Hand FlyingOne 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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