The World's Only Flying Mammals
Did you know that there is only one mammal in the world that can fly? Many people think that flying squirrels, flying possums and colugos (flying lemurs, which actually aren't even lemurs) can fly. But, the can't. They can only glide for limited distances. The only mammal that is actually capable of powered flight is the bat.
All bats belong to the order Chiroptera. The word Chiroptera comes from the Greek words cheir meaning "hand" and pteron, which means "wing" because the structure of the open wing is very similar to an outspread human hand. But, the bones of the arm and four of the fingers are light, slender and lengthened as an adaptation for flight. They are used to support, spread and manipulate the membrane. The only digit not attached to the wing is the thumb. It remains free for clinging to various surfaces. The knees on the hind limbs bend backward, but the feet face forward.
The bat's wing is an extension of the skin on its body. It is two tightly stretched layers of skin membranes connected together by connective tissue without any flesh between the layers. This is why a bat's wings are translucent, which means you can see through them. This formation of skin membranes is called the patagium. Bat wings usually run from the shoulder region to the ankle, or in some cases, to the toes themselves. The wing membrane joins the body along the sides, except in a few cases in which it arises near the middle of the back. When the wing is not extended the membrane folds up along countless creases more efficiently than an umbrella.
Bat biologists use different names to refer to different parts of the patagium. Here is a simplified illustration of a bat's wing. Roll your mouse over the image below to see the different parts of the membrane with descriptions.
Wing and tail membranes appear naked on most bats. However, when you look closely you'll see that they are covered with tiny little hairs. Some species have distinctive tufts and fringes of hairs. A few species have a thick layer of fur on their tail membranes and some have thick hair on parts of their wings.
While flapping their wings, bats move upward and downward through the air by moving the membrane between the body and fifth finger. This is called "lift." Bats move forward (called "thrust") by changing the shape of the membrane between the second and fifth fingers. With their flexible wings, bats can change the degree and direction of lift and thrust very quickly by positioning their wings into different shapes. They can also fold one of their wings to steer and brake. This is how they are able to weave and dive in the air like no other animal can.
The rigid bird wing is more efficient at providing lift, but the flexible bat wing allows for greater maneuverability. Bats fly by using their hands and wings in a "rowing motion" through the air. Many bats may fold their wings on the upstroke. This helps to reduce air resistance. Both thrust and lift occur during the down stroke.
Another way bat's wings are different than bird's wings is the fact that most of the flight muscles controlling the wing beat are attached to the shoulder blades. With birds, the muscles are braced against the rib cage, and they typically have a prominent breast keel where heavy muscles are attached. Bats only have one muscle attached to the breatbone, the ribs are flattened, and there is some fusing of the vertebral joints. These additional adaptations help make the bat incredibly agile in the air.
The picture below gives you an inside view of how bats extend and fold their wings. You'll also see the heavy muscles attached to the shoulder blades. Click the left arrow to fold the wing and the right one to extend it.
Can bats take off from the ground?
Bats with longer, narrower wings cannot take off from the ground. They'll take off by dropping from a hanging position. But some bats, like some species of horseshoe bat and the common vampire bat, can take off from the ground. Most bats land by slowing down until they stall and grabbing hold of a branch or other surface. Common vampire bats land on the ground, so they can sneak up on their prey. Many species of flower-visiting bats and some species that take animal prey from the ground can hover.
Bats can also move around on the ground.
The first digit of the bat wing (similar to our thumb) is small and clawed. Bats use it to climb or to walk on the ground. Several species of bat are known to use their strong forelimbs and weak hind legs to crawl along the ground. Some insectivorous species feed on ground-dwelling insects as much or more than they feed on insects captured in flight. Common vampire bats also get around quite well on the ground.
Cornell University biologist Daniel Riskin dropped a common vampire bat onto a treadmill. He was quite stunned when it broke into a bounding run. Common vampire bats of Central and South America can run up to about 2.5 miles per hour. New Zealand common vampire bats can get around pretty well on the ground, but they can't run. Some bats can also swim. There have been photos of the flying fox actually swimming, using its wings and feet to reach land rather than floating or paddling.
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