How Bats Work

The German word for bats is "Fledermäuse," which translates as "flying mice." It's easy to see where this name came from -- many bat species do look a lot like flying rodents. But in fact, bats are more closely related to humans than they are to mice and rats. If you look closely at a bat's wings, you can see the resemblance.

The illustration above shows the bone structure in a bat's wing, a bird's wing and a human arm. The bird's wing has a fairly rigid bone structure, and the main flying muscles move the bones at the point where the wing connects to the body. A bat has a much more flexible wing structure. It is very much like a human arm and hand, except it has a thin membrane of skin (called the patagium) extending between the "hand" and the body, and between each finger bone. Bats can move the wing like a hand, essentially "swimming" through the air. The "thumb" extends out of the wing as a small claw, which bats use to climb up trees and other structures. This helps them reach a high "launching point" for flight takeoff. Appropriately, the order of bats is called Chiroptera, Greek for "hand-wing."

Scientists believe that bats evolved from a non-flying mammal that lived mostly in the trees, roughly a hundred million years ago. Like a lemur or squirrel, this animal would leap through the air from branch to branch. Some individual members of this species were born with more skin extending between their arms and body, which gave them just a little bit more lift as they leaped through the air (some modern lemurs and squirrels have developed this same sort of physiology). The individual bats with this mutation had slightly greater mobility than others in the species and so were more likely to thrive and reproduce. In this way, nature selected for wider and wider skin membranes over time, eventually leading to fully functional wings.

The rigid bird wing is more efficient at providing lift, but the flexible bat wing allows for greater maneuverability. Bats can position their wings into different shapes, changing the degree and direction of lift very quickly. This lets them weave and dive in the air like no other animal, giving them a distinct advantage in hunting prey.

In the next section, we'll learn how bats get around in the dark.

In the last section, we saw that the unique wing structure of bats gives them a great deal of flight maneuverability. This is crucial to a bat's survival, as their main prey are small, quick-moving insects. The task of hunting is made even more difficult for bats because they are only active at night, dusk and dawn. Bats have adapted to this lifestyle to avoid the fierce flying predators that are active in the daytime, and also to take advantage of the abundance of insect species that are active at night.

To help them find their prey in the dark, most bat species have developed a remarkable navigation system called echolocation. To understand how echolocation works, imagine an "echo canyon." If you stand on the edge of a canyon and shout "hello," you'll hear your own voice coming back to you an instant later.

The process that makes this happen is pretty simple. You produced sound by rushing air from your lungs past your vibrating vocal chords. These vibrations caused fluctuations in the rushing air, which formed a sound wave. A sound wave is just a moving pattern of fluctuations in air pressure. The changing air pressure pushes surrounding air particles out and then pulls them back in. These particles then push and pull the particles next to them, passing on the energy and pattern of the sound. In this way, sound can travel long distances through the air. The pitch and tone of the sound are determined by the frequency of the air-pressure fluctuations, which is determined by the way you move your vocal chords.

When you shout, you produce a sound wave that travels across the canyon. The rock face on the opposite side of the canyon deflects the air-pressure energy of the sound wave so that it begins moving in the opposite direction, heading back to you. In an area where atmospheric air pressure and air composition is constant, sound waves always move at the same speed. If you knew the speed of sound in the area, and you had a very precise stopwatch, you could use sound to determine the distance across the canyon.

Let's say you're at sea level, and the air is relatively dry. In these conditions, sound waves travel at 741.1 miles per hour (1,193 kph), or 0.2 miles per second (0.32 kps). To figure out the distance across the canyon, you would clock the time between when you first started shouting and when you first heard your echo. Let's say this took exactly 3 seconds. If the sound wave were moving at 0.2 miles per second for 3 seconds, it would have travelled 0.6 miles (0.97 km). This is the distance of the total trip, across the canyon and back. Dividing the total by two, you get 0.3 miles (0.48 km) as the one-way distance.

This is the basic principle of echolocation. Bats make sounds the same way we do, by moving air past their vibrating vocal chords. Some bats emit the sounds from their mouth, which they hold open as they fly. Others emit sound through their nose. It's not fully understood how the bat's sound production works, but scientists believe that the strange nose structure found in some bats serves to focus the noise for more accurate pin-pointing of insects and other prey.

