Echolocation gives bats their own way of “seeing” the world – as a mosaic of reflected sound.
Many bats are able to form images of their world using echoes reflected from their surroundings. Using echolocation to ’see’ with sound instead of light, bats can fly in total darkness. Echolocation is such a powerful sense that bats can hunt even the smallest flying insects in the night sky.
Bats use high-pitched sound pulses for echolocation. This type of sound is called ‘ultrasound’, because it is pitched well above the limit of human hearing. Bats also makes audible squeaks, but these are not used for navigation. Typically, every pulse sweeps from a high note to a lower note. As a result the p sounulse contains a range of frequencies, and the way the different frequencies are reflected may help probe the texture of the target. Each pulse corresponds to a wingbeat, and is rather like the grunt made by some tennis players when serving.
As the bat closes in on its prey it flutters its wings to speed up the pulses, subdividing each one to multiply the effect.
Some bats call through the mouth, others through the nose, which has elaborate leaves and flaps to focus and aim sounds. The ears on a bat also have ridges and channels, which make them extra-sensitive to the echoes. Some have enormous ears, and aer particularly good at picking out prey from a background of dense foliage or other ‘clutter’.
Many animals use sound to hunt. A fox can pinpoint a mouse in the dark by the noise it makes. But if the mouse keeps quiet it may avoid detection. An insect being hunted by a bat, though, would not be so lucky, as the sound that gives it away is made my the bat itself.
As it hunts, a bat emits a rapid stream of pulses that spread out through the air like ripples on a pond. These ‘illuminate’ the bat’s surroundings and are reflected off anything they strike.
Echoes from a bat’s sound pulses carry very detailed information. A bat’s ears enable it to sense the direction of an object that caused the echo, the delay between pulse and echo gives its range, and the way the echo is modified by a reflective surface gives its size and texture.
A sound image is built up like a mosaic, from many tiny pieces. The more pieces the better, so the more pulses a bat makes, the finer the picture. But generating sound uses energy, so bats vary their call rate. The noctule bat makes a second when flying high, but increases the rate when nearer the ground. Daubenton’s bat, which often flies low over water, makes about 13 pulses a second to keep a check on its height above the water. When hunting, the call rate speeds up to a frenzied buzz as a bat homes in for the kill.
There are those bats who do not use their natural echolocation methods to hunt. These includes the Fruit bats who have a highly developed sense of smell.
And on a last note. Bats are not the only animals that make use of echolocation for hunting or navigation. Many toothed whales, such as dolphins and killer whales, share this facility, as do some birds, such as the cave swiftlets of Southeast Asia and the oilbird of Central and South America.