Most bats echolocate to navigate through their environment, to locate and maneuver in the roost and to locate and pursue prey. Not all bats hunt by means of echolocation. The Old World, fruit-eating flying foxes use their senses of sight and smell to find food.
Echolocation is a means of close-range navigation. (High-frequency sound dissipates or is absorbed in air quickly.) Canadian chiropterologist Brock Fenton calculated that a big brown bat can first detect an object in its path at a distance of about 43 times the length of its body. If human vision provided similar performance, a 6-foot-tall person would first see a stop sign at 85 or 90 yards.
Echolocation involves two processes. First, the bat must produce and transmit calls; then it must receive and interpret echoes. Calls of most bats are produced in the larynx or voice box. As with other mammals, bats can change the frequency, or pitch, of the calls produced by their vocal cords just as we do while speaking or singing. Bats may use more than one frequency during echolocation. They can use FM (frequency modulated) calls that start at one frequency and drop or sweep down to another. Or, they can use constant frequency (CF) calls, dominated by one frequency. Most species use a combination of FM and CF calls.
Most bats emit vocalizations through their mouths, which explains why most bats have their mouths wide open while in flight. However, some bats, including the New World leaf-nosed bats, emit vocalizations through their nostrils. Vocalizations can differ in intensity between species of bats. Low-intensity calls can be compared to someone whispering in your ear, while high-intensity calls are worse than someone screaming. A little brown bat generates sound with the intensity of a smoke detector. Fortunately, these calls are ultrasonic, that is, above the range of human hearing. Imagine the cacophony that would fill the night air if we could hear the chorus of echo locating bats.
The duration of pulses varies. Longer pulses are better suited to locating smaller objects. Shorter calls are better for collecting detailed information about a target. A combination of the two is used by most species that hunt airborne insects, switching from long to short pulses as they detect and then lock on to their prey. Hoary bats have a call rate of 3-10 calls per second while searching for prey. This increases to 200 per second as a bat closes in on its prey. This high-rate vocalization is referred to as a feeding buzz.
Receiving and analyzing echoes depend on a bat's ears and brain. Their ears exhibit a wide variety of sizes and shapes, apparently specialized to increase effectiveness by focusing and amplifying returning echoes. A bat can obtain much information from an echo, including size, shape, texture, speed and direction of movement of the target.
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