In the case of most bats, the echolocation sound has an extremely high pitch -- so high that it is beyond the human hearing range. But the sound behaves the same way as the sound of your shout. It travels through the air as a wave, and the energy of this wave bounces off any object it comes across. A bat emits a sound wave and listens carefully to the echoes that return to it. The bat's brain processes the returning information the same way we processed our shouting sound using a stopwatch and calculator. By determining how long it takes a noise to return, the bat's brain figures out how far away an object is.

The bat can also determine where the object is, how big it is and in what direction it is moving. The bat can tell if an insect is to the right or left by comparing when the sound reaches its right ear to when the sound reaches its left ear: If the sound of the echo reaches the right ear before it reaches the left ear, the insect is obviously to the right. The bat's ears have a complex collection of folds that help it determine an insect's vertical position. Echoes coming from below will hit the folds of the outer ear at a different point than sounds coming from above, and so will sound different when they reach the bat's inner ear.

A bat can tell how big an insect is based on the intensity of the echo. A smaller object will reflect less of the sound wave, and so will produce a less intense echo. The­ bat can sense in which direction the insect is moving based on the pitch of the echo. If the insect is moving away from the bat, the returning echo will have a lower pitch than the original sound, while the echo from an insect moving toward the bat will have a higher pitch. This difference is due to the Doppler effect, which you can read about in How Radar Works.

A bat processes all of this information unconsciously, the same way we process the visual and aural information we gather with our eyes and ears. A bat forms an echolocation image in its head that is something like the image you form in your head based on visual information. Bats also process visual information -- contrary to popular belief, most bats have fairly acute vision. They use echolocation in conjunction with vision, not instead of it.

In the next section, we'll look at the other part of a bat's life, the things they do during the daytime. As we'll see, a bat's daytime life couldn't be more different from its night life, but it is just as phenomenal.

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I­n the last couple of sections, we looked at the unique adaptations that help bats zip around, catching bugs all night. For the other half of the day, during daylight hours, bats lead a completely different life. A bat will pass the time hanging upside down from a secluded spot, such as the roof of a cave, the underside of a bridge or the inside of a hollowed-out tree.

There are a couple of different reasons why bats roost this way. First of all, it puts them in an ideal position for takeoff. Unlike birds, bats can't launch themselves into the air from the ground. Their wings don't produce enough lift to take off from a dead stop, and their hind legs are so small and underdeveloped that they can't run to build up the necessary takeoff speed. Instead, they use their front claws to climb to a high spot, and then fall into flight. By sleeping upside down in a high location, they are all set to launch if they need to escape the roost.

Hanging upside down is also a great way to hide from danger. During the hours when most predators are active (particularly birds of prey), bats congregate where few animals would think to look and most can't reach. Effectively, this allows them to disappear from the world until night comes again. There's also little competition for these roosting spots, as other flying animals don't have the ability to hang upside down.

Bats have a special physiological adaptation that enables them to hang around this way. For you to clench your fist around an object, you contract several muscles in your arm, which are connected to your fingers by tendons; as one muscle contracts, it pulls a tendon, which pulls one of your fingers closed. A bat's talons close in the same way, except that their tendons are connected only to the upper body, not to a muscle.

To hang upside down, a bat flies into position, opens its claws and finds a surface to grip. To get the talons to grab hold of the surface, the bat simply lets its body relax. The weight of the upper body pulls down on the tendons connected to the talons, causing them to clench. Since it is gravity that keeps the talons closed, instead of a contracted muscle, the bat doesn't have to exert any energy to hang upside down. In fact, a bat will continue to hang upside down if it dies in that position. To release the surface it is gripping, the bat flexes other muscles that pull its talons open.

Most bat species will roost in the same location every night, joining a large colony of bats that cluster together for warmth and security. Bats have been known to demonstrate remarkable acts of altruism to support the colony. In some cases, when a bat is ill and cannot hunt for its own food, other bats from the colony will bring food back to it. Scientists don't fully understand the dynamics of bat colonies, but they are clearly complex, tight-knit social communities.

