How Do Bats Use Echolocation? Seeing With Sound Explained

How echolocation works

Echolocation is a natural form of sonar. An insect-eating bat produces calls in its larynx, the voice box, and sends them out through its open mouth or, in some groups such as horseshoe bats, through its nose. When a call hits a moth or a branch, part of the sound reflects back to the bat’s large, sensitive ears.

The bat’s brain then reads the echo in several ways:

• Distance: the time between the call and the echo shows how far away the object is. Sound travels about 34 cm (13 in) per millisecond, so an echo returning after 10 milliseconds comes from an object about 1.7 m (5.6 ft) away.
• Direction: small differences in timing and loudness between the two ears show where the echo came from. The tragus, a flap of skin in front of the ear, helps judge height.
• Size and texture: larger objects return stronger echoes, and the pattern of frequencies in the echo reveals shape and surface.
• Movement: a beating insect wing makes the echo flicker and shifts its pitch slightly, the Doppler effect, which tells the bat its target is alive and flying.

Laboratory experiments have shown that some bats can tell apart two targets that differ in distance by less than a millimeter. Bats do this in a fraction of a second while flying fast, which is why echolocation remains a model for engineers who design sonar and radar.

How high are bat calls?

Most echolocation calls are ultrasonic, too high for people to hear. Human hearing reaches about 20 kHz (20,000 vibrations per second) at best, while bat calls range from roughly 11 kHz to more than 200 kHz, with most insect-eaters calling between about 20 and 60 kHz.

Each species has typical call shapes and frequencies. The