Bats After Dark

Wings and Echoes

A bat is a mammal, which means it has fur, breathes air, and feeds its babies milk, just like a dog or a person does. But it is also the only mammal in the world that can truly fly. Flying squirrels can only glide from tree to tree, while a bat beats its wings, steers, and turns in the air. There are more than 1,400 kinds of bats, and they live on every continent except Antarctica. That is about one out of every five kinds of mammal on Earth, which makes bats one of the largest and most successful groups of animals on the planet.

The scientific name for bats, Chiroptera, comes from Greek words that mean hand wing, and the name is exactly right. A bat's wing is built like a very stretched hand. It has an arm, a wrist, a short thumb with a small claw, and four extremely long finger bones. A thin, stretchy skin runs between the fingers like the cloth of an umbrella. Because the finger bones can bend, a bat can change the shape of its wing in the middle of a single flap, and that flexibility lets it twist and swerve sharply to capture a tiny insect, which is why bats are such skillful fliers.

Most bats navigate in complete darkness by using sound. A bat sends out very high-pitched squeaks, far too high for people to hear, and then listens for the echoes that bounce back from the things around it. The echo tells the bat how far away an object is, how big it is, and even which way it is moving. This skill is called echolocation. Some bats make their calls through the mouth, and others through the nose, which is why a few kinds have strange, leaf-shaped noses. With echoes alone, a bat can catch an insect as small as a mosquito, even in total darkness.

You may have heard the saying blind as a bat, but bats are not blind. Every kind of bat can see, and many see about as well as a person does in dim light. The big fruit bats of Africa, Asia, and Australia have large eyes and use their eyesight and sense of smell to find ripe fruit, and most of them do not use echolocation the way smaller bats do. So the saying is not an accurate description of any bat. Bats simply have several different senses, and they use whichever combination works best for the situation at hand, whether it is day or night.

Roosts and Families

Bats include some of the smallest and some of the largest mammals that fly. The bumblebee bat of Thailand and Myanmar is about the size of a large bumblebee and weighs approximately two grams, which is less than a penny. At the other end, the giant golden-crowned flying fox of the Philippines can have a wingspan of about 1.7 meters, which is nearly as wide as an average adult is tall. Flying foxes are called that because their faces look a little like a fox's face, with a long nose and pointed ears, but they eat fruit and flower nectar and are completely harmless to people.

Bats rest hanging upside down, and they do it without trying. The tendons in a bat's feet work like a lock, so when its body weight pulls down, the toes close tightly around the perch and hold on by themselves. A bat can even sleep this way without using any muscle strength at all. The upside-down position also makes takeoff effortless, because the bat only has to let go and spread its wings. Many bats sleep most of the day, and some sleep for about twenty hours out of every twenty-four, waking up at dusk to go out and feed.

In central Texas there is a place called Bracken Cave, and each summer it holds about fifteen million Mexican free-tailed bats. It is the largest known gathering of mammals anywhere on Earth. At sunset the bats pour out of the cave in a swirling dark cloud that can take hours to leave, and the cloud is so large that weather radar can pick it up in the sky. Together the bats eat many tons of insects every night, including several kinds of moths that damage farmers' crops. When the mothers come back at dawn, each one finds her own baby among millions by its voice and smell.

A mother bat usually has just one baby each year. The baby is called a pup, and it is born without any ability to fly, so it clings to its mother's fur or waits for her in a nursery roost. After a few weeks it learns to flap and glide, and then to fly and hunt for itself. Bats live remarkably long for such small animals. Many kinds live for more than ten years, and some live for more than thirty. One Brandt's bat in Siberia was recorded at forty-one years old, which is an extraordinary lifespan for an animal that can weigh less than a coin.

Bats at Work

Many bats do not eat insects at all. Some feed on the nectar of flowers, and others eat fruit. Flower bats are extremely important to many plants because they carry pollen on their fur from one flower to the next. Plants such as wild bananas, durians, and agave are pollinated by bats, and flowers that depend on bats often open at night and have a pale color and a strong smell. Fruit bats help forests grow in another way. They carry fruit away from the tree as they fly and drop the seeds far from the parent tree, which helps new forests regrow in places where the forest has been cut down.

In cold places the insects disappear in winter, so many bats go into a deep sleep called hibernation. A hibernating bat's body temperature falls close to the temperature of the cave around it. Its heartbeat slows from hundreds of beats a minute while flying to only a few beats a minute, and it can go for months without eating, living off the fat it stored in the fall. Some bats solve the problem differently and migrate instead, flying hundreds of miles to a warmer place, the way many birds do. Mexican free-tailed bats from Texas fly south into Mexico for the winter.

Only three of the more than 1,400 kinds of bats drink blood, and they live in Mexico and in Central and South America, where they feed mostly on cattle and other farm animals. Vampire bats are surprisingly cooperative animals. A vampire bat that has fed well will often share some of its meal with a roost mate that went hungry, and bats that have been helped in the past are the ones most likely to be helped again. In this way, a roost of these small animals works as a team, which is a behavior that scientists did not expect to discover in any bat.

You can find bats near you on a warm evening in summer. Go outside at dusk, near a pond, a streetlight, or the edge of some trees, and watch the sky for small dark shapes that flicker and swerve. A bat flies in quick zigzags as it follows insects, while a bird usually flies in smoother curves. Right now, those bats are sending out squeaks that you cannot hear, and listening to the echoes that come back, and constructing a detailed picture of the night out of sound. That is the most amazing fact of all, and it is happening right above your head.