Echolocation in Bats

In a process known as echolocation,some animals use reflected sound waves to assess their surroundings.Echolocation has been best studied in bats.Today the general consensus among scientists is that the functions of bat echolocation are primarily to acquire food (for example,flying insects)and secondarily to orient in darkness. Echolocation and other sounds are measured in frequency (vibrations per second),using a unit of measure called a hertz (Hz).Sounds emitted by bats can range between 100 and 200,000 Hz,but those used for echolocation are generally between 25,000 and 140,000 Hz. Because humans typically hear sounds between about 20 and 20,000 Hz,echolocation in bats is truly ultrasonic from a human standpoint.

Why are ultrasonic signals produced by bats so high in frequency? Bats almost certainly did not evolve ultrasounds in order to fly undetected by humans and other potential predators;instead, evolution probably had a lot to do with prey capture and navigation.A bat flying at 30 kilometers per hour in relative darkness is able to detect and capture flying insects several meters away that are about eleven millimeters or smaller.Under these circumstances,a rapid series of signals of about 30,000 Hz allows the bat to produce wavelengths eleven millimeters in size that match the body size of an eleven-millimeter flying insect.When the wavelength of the signals emitted by a bat is the same as that of the intended prey,a better echo returns to the bat,thereby giving the bat more precise information on the location of the prey.The detection of insects smaller than eleven millimeters would require bats to emit signals even higher than 30,000 Hz in frequency.

A challenge for an echolocating bat in flight is to distinguish quickly between outgoing signals and incoming signals that echo off a flying insect or obstacle.Reflected incoming signals return to the bat in milliseconds,so signals emitted by the bat must be not only brief but also spaced often enough to detect immediate changes in the location of the intended object in the bat’s echolocation field.The little brown myotis bat,for example,achieves this by using a normal rapid- fire series of pulses emitted approximately every twenty-five seconds when searching for prey.When prey is detected,the pulse duration is shortened,and the pulse rate is increased to around 200 pulses per second as the bat closes in on the prey.This adjustment of pulse duration and rate also allows the bat to better determine the characteristics of a prey item,such as its size.Interestingly,the little brown myotis (and perhaps other bats)is capable of eavesdropping on the calls of members of the same species to locate better foraging areas.

The fleshy projection in the ear of bats,termed the tragus,plays an important role in echolocation.It functions as a reflecting surface for incoming signals,providing a second and slightly longer path for the sound to travel down the ear canal of a bat.This second,phased signal enables bats,such as the big brown bat,to distinguish very slight changes (3)in the vertical positioning of an object,such as a flying insect or an inanimate obstacle to the bat’s flight path.

Each species of bat produces a vocal signature when echolocating.For instance,the little brown myotis uses signals slightly below 40,000 Hz,the eastern red bat emits signals between 35,000 and 40,000 Hz,and the big brown bat echolocates at frequencies between 25,000 and 30,000 Hz.Thus,wildlife biologists can identify bat species in the field using relatively low-cost,portable ultrasonic bat detectors that distinguish differences in frequencies and other features of these species-specific signals.Just as each of our voices has unique characteristics,researchers have noted that echolocation calls can vary among individual bats of the same species.This individual variation in signal characteristics within a species has posed some difficulties in identifying species with the use of ultrasonic detectors in the field.In one study involving ultrasonic detectors,only 70 percent of the calls of big brown and silver-haired bats living in the same geographical area were correctly identified to species by field researchers.?

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?In a process known as echolocation,some animals use reflected sound waves to assess their surroundings.Echolocation has been best studied in bats.Today the general consensus among scientists is that the functions of bat echolocation are primarily to acquire food (for example,flying insects)and secondarily to orient in darkness. Echolocation and other sounds are measured in frequency (vibrations per second),using a unit of measure called a hertz (Hz).Sounds emitted by bats can range between 100 and 200,000 Hz,but those used for echolocation are generally between 25,000 and 140,000 Hz. Because humans typically hear sounds between about 20 and 20,000 Hz,echolocation in bats is truly ultrasonic from a human standpoint.

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