|Bats see the world in “sound pictures.” From the
echoes of the complex, high frequency sounds they produce, they can tell
the size, shape, distance, and even texture of objects around them. For many
years, it was assumed that exceptionally keen sight allowed bats to fly and
hunt in total darkness - a feat even sharp-eyed owls cannot match. It is
now known that most bats have a highly sophisticated method of using sound
instead of light to perceive the world around them – a method that is just
as effective as sight.
A Mystery Unravelled: For centuries scientists did not know how bats could fly at night between finger sized tree branches and accurately catch tiny flying insects. They originally thought that the bat, like other nocturnal animals, had super sensitive eyesight. Late eighteenth-century Italian scientist Lazzaro Spallanzani found that owls could not fly in a completely dark room while bats could fly normally. He discovered that blinded bats could fly and capture prey, but those fitted with ear plugs were grounded.
Later, after the Titanic hit an iceberg in 1912, machine gun inventor Hiram Maxim began investigating sonar (the use of sound for navigation and ranging). Maxim believed that bats used sonar, but mistakenly thought they used low frequency sounds generated by wing movements. Not until the 1930s, when G.W. Pierce developed a high frequency sound detector, were bat sounds first heard.
Seeing with Sound: Bats build up an image of the world from reflected sound waves, or echoes, much as humans use reflected waves of light to see. Humans depend on external light sources such as the sun, but the bat’s sound pictures come from the echoes of calls that it produces itself. To “see” in the dark, bats produce a short series of high frequency sound pulses that spread out into a cone of sound like light from a flashlight beam. When the sound hits an object, an echo bounces back to the bat. This is called echolocation.
Bats use high frequency sound because it has a shorter wavelength that can detect small insects better. Low frequency sounds of long wavelength wash around small objects and do not send back an echo. Bats can distinguish high frequency sounds out of background noise, which is mostly at low frequency. By using sounds pitched higher than most other animals can hear, the bat can hunt without being detected by its prey or by other predators.
The Nature of Sound: Each bat call has a mixture of one or more frequencies and up to five variations of tone. The call may sweep through a frequency range or be at a constant frequency. It may last from 1 to 100 thousandths of a second. Using calls of varying frequency, the bat recognizes the order in which echoes return. The bat then builds a detailed “picture” of its surroundings, listening for changes. To tell echoes apart, individual species – and even individual bats – have distinctive voices. By keeping calls short, the bat avoids confusion from “sending” and “receiving” at the same time. The bat can vary the volume and quality of sounds to suit the habitat it is flying through. When flying close to trees, it uses lower volume calls to avoid a confusing mix of echoes. When flying from one place to another, the bat uses infrequent, simple calls. The number and complexity increases for hunting.
How Well Can Bats “See”? It is hard for humans to imagine how bats use only their sense of sound to obtain as detailed a view of their surroundings as we do with sight. Experiments show that bats “see” extremely well. They can tell distance, speed, and direction of movement. The only quality of an object the bat cannot perceive by sound is color.
Flies and fine wires: The little brown bat detects wires as fine as 0.01 inch from over three feet away, and it spots wires 0.12 inch thick over six feet away. The Mediterranean horseshoe bat navigates between wires thinner than a human hair. When hunting, the little brown bat can spot fruit flies no larger than 0.12 inch from two feet away. It detects and eats from 500 to 1,200 of them in an hour.
On Target: Experiments show that the big brown bat discriminates between targets as close together as half an inch and can tell one triangle measuring 4 inches x 4 inches x 2 inches from another measuring 3 ½ inches x 3 ½ inches x 2 inches. It has also been able to tell the difference between plastic plates with holes drilled into them at depths varying by only 0.03 inch.
Sending a Signal: The bat produces sounds in pulses, timing the delay between sending out the sound and receiving the echoes that bounce back from its target.
Receiving a Signal: Echoes from the target are picked up by each ear and combined in the bat’s brain to form an accurate “picture” of the prey’s location and size.
Producing the Sound: Bats produce their calls using a voicebox that is proportionally larger than that of other mammals. To make sounds, the bat forces air between two membranes that vibrate. Muscles tense the membranes, producing different pitches that are either amplified or filtered through various chambers before exiting the bat’s mouth or nose. Bats that call through their nose have nose leaves that direct the sound. It is mainly insect and nectar eating bats that use this method of echolocation. But one known species of cave dwelling fruit bat uses a more basic form of echolocation, clicking its tongue to produce high frequency sounds.
Specialized Ears: The bat’s ears are sensitive to the direction and quality of its echoed calls. The external ears collect sound well, and some species’ ears are huge. Bats move their ears to locate sound. The sounds are picked up by each ear and combined in the bat’s brain.
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