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Authors: Warner Shedd

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Predation is by no means the only danger faced by squirrels. A variety of parasites and diseases can also prove fatal, and accidents happen, as well. Remarkably, however, in view of lives spent racing around through the trees, leaping from slender branch to slender branch, squirrels are rarely injured in falls. Partly this is because squirrels are so agile and adept in the trees that they seldom fall, and partly it’s due to their ability to survive a fall without serious injury.

My mother had a bird feeder at her upper-story bedroom window, and she frequently became incensed at the sight of hog-fat gray squirrels gobbling up the seeds she intended for the birds. At such times she would shoo the grays off the feeder. They, without the slightest hesitation, would launch themselves outward to fall two and a half stories onto the bare ground below. Not once did one of these daredevils ever show any visible sign of injury. No doubt this ability is due to a combination of light weight and substantial wind resistance, which, much in the manner of a parachute, slows the squirrel’s descent greatly. This wind resistance, in turn, is the result of the squirrel’s long, bushy tail and the way it flattens itself out, much in the manner of a flying squirrel.

Despite the dangers imposed by all these hazards, both gray squirrels and reds are prolific enough to cope with this steady attrition and assure their continued abundance. It seems likely, therefore, that this tree-dwelling odd couple will continue to frequent our forests, just as they’ve been doing successfully for so many thousands of years.

4

Nature’s Gliding Machines: Flying Squírrels

MYTHS

Flying squirrels truly fly.

Flying squirrels are active only at night.

ALTHOUGH THEY’RE COMMON ENOUGH IN MANY FORESTED AREAS, THESE LITTLE CREATURES ARE SEEN SO SELDOM THAT THEY MIGHT ALMOST BE CONSIDERED WOODLAND GHOSTS. At night, perhaps, car headlights may for a fleeting second reveal what looks like a large, square leaf passing overhead, leaving the occupants wondering exactly what it was that they saw. More than likely, they glimpsed a flying squirrel—possibly the only view they would ever have of one, outside a museum or zoo. Reclusive as these little rodents are, however, it’s very much worth the effort to learn about them, for they are wonderfully adapted for the sort of life that they lead.

The most basic fact about flying squirrels is that they don’t! That is, despite their name, flying squirrels don’t truly fly, as bats and birds do. Rather, they use a remarkable adaptation that enables them to glide, and they could more accurately be called
gliding
squirrels.

And just how can a squirrel manage to glide? Very easily and well, as it turns out, thanks to a most unusual and extraordinarily useful adaptation. A wide, loose flap of fur-covered skin extends outward from each side of the flying squirrel’s body and stretches from the front ankle back to the rear one. When the squirrel wishes to “fly,” it simply launches itself into the air from high up in a tree and spreads its legs to the widest extent possible. This action stretches out the flap of skin on each side, making the squirrel resemble a large, square pancake with head, feet, and a tail. Stretched out in this fashion, the squirrel, which weighs very little, has a large surface area to support it. This adaptation enables the creature to glide laterally for a considerable distance before reaching the ground or a lower point on another tree.

Northern flying squirrel

Just how far can a flying squirrel glide in terms of horizontal distance? There’s no absolute answer to this question because there are so many variables. These include the height from which the squirrel launched; the slope of the land, if any; and wind velocity and direction. Height is the most important of these, because the glide ratio of a flying squirrel is about three feet horizontally for each foot of height.

Our North American flying squirrels can certainly glide two hundred feet or more, and some authorities say as much as three hundred feet. Considering the glide ratio, that would mean launching from a height of one hundred feet—certainly possible in many situations, although most glides are a good deal shorter. In theory, at least, the only limit to a flying squirrel’s length of glide is the height from which it started.

Because a flying squirrel in a long glide acquires a certain amount of velocity, one might think that the little animal would injure itself by crashing into its landing site on the trunk of a tree. But in fact this presents no problem for the squirrel, which, as it prepares to land, drops its tail and raises its forepaws, thus creating wind resistance much as an airplane does when it lowers its flaps during a landing. The squirrel, now almost in a vertical position, lands lightly against the target tree and promptly scurries off about its business.

Nor is a soft landing the only talent that flying squirrels exhibit while gliding. By using its tail as its main rudder, and also by moving its legs, thereby tightening or loosening the flaps of skin on one side or the other, the flying squirrel can maneuver well enough to avoid branches and other obstacles during a glide. In fact, a flying squirrel has been observed banking through an arc of as much as 180 degrees during a glide.

It’s widely assumed that flying squirrels are almost entirely nocturnal, but twice I’ve seen the little creatures abroad in daylight. One afternoon, while it was still full daylight, I saw a flying squirrel glide from a large maple tree to a tall balsam fir on the edge of our woods. The squirrel landed a few feet above the ground, scampered up the trunk, and, to my surprise, disappeared into a small hole that I hadn’t previously noticed. Though I watched for some time, the squirrel failed to reappear; almost certainly the hole was the entrance to its den. Where it had come from or what it had been doing is anyone’s guess.

On another bright, sunny day, two of us were doing forestry work. As we sat on the ground that noon, munching our sandwiches, my companion suddenly whispered, “Look, there’s a flying squirrel coming toward us.”

Sure enough, a flying squirrel on the ground was headed in our direction; its movements, impeded somewhat by the skin flaps, were far less graceful than the glides normally associated with its species. There was a small clearing in front of us, with mushrooms growing here and there, and these fungi were an evident attraction for the little squirrel. While we watched, it eagerly ate parts of several mushrooms, gradually drawing closer to us.

