Echolocation

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Echolocation is the process of using sound waves to locate objects that may be invisible or at a distance. Some bats use sound to locate their insect prey. Bats have vocal chords modified to emit the high-frequency soundsneeded for good resolution and specially adapted ears to receive the sound. Animals also use echolocation for orientation, avoiding obstacles, finding food, and for social interactions. The animal produces sounds and listens for the echoes reflected from surfaces and objects in its environment. By analyzing the information contained in these echoes, the animal can perceive the objects.

In all species that use echolocation, the sound pulses are short bursts at relatively high frequencies, ranging from about 1,000 Hz in birds to at least 200,000 Hz in whales. Bats use frequencies from about 30,000 Hz to about 120,000 Hz. The pulses are repeated at varying rates depending on what the animal is doing. A flying bat will emit about one pulse per second. In a hunting bat close to its target, the rate may increase to several hundred pulses per second.

Most bats, including all small bats (suborder Microchiroptera) and one genus of large bats (Megachiroptera) use echolocation. Other animals thought to use echolocation are a few species of shrews and two kinds of birds. Echolocation is also used by most toothed whales and porpoises (Odontoceti). Baleen whales (those that exist primarily on krill and similar organisms) do not use echolocation.

There are two groups of bats, large bats and small bats. Large bats eat fruit and find their way around using their excellent eyesight. Small bats are mostly insectivores that find their flying prey at dusk using echolocation. Bats produce sounds with their larynx, or voice box, which has adapted to produce loud, high-frequency sounds. The quality, frequency, duration, and repetition rate of the sounds produced varies with the species of bat and with the situation. For example, as a bat closes in on its prey, it will repeat the pulses of sound more frequently.

Although the frequency of bat cries varies among species, the cries usually occur in a range between 30,000 and 80,000 Hz. The use of such high frequencies is an essential feature of the bat's sonar system. Because the target object (a moth or other small insect) is so small, a high-frequency, very short-wavelength sound must be used. A sound wave with a frequency of 80,000 Hz has a wavelength of around 4 millimeters (1/8 inch), which is suitable for locating a small moth.

Bat ears are well adapted to receive high-frequency sounds. In most bat species, the size of the outer ear is large relative to the size of the head. In some species that use relatively faint sounds, the outer ear is twice the size of the rest of the head. The large surface of the outer ear acts as an efficient collector of sounds. The outer ear is tuned to receive the frequencies emitted by the bat larynx, which helps the bat to hear the sounds it produces and tune out other sounds. The outer ear is also very mobile and can be rotated and tilted in various ways. The bat ear canal also contains a special organ that allows the canal to be closed to reduce the entrance of excessively loud sounds.

Neurophysiological and behavioral studies of bat hearing have revealed some curious features. One such feature is that bats do not respond behaviorally to frequencies below 10,000 Hz, although studies demonstrate thatthey can hear these frequencies. This lack of a response is probably due to the bat's dependency on hearing for echolocation. Below 10,000 Hz, the wavelength is too long to be of any use in finding prey. It is also a frequency range where environmental noise is likely to occur, so bats have evolved the ability to selectively ignore sounds that are distracting and are not useful in finding prey. Researchers have also observed that bats are not easily disturbed by extraneous sounds of low frequencies, even very loud sounds. This peculiarity of hearing in bats may account for their resistance to distracting sounds.

Dolphins and toothed whales use echolocation to orient themselves and locate objects in the water. These animals probably rely on sound production and reception to navigate, communicate, and hunt in dark or murky waters where sight is of little use. They produce sounds with their larynx and a complex system of cavities connected to their blowhole. The sounds used in echolocation are a rapid series of clicks. The clicks contain a wide range of frequencies, but most of the sound energy is in the 50,000 to 200,000 Hz range. These high frequencies are necessary for echolocation in water. Because the speed of sound in water is five times greater than in air, the wavelength of a sound of a given frequency is five times longer in water than in air. To achieve the same resolution, the frequency must be five times higher.

All toothed whales, including dolphins, have a fat-filled organ in the front part of the head called a melon. The melon acts like a lens for sound waves, focusing the sound waves into a narrow beam. Dolphins and other toothed whales generate a wide variety of clicks, whistles, and other noises used in communication and echolocation. The clicks they use for echolocation are of a higher frequency than those used for other forms of communication. This improves resolution and allows smaller prey to be located. The clicks are generated in a series of interconnected passages behind the melon. When the sound strikes an object such as a prey fish, some of the sound is reflected back toward the dolphin. Another fat-filled cavity in the dolphin's lower jawacts as a receptor for this sound. The sound is carried from the fat-filled cavity to the middle ear and perceived by the animal's brain.

As soon as an echo from one click is received, the dolphin generates another click. The time lapse between click and echo enables the dolphin to determine the distance between it and the object. The difference in sound intensity received by each ear allows the animal to determine the direction. By emitting a series of clicks and listening to the echoes, the dolphin is able to locate and follow its prey.

Acoustic Signals.

Allen, Glover M. Bats. Cambridge, MA: Harvard University Press, 1939. Reprint, New York: Dover Publications Inc., 1962.

Au, Whitlow W. L. The Sonar of Dolphins. New York: Springer-Verlag, 1993.

Bonner, Nigel. Whales. London: Butter and Tanner Ltd., 1980.

Fenton, M. Brock. Bats. New York: Facts on File Inc., 1992.

Gaskin, David Edward. The Ecology of Whales and Dolphins. London: Heinemann Educational Books Ltd., 1982.

Graham, Gary Lynn. Bats of the World: A Golden Guide. New York: Golden Press, 1994.

Harrison, Richard John, and Michael M. Bryden, eds. Whales, Dolphins, and Porpoises. New York: Facts on File, 1988.

Tuttle, Merlin D. America's Neighborhood Bats, rev. ed. Austin: University of Texas Press, 1997.

Wilson, Don E. Bats in Question: The Smithsonian Answer Book. Washington, D.C.: Smithsonian Institution Press, 1997.

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