How Do Bats See in the Dark? Echolocation Explained

At dusk a bat zigzags over a pond, snapping up mosquitoes it could never see in the gloom. It hunts with sound. For a long time nobody could explain how, until scientists realised that bats are shouting into the night and listening for the answer.

A discovery that took 150 years

In the 1790s the Italian scientist Lazzaro Spallanzani found that blindfolded bats could still fly through rooms strung with wires, but bats with blocked ears crashed. He concluded that bats somehow navigate with their ears, but nobody could hear any sound.

The mystery was solved in the late 1930s and 1940s by Donald Griffin, then a student at Harvard. Using a new microphone that could pick up ultrasound, he detected the bats’ high-pitched calls and coined the word echolocation.

How echolocation works

A bat produces short calls, usually in its larynx, and emits them through its mouth or nose. Most calls are ultrasonic, between about 20,000 and 120,000 hertz, beyond the upper limit of human hearing at around 20,000 hertz. Some species go even higher.

When a call hits an object, part of the sound bounces back as an echo. From that echo, the bat’s brain extracts several kinds of information:

• Distance: sound travels about 340 metres per second, so an echo returning after 6 milliseconds means an object about 1 metre away.
• Direction: the echo reaches the two ears at slightly different times and loudness.
• Size and texture: a large target returns a stronger echo; a fluttering wing makes the echo flicker.
• Movement: if an insect is flying towards or away from the bat, the pitch of the echo shifts slightly, the same Doppler effect that changes the sound of a passing ambulance.

Horseshoe bats are specialists in the Doppler trick. They use long calls at a constant pitch and can detect the tiny flicker of an insect’s beating wings.

Faster and faster: the feeding buzz

Echolocation has a built-in problem: a bat must wait for each echo before the next call, or the sounds overlap. So bats change their calls as they hunt.

While searching, a bat may call about 10 times a second. Once it detects prey, it speeds up. In the final moment before capture, it produces a rapid burst called the feeding buzz, reaching about 200 calls per second.

In 2011 Coen Elemans and colleagues discovered how bats manage this. Their vocal muscles are among the fastest muscles known in mammals, able to contract and relax up to around 200 times a second. Even so, the buzz is limited by these muscles.

Loud, but not to us

Bat calls can be astonishingly loud, up to about 140 decibels close to the bat’s mouth, as loud as a jet engine nearby. To avoid deafening themselves, many bats contract tiny muscles in the middle ear at the instant they call, briefly reducing their sensitivity, and relax them in time to catch the echo.

An evolutionary arms race with moths

Moths are a favourite prey, and many have fought back. Numerous moths have ears tuned to bat frequencies. When they hear a distant bat, they turn away; when a bat is very close, they dive, spiral or drop to the ground.

Some tiger moths go further and produce their own ultrasonic clicks. In some species the clicks warn bats that the moth tastes bad; in others they seem to jam the bat’s sonar at the critical moment. Some bats, in turn, call at frequencies or volumes that moths hear poorly.

Bats are not blind

The phrase “blind as a bat” is wrong. All bats have functioning eyes. Many insect-eating bats use vision for long-distance navigation, for example by recognising the horizon or landmarks at dusk.

Of the roughly 1,400 bat species, the large fruit bats of Africa, Asia and Australia mostly do not echolocate with their larynx. They have big eyes and an excellent sense of smell. A few, such as the Egyptian fruit bat, which also lives in southern Turkey, make simple echolocation clicks with their tongue.

In short

• Most bats use echolocation: they make ultrasonic calls and listen to the echoes.
• Echo delay gives distance, ear differences give direction and pitch changes reveal movement.
• When attacking, bats call up to about 200 times a second in a feeding buzz, powered by superfast muscles.
• Calls can reach about 140 decibels, but they are too high-pitched for us to hear.
• Many moths have evolved ears and even sonar-jamming clicks.
• Bats are not blind; fruit bats rely mostly on sight and smell.