In the animal kingdom, bats are classed as mammals. Like all mammals, bats nurse their young on milk. Like most mammals, they have hair and bear living young. But in one, way bats are different from all other mammals: bats can fly.
There are mammals known as flying squirrels and flying lemurs, but they do not truly fly. Rather they glide from tree to tree. Bats are the only mammals that move through the air with wings.
A bat's wing is not like a bird's wing. While the bird's wing is formed chiefly of feathers, the bat's wing is a double layer of skin stretched over the thin bones of its arm and fingers. A bat's skeleton is the framework for these wings. The arm extends from a shoulder socket, bends at the elbow, and ends with long, slender fingers. The fingers are almost as long as the rest of the body. They support the main part of the wing and are covered with skin.
The wing covers all fingers except a short thumb, which is left free. A sharp claw on the end of the thumb forms a hook at the top of the wing. When its wings are folded, the bat uses its hooks to climb tree trunks, rocky walls, and other rough surfaces.
The skin connecting the webbed fingers is also attached to the bat's clawed feet. This makes the back part of the wings. Most bats have an extra flap of skin connecting their feet. While flying, many can fold this flap into a pocket for catching insects.
Since its leg bones and leg muscles are included in its wings, a bat can fly more easily than it can walk. But the feet are far from useless. Like human hands, a bat's feet can turn inward, which enables them to grasp objects like twigs and branches. Sharp claws hook securely into cracks or around bumps in the wall or ceiling of a cave. These claws are so strong that they support the bat's whole weight, even during sleep.
When resting, a bat can use its claws to cling to a wall or tree trunk — or it may hang upside down, suspended by its feet. Bats hang upside down because it is easier for them than perching upright.
Bats may be less than 1 inch (2.5 centimeters) in length or as long as 15 inches (38 centimeters). Their wing spread can be as wide as 5 feet (1.5 meters). Bats have fur that may be white, red, brown, gray, or black.
How Bats Live
There are nearly a thousand different species, or kinds, of bats. They live in almost every part of the world except the polar regions. Each kind of bat has different habits, though the various species also have many things in common.
Bats are usually social animals. That is, they live in groups. Often they can be found living in caves. If you see one bat flying into a cave, you can be reasonably sure there are other bats inside. In some caves thousands of bats crowd together on walls or ceilings. Smaller bat colonies numbering only ten or twelve bats may live in a hollow tree.
Caves and hollow trees are not the only places where bats live. Some bats simply roost in trees, hanging like leaves from twigs and branches. Two kinds of tropical bats make tents from palm leaves. Such a bat slits the leaf with its teeth, then hangs inside the folds.
Bats live in the pyramids of Egypt and in the fruit trees of Australia. In North America and Europe people sometimes share a house with bats and never know they are there. A bat can squeeze through narrow cracks and roost between layers of wall and ceiling.
Most bats are nocturnal. This means that they are active only at night. They sleep in the daytime and come out at night to find food. Only a few kinds of bats venture out in bright sunlight. Bats are probably night creatures for the same reasons that most small mammals are. A small animal is in less danger at night. In the daytime it is in constant danger of being eaten by larger animals that sleep at night. Also, at night bats can catch insects with less competition from birds.
The Search for Food
Most bats live on insects alone. Some eat only fruit. Some eat both insects and fruit. A few kinds of bats eat other things — meat, fish, and even flower nectar.
In Canada and the United States, the most familiar bats are insect eaters, though there are nectar-feeding bats in Arizona and California. Probably the best-known fruit bats are the huge flying foxes. In Australia these giant bats have become a serious nuisance to fruit growers. They swarm over the orchards, devouring fruit at night and roosting in the trees by day.
In India one kind of bat has been seen eating mice, birds, and lizards. When captured, the large spear-nosed bats of tropical America will eat almost anything. They have been fed bananas, horsemeat, liver, and hamburger. They will even eat smaller bats.
The bats with the most unusual diets are found in the tropics. Noctilio bats of South and Middle America eat fish. They skim over a pond or lake, dragging their sharp claws through the water to catch small fish swimming near the surface. Another group of jungle bats, the tiny hummingbird bats, eat chiefly the pollen and nectar of flowers.
