Echolocation

Echolocation in bats was first clearly described in 1945 in a seminal paper by Griffin and Galambos entitled Development of the Concept of Echolocation. Bats that eat frogs, fish, and insects use echolocation to find their prey in total or near-total darkness. After emitting a sound, these bats can tell the distance, direction, size, surface texture, and material of an object from information in the returning echo. Although the sounds emitted by bats are at high frequencies, out of the range of human hearing, these sounds are very loud—as high as 100 decibels, which is as loud as a chainsaw or jackhammer. People may hear the calls as clicks or chirps. The fruit-eating and nectar-loving bats do not use echolocation. These daytime and dusk-active bats have strong eyes and noses for finding food.

Bats use echolocation to hunt for food and to avoid collisions. A group of insect-eating bats was trained to tell the difference between insect larvae with fuzzy bristles and larvae that had their bristles removed. Researchers injected the bristle-less larvae with a chemical that made them taste bitter, then offered both the normal and the nonfuzzy larvae to the bats; the bats could instantly distinguish between them. In another experiment, a group of bats was conditioned to detect very thin wires in total darkness.

One question that puzzled scientists is how a bat can hear the echo of one sound while it is emitting another sound; why is the bat not deafened or distracted by its own sounds? The answer is that the bat is deafened—but only for a moment. Every time a bat lets out a call, part of its middle ear moves, preventing sounds from being heard. Once the bat's call is made, this structure moves back, allowing the bat to hear the echo from the previous call.

One family of bats, the Vespertilionidae, emits ultrasonic sound pulses from their mouths in a narrow directed beam and uses their large ears to detect the returning echoes. Each sound pulse lasts 5-10 milliseconds and decreases in frequency from about 100,000 Hz at the beginning down to about 30,000 Hz. This change in frequency (or frequency modulation) is roughly equivalent to a human looking at an object under a range of different colors of light. When the bat is just "looking around" it puts out approximately 10 pulses per second. If it hears something interesting, it takes a closer look by increasing the number of pulses per second to approximately 200. This is roughly equivalent to a person shining a brighter light on an object under investigation.