Paul Fiske, my partner, had a pair of leather work gloves which he had laid on the ground beside him. Slowly and quietly slipping them on, he whispered, “I’m going to try to catch it.”

Moving with great caution, Paul got to his feet; then he leaped up and ran at full speed toward the squirrel. With nothing close by to climb, the squirrel could only run for the nearest tree. Hampered by its skin flaps, the squirrel was a slow runner, and Paul scooped it up after a few long strides. I ran to join him, and together we inspected the soft, big-eyed creature that Paul was holding very gently. After we had satisfied our curiosity, the squirrel was placed on the ground and released, whereupon it ran to the nearest tree, climbed it, and glided off into the forest.

Unquestionably, flying squirrels are predominantly nocturnal. Because I’ve seen them abroad twice in full daylight, however, it’s evident that these furry gliders are more diurnal than is generally believed. After all, my two daylight encounters with them can hardly be unique, and no doubt many other people have occasionally observed flying squirrels active in the daytime.

There are two species of flying squirrels native to North America, the northern flying squirrel
(Glaucomys sabrinus)
and the southern flying squirrel
(Glaucomys volans),
as well as some subspecies. Although these two species are somewhat different in size and in some of their habits, they’re very similar in at least three characteristics.

First, the eyes are large, very dark, and prominent—hence the genus name of
Glaucomys.
This name ultimately stems from the Greek word
glaukos,
meaning bluish gray, probably because of the way the flying squirrel’s eyes reflect light. Second, both species have wonderfully fine, soft fur, far softer than the fur of their tree squirrel relatives, such as the red and the gray. And third, of course, both species glide whenever possible, and are much more at home in the trees than on the ground.

Despite these major similarities, there are a few noteworthy differences between the two species. Size is one. The northern flying squirrel can have a total length of as much as thirteen inches, although eleven inches or so would be more typical. Of this, the tail takes up roughly four to five inches, while the remainder is head and body. In total length, then, the northern flying squirrel is only about an inch shorter than the red squirrel, although it appears considerably smaller. That’s because the red is stockier and weighs from 24 to 50 percent more; the red weighs in at seven to eight ounces or more, while the northern flying squirrel only tips the scales at a distinctly lightweight four to six ounces.

The little southern flying squirrel is considerably smaller than its more northerly cousin. In fact, it’s the smallest of all our North American tree squirrels. With a total length of about eight to nine inches—three to four and a half inches of it being tail—this diminutive glider weighs only two to two and a half ounces. By way of comparison, this is about half of the northern flying squirrel’s weight, and only one-third of the red squirrel’s.

Even our northern flying squirrel is small, however, when compared to some of the giant Asian flying squirrels. One, the red giant flying squirrel
(Petaurista petaurista)
of Southeast Asia, has a combined head and body length of about sixteen inches, plus a tail slightly longer than that—a total length approaching three feet! So large is this squirrel that its thick pelt of mahogany-red hair is occasionally a commercial item. This squirrel can reputedly glide for very long distances, but
Walker’s Mammals of the World,
a highly respected source, only lists a known glide distance of about 225 feet.

Although the ranges of our two native flying squirrels show considerable overlap, their habits and habitat requirements vary considerably. The northern flying squirrel, as its name implies, inhabits much of Canada and Alaska. It also spills down into the United States through New England and New York and all the way along the spine of the Appalachians into Tennessee and a bit of western North Carolina; into the northern portions of the Great Lakes states; southward through the Rocky Mountains; and down the Pacific coast into a considerable piece of California.

In contrast, the southern flying squirrel is found in much of New England and nearly all the rest of the eastern United States as far west as the edge of the Great Plains. These ranges alone are a clue to the different habitat and food requirements of the two species. In the far north, where much of the northern flying squirrel’s habitat lies, forests are almost exclusively coniferous. Conversely, throughout much of the southern flying squirrel’s range, deciduous forests are very much the rule. Where their territories overlap, forests contain areas of conifers, mixed growth, and deciduous stands.

These differences in habitat have considerable implications as far as food is concerned. In much of the northern flying squirrel’s range, nut trees are absent. Although this squirrel devours nuts avidly when they’re available, it can make do without them. In summer, lichens and fungi make up much of its diet; in winter, the northern flying squirrel is known to raid the caches of red squirrels. In the latter instance, it seems doubtful that the red even realizes it’s being robbed, since it is sound asleep when the nocturnal robber pillages its hoard of cones. Arboreal lichens are also an important winter food source.

Berries, seeds, and fruit are also prominent in the northern flying squirrel’s diet. Although we don’t think of flying squirrels as carnivorous, like red and gray squirrels (see chapter 3) they eat meat whenever it’s available. Mostly, their meat supply consists of nestling birds (they consume the eggs, as well) and the young of mice, voles, and shrews.

Not unexpectedly, southern flying squirrels are more dependent on nuts, but they eat many of the same foods—fungi, seeds, fruit, and berries. They also consume a substantial amount of meat. In fact, the gentle appearance of this tiny glider is quite deceptive, and a number of experts regard it as the most carnivorous of all the tree squirrels. Nestlings and birds’ eggs, carrion, baby mice and voles, and even adult mice and shrews are killed and consumed by this little terror. Moreover, as it turns out, the southern flying squirrel is considerably more aggressive than its much larger northern cousin and generally is dominant wherever the two species inhabit the same tract of forest.

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