The Night is Alive With the Sound of Echoes.

The evening was peaceful as the sun settled below the horizon. Suddenly, the orange and red sky was dotted with flying blue-grey silhouettes zigzagging above Lake Union. Bats! Some bats dipped low over the water's surface to drink in a manner similar to swallows, while others followed erratic courses above, consuming mosquitoes and other insects in flight.

This graphic reveals how echolocation is used by a bat to reveal the location of a tasty meal, such as a moth. [Graphic courtesy of Bob and Rita at bats4kids]

How do bats find and capture insects in darkness? All insect-eating bats produce sounds -- either with open mouths or through an elaborate nose-leaf that functions like a megaphone -- that are reflected as faint echoes from objects in the environment, providing bats an accurate picture of their environment and allowing them to maneuver in complete darkness.

We didn't always know that bats rely on sound to move through dark surroundings. In the late 1800s, Swiss zoologist Charles Jurine's suggestion that bats could "see" using their ears was rejected as preposterous by most of his colleagues. However, in 1944, Donald Griffin, then an undergrad at Harvard University, used special microphones to show that bats generate ultrasonic sounds above the range of human hearing to create echoes that reveal the locations of objects. As a result, Griffin coined the term "echolocation" to describe this phenomenon.

All bats communicate using a variety of squeaks and squeals that human ears can hear, but bat species in the taxonomic suborder Microchiroptera emit high-frequency (high-pitched) sounds for echolocation. Even though low frequency sounds travel farther, only small ultrasonic sound waves used for echolocation can provide detailed information about tiny objects in the environment, such as flying mosquitoes.

Echolocation sounds are not the same pitch for all bat species. High frequency echolocation calls produced by bats range from 20-200 kilohertz (kHz) or more (humans cannot hear sounds above 20kHz). There are differences in the pattern of echolocation calls, as well. Some species produce a constant frequency (CF) cry, while others use a frequency modulated (FM) call that descends in pitch. However, most bat species can choose to use either type of call.

CF and FM sounds reveal different types of information to the bat. CF is used to detect objects while FM provides distance and other finely detailed data about the nature of the object. For example, by comparing the time elapsed between outgoing calls and returning echoes, bats accurately estimate distance to an object. Deviations in echo intensity and pitch reveal important details about the target, such as direction, size, shape and velocity. Most bat species produce a complicated sequence of CF and FM calls that they modify apparently depending upon differences in habitat, such as open areas versus dense vegetation.

The Little Brown Bat, Myotis lucifugus, is the most common bat in the Seattle area. It typically roosts in attics or barns near water. It is a "shouting bat." [Photo copyright by the Missouri Department of Conservation.]

Because air absorbs energy contained in sound waves, the usefulness of high-pitched (short wave) sounds such as those produced by echolocating bats are limited to a range of 50 feet or less, making echolocationg bats nearsighted. To partially compensate for this energy loss and to increase their range, most bats produce high intensity sounds of up to 120 decibels, which is as loud as a smoke detector held four inches from your ear. These "shouting" bat species include the Little Brown Bat, Myotis lucifugus, and the Big Brown Bat, Eptesicus fuscus, which are common in Seattle neighborhoods.

How do bats make such loud sounds without deafening themselves? Shortly before the bat's larynx muscles contract to produce an ultrasonic call, special muscles in the middle ear contract, separating the three inner ear bones (the malleus, incus and stapes or "hammer," "anvil" and "stirrup"), causing momentary deafness. After the call has been made, the middle ear muscles relax, restoring the bat's hearing in time to receive echoes from objects that are as close as one meter away.

There still are many unanswered questions about the acoustic abilities of bats. However, research into echolocation is already yielding practical benefits for humans. For example, sonar is a primitive form of echolocation (from a bat's perspective!) that is widely used for navigation, tracking aircraft, ships, submarines and missiles, and for forecasting weather.

The author acknowledges Bob and Rita from the website, bats4kids, who graciously provided permission to use their echolocation graphic, and the Missouri Department of Conservation for their excellent photograph of the Little Brown Bat in flight. The author also thanks Dr. Van Tiggelen from the Belgian Museum, who read this article and kindly corrected my misspelling of Donald Griffin's name.

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[10 October 2001]