints. For example, the squirrels were frequently recorded moving through the air with extraordinarily high “angles of attack,” which is the angle between the wing—in this case the patagium—and the direction of oncoming airflow. Aircraft typically stall when their angle of attack reaches 15 to 20 degrees. Flying squirrels routinely reach 60 degrees, far exceeding values that would result in the stall and crash of even the most advanced military jets. Stalling is caused by a loss of lift. This occurs when the main source of lift, air vortices—the swirls of air that form at the leading edge of a wing as a result of differences in the pressure below and above the wing—essentially slide down the wing surface at high angles. Except, evidently, when the wing belongs to a flying squirrel…

The photographs alone are worth the visit to this article’s home, but the text is also excellent.

Read the whole article here.

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