Probably the most famous tropical bats are the vampires, found only in South and Middle America. The vampire bat has inspired legends, superstitions, and horror tales — all of them false. A vampire bat does bite other animals and drinks their blood. But a vampire bat may bite a sleeping horse, cow, or goat — or even a person — without being noticed. Its sharp teeth make a shallow cut. Then the bat simply laps up a small amount of blood and flies away. The chief danger to the victim is not loss of blood but rather infection. Vampire bats — as well as several other species — are known carriers of rabies.
Migration and Hibernation
Bats cannot survive in extremely cold weather. So some fly to warmer climates for the winter. When spring comes, these bats return to a favorite roost. They may make long flights, flying as far as 800 miles (1,287 kilometers) during a migration.
Instead of migrating, other bats avoid winter conditions by hibernating deep in caves, where the temperature changes very little from season to season. For many weeks hibernating bats hang head downward, sometimes packed together in thick clusters. They each have an extra layer of body fat that provides the fuel they need to keep alive until spring.
Birth of Young
Bats mate in the fall, before hibernating or flying away for the winter. When the weather begins to warm up, female bats gather in the roosts that will become nurseries. In late spring or early summer, the baby bats are born. Most mother bats have just one baby at a time. (Some have up to four, but this is as rare as twins in human families.)
When a baby bat is born, the mother forms a living cradle by hanging belly-up from the ceiling or a branch. She may hang by the hooks at the top of her wings as well as by her feet. The newborn bat rests in this cradle. The baby bat is able to hang on to its mother's fur, using its own sharp teeth and claws.
Young bats grow so rapidly that a 10-day-old bat can be too heavy for its mother to carry. Within a month after birth, the baby bat has grown to its full size. A bat born in June is flying on nightly hunting trips by August. It may live as long as 10 to 14 years, a remarkably long lifetime for a small mammal.
Ancestors of today's bats were flying about the Earth at least 50 million years ago. Since that time some bats have changed very little. One 40-million-year-old bat fossil found in Europe looks very much like the skeleton of a modern bat.
Some scientists think that the first bats may have evolved from a tree-climbing mammal that could leap and glide after insects. Over millions of years the limbs of some gliding animals may have developed into wings. These winged animals would have been the first bats. Scientists are still looking for fossils that would prove this theory.
Finding Their Way
For centuries, people who studied bats wondered how they find their way in the dark. Many people thought that bats had unusually keen eyesight and could see by light too faint for human eyes to detect. Scientists now know that a bat's ability to navigate depends not on its eyes but on its ears and vocal organs.
The first steps toward understanding how bats navigate were taken in the 1780's. An Italian zoologist named Lazzaro Spallanzani suspected that bats could not see in the dark. To find out, he blinded some bats and released them into a room crisscrossed with silk threads. The bats flew through the maze without touching the threads. Then he tried plugging their ears with wax. The animals blundered about, flapping their wings helplessly and becoming entangled in the threads.
In 1920, Cambridge University professor H. Hartridge suggested that bats sent out signals that were beyond the range of human hearing. (Such sounds are called ultrasonic.) He thought bats might use the echoes of these signals to navigate in the dark. But Hartridge could not prove his theory because he had no way of listening to ultrasonic sounds.
In 1941 two scientists in the United States proved that Spallanzani and Hartridge had been on the right track. Donald R. Griffin and Robert Galambos of Harvard University placed bats in front of a new electronic instrument that could detect ultrasonic sounds. The men could hear no sounds, but patterns on a screen showed that the bats were uttering high-pitched cries.
Griffin and Galambos strung a room with a network of wire and repeated some of Spallanzani's experiments. They added their own modern equipment — microphones and recording devices. Patterns on the electronic screen showed that the bats were constantly squeaking as they flew successfully through the maze of wires.
A bat sends out signals — high-pitched squeaks — that bounce off anything in its path. A sound that is bounced back, or reflected, is called an echo. The bat uses echoes to locate things in the dark. So scientists call this system echolocation. It is often compared to the radar and sonar systems used by people, which also use reflected signals to locate objects. But radar and sonar are newcomers. Bats have been using echolocation for millions of years.