Hand Flying
One of the more fascinating creatures on the earth is the
bat. Bats
make up a huge percentage of all mammals on the earth--nearly 25%. One
of the interesting characteristics of bats is that they actually fly
with their hands. In fact, the scientific name given to the order of
bats is Chiroptera which means "handwing." Some bats are as small as a
bumblebee (the hog-nosed bat), and some are as large as an
average-sized dog (the Malayan flying fox).
When you look carefully at a bat's wing, you find that it has five
fingers on the end of the wing. These are attached to a forearm like
our own that has two bones--a radius and an ulna--attached to an upper
arm bone (the humerus) just as ours is (see picture at left).
What is different
about the flight of bat is that it does not fly by
using a rigid wing as our airplanes do. The bones of a bat wing are
designed to be able to bend. The digits of a bat wing (see picture at
right) lack calcium toward the tips of the fingers. This gives the end
of the bat wing the ability to bend without breaking or splintering. In
addition to this, the bones are not circular, but flat. This is another
design feature that gives the bone the ability to bend. If you want to
see this, take a drinking straw and try to bend it. Then step on the
straw and try again.
Bats do not just bend their wings up and down. Bats actually row,
using the air to lift by cupping their wings and pushing against the
air in a very complex movement. The flexing of the bones provides the
extra lift needed to fly. In addition to the bones bending, bat skin is
a most unusual material. Instead of stretching the same amount in all
directions, bat skin stretches much more along the wing from the body
to the wing tip than it does from the front to the back of the wing.
This means the skin billows out and transmits lift force along the wing
generating lift. The skin is stretchy enough not to break any bones in
the wings, but still enough to give enormous lift.
These materials are engineered to an incredible degree. The wing
bones only make up 5% of the animal's weight. If they weighed twice as
much, the weight would be too much for safe flight to happen. Man has
done a fair job of copying the flight of birds, but we have a very long
way to go to emulate the flight of the bat.
--Reference: Natural History,
February, 2003, pages 30-31
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