Long Live Flying Squirrels!
(10/13/2000)
I was told by more than one biologist that the easiest way to see flying
squirrels is to go to a forest with oak trees and rap on the trunks of trees
that have holes. The squirrels will come right out to see what's up, they said.
You may even get to see them glide away to another tree if you're lucky, they
said. Well, I rapped on dozens of trees on several occasions in several suitable
forests with not one squirrel appearance. Either the squirrels had advance
warning of my rapping expeditions, or there is a room full of Candid Camera
biologists somewhere laughing hysterically.
I guess I shouldn't be too surprised that I have yet to see a wild flying
squirrel. My interest in them was piqued when I read that in spite of the fact
that flying squirrels are quite common, few people ever see them. The Southern
flying squirrel, the most common of the two North American species, is probably
at least  as
populous as the ubiquitous gray squirrel and the Eastern chipmunk. Some wildlife
biologists think the Southern flying squirrel is the most common squirrel on the
continent. (The Northern Flying Squirrel, however, is becoming more rare,
and in some states is considered to be threatened.)
The reason no one ever sees our two flying squirrel species is that they are
the only nocturnal squirrels in North America. Their other distinction, is, of
course, that they "fly." They don't really fly, though; what they do is more
accurately described as gliding. When a flying squirrel wants to get somewhere
in a hurry, or more commonly, when it wants to get away from somewhere in a
hurry, it launches itself from a tree, spreads its legs and glides on its "patagium,"
the folds of skin that stretch between its legs. There are claims that a
squirrel can glide a hundred yards this way, but Gary Heidt, professor of
biology at the University of Arkansas at Little Rock, who has studied flying
squirrels for more than 25 years, says that 50-60 yards is a more typical long
glide, and most glides are much shorter than that. "They have memorized escape
routes that usually lead to another hole in a nearby tree," Dr. Heidt says.
"When the squirrels jump out of their nest, they know exactly where they are
going and once they land, they scurry up or down the trunk pretty much like any
other squirrel."
There is a tendency to view an animal like a flying squirrel as somehow
halfway between -- in this case, halfway between a strictly arboreal animal,
such as a red squirrel, and something that flies, such as a bat. With a few more
million years of evolution, the thinking goes, the descendants of today's
gliding squirrels will fly. That might be true, but not necessarily. Gliding
doesn't have to be halfway to anything. It's an effective way of life just as it
is.
There are great costs involved in flying, after all. The bodies of birds and
bats, for example, are highly modified to accommodate flying, making them
generally less apt for things like climbing or digging. Flying is extremely
energetic, and animals that fly must constantly stoke their high metabolisms
with food. There are also reproductive difficulties, especially for a flying
mammal. Nearly all bat species have only one offspring at a time – most likely
because it's hard enough to fly while carrying one developing fetus, never mind
two or three.
Gliders get a large part of the benefit of flying, the increased mobility and
the ability to escape predators, without the high cost of true flight. The
metabolism of a flying squirrel is not much higher than that of a non-gliding
squirrel. Flying squirrels, because they aren't as limited by the weight rules
of true flight, are able to have three or four offspring at a time. Gliding is
such an effective way to solve the problems of survival that it has arisen in
nature a great many times. A National Geographic Magazine feature on the gliding
animals of Borneo showed two different families of lizards, gliding frogs, a
species of flying squirrel, a mammal called a colugo, and, believe it or not, a
gliding snake.
Peter Zahler, a University of Massachusetts graduate student who studies the
four-foot-long woolly flying squirrel of Pakistan, explains that gliding in
mammals seems to have arisen in four unrelated groups. There is a family of
gliding marsupials, of which the sugar glider of Australia is the most well
known. There is the lemur-like colugo. There is a family of gliding rodents in
Africa. And then there are the gliding squirrels. There are more than 40 species
of gliding squirrels, most of them found in Asia. We have two in North America,
the Southern flying squirrel and the Northern flying squirrel. The Southern
flying squirrel that nests in tree holes from which it is rumored to readily
emerge when disturbed, is a fairly small animal. They weigh about 2.5 oz., a
little smaller than a chipmunk, and less than half the weight of a red squirrel.
Southern flying squirrels will eat just about anything, but most of their diet
is acorns and nuts. They have silky fur and huge eyes that facilitate their
nocturnal existence, and also make them adorable. Tom French, from the
Massachusetts Division of Fisheries and Wildlife, told me, "Nothing has bigger, browner eyes with the possible exception of a baby seal.
Southern flying squirrels just look lovable." This is unusual adoration from a
hardcore wildlife management biologist. But French is also one of those who told
me to rap on tree trunks, so maybe I should accept his assertion with caution.
In the eastern U.S. down into Florida, the flying squirrel you are most
likely to see (or not see) is the Southern flying squirrel. When people
encounter them, it is most often in an attic in winter, where the squirrels
congregate for warmth. Because our flying squirrel is a very social animal, a
dozen or more may live in a single attic. And once flying squirrels get
interested in your attic, they can be there a long time, because they can live a
relatively long time. Flying squirrels in captivity have lived as long as 17
years. This is a very long lifespan for such a small rodent. Chipmunks, for
example, only live to be about eight in captivity, and red squirrels about ten.
Why do flying squirrels live longer than similar rodents? Steve Austad, a
biologist at the University of Idaho, thinks it's because of gliding. Dr. Austad
studies the evolutionary question of aging. Why do animals, including us, age?
It seems reasonable to expect that natural selection would favor animals that
maintain their health and reproductive capability as long as they live. The
Evolutionary Senescence Theory suggests that the health and reproductive ability
of animals decreases as they grow older because the force of natural selection
grows weaker with increasing age. The simplest way to understand this is to
realize that most animals get eaten, or die of other causes, long before they
get a chance to grow old. If a species has genes that would allow it to remain
youthful a long time, those genes are largely wasted, because so few individuals
survive to old age. If, on the other hand, the genes are coded to put all
available resources into breeding as early and often as possible, that benefit
is much more likely to be useful. This helps to explain the general correlation
between long life and size. Larger species generally live longer because, for
the most part, larger animals suffer less predation. They have more chance to be
alive to reproduce later in life, and natural selection will favor those with a
genetic predisposition to stay healthy despite the passage of time.
The theory can also explain the exceptions to the rule that connects longer
lifespan and larger size. Our tiny flying squirrels, for example, live almost
twice as long as larger chipmunks and red squirrels. Dr. Austad suggests that
because gliding lowers the predation rate for flying squirrels, enough of them
survive long enough to benefit from genes that might lead to slower aging. His
data shows that gliding animals in general age more slowly than similar sized
non-gliding animals.
So maybe the reason I didn't get to see any flying squirrels when I rapped on
tree trunks is that all the squirrels are wily old-timers who have experienced
far too many biologists rapping on their trees. Maybe, after all these years,
they have learned to sit tight and save their gliding for real danger. |
|