Like all mammals, bats are warm-blooded, meaning they maintain their body temperature internally. But unlike most mammals, bats allow their body temperature to sink to the ambient temperature when they are not active. As their temperature drops, they enter a torpor state, in which their metabolism slows down. By reducing their biological activity and not maintaining a warm body temperature, bats conserve energy. This is important, as flying all night is extremely hard work.

During the winter, when temperatures are cold for months at a time, some bats will enter a deeper torpor state called hibernation. This allows them to live through the months in which food is very scarce. Other bat species follow a yearly migration pattern, traveling to cooler climates in the warm months and warmer climates in the cool months. This is why some regions experience "bat seasons" every year.

When bats do come to town, a lot of people are made uneasy in the evening and at night. They worry about bats biting, sucking blood and even getting caught up in people's hair. But as it turns out, all of these occurrences are extremely rare. As we'll see in the next section, bats are usually harmless to people, and many species are actually beneficial.

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Many people have a negative reaction to bats, and it's easy to see why. Just by virtue of their appearance and behavior, bats play into a number of human fears. First of all, they only come out at night, a time that is full of danger and mystery for humans. Additionally, their leathery wings and odd facial structures coincidentally resemble the grimaces of mythological ghouls and demons.

People are also wary of bats because of the vampire legend. Vampires are a mixture of fact and fiction. There are in fact a few species of vampire bats, and they do feed on blood, but they are not bloodthirsty man-hunters. Vampire bats merely prick an animal or human and lap up the blood that flows out. A powerful anticoagulant in the bat's saliva keeps the blood from clotting, so it will come out in a trickle that the bat can drink from. Vampire bats only need about two tablespoons of blood per day to survive, so they never consume enough to kill their prey, which is generally limited to large animals such as cows and, occasionally, people.

Vampire bats can be dangerous, however, because they sometimes carry rabies and can pass it on to their host. Vampire bats are only found in South America and Central America, and even there the risk to humans is minimal. You are much more likely to die from a bee sting or dog attack than from a vampire-bat bite.

Most bat species are not only harmless to humans, but actually beneficial. Insectivorous bats are far and away the best bug-killers on the planet. The little brown bat, one of the most common North American bat species, can catch and eat as many as 1,200 mosquitoes in one hour. The famous colony of Mexican free-tail bats that lives underneath the Congress Avenue Bridge in Austin, Texas, will eat up to 30,000 pounds of insects in a single night. A Mexican free-tail colony in Bracken Cave, Texas, containing more than 20 million bats, will eat roughly 200 tons of insects in a night. These bats, and many other species, feed on insects that destroy crops, providing an invaluable service to farmers.

When there is an outbreak of rabies in an area, people often take extreme and ill-informed measures. In Central America, where vampire bats can be a problem, locals find bat caves and blow them up, killing entire colonies. But the bats that are easiest to find are the beneficial ones -- vampire bats roost in small groups and conceal themselves very well. Considering that just one of these harmless colonies might contain millions of insect-eating bats, this sort of destruction is a devastating loss to the environment.

Bats are also beneficial as plant pollinators. Many species, particularly in the tropical rainforest, feed on plant nectar, gathering pollen on their bodies as they feed. When they fly away, they spread the pollen, helping the plant disperse its seed. Bats are major pollinators of many plants used by humans, including bananas, figs, mangoes, cashews and agave, which is used to make tequila.

One of the stranger ways in which bats help us out is with their bodily waste. Bat feces, called guano, is rich in nitrogen, making it a powerful plant fertilizer. In the past, people also used this nitrogen to make explosives. More recently, scientists have discovered that a number of enzymes found in bat guano work well as cleaning agents in laundry detergent and other products.

Bats are extremely susceptible to extinction because of their reproductive habits. Most bat species give birth to only one baby per year, so they multiply at a relatively slow pace. Since bats have a fairly long life span (as long as 30 years in some species), the loss of one female bat has a major effect on the rate of reproduction.

Bats are some of the most amazing animals on Earth. They are so well adapted to their environment that they have survived as a group for more than 50 million years, longer than most other modern animals. To learn more about bats, including bat research and bat preservation, check out some of the links on the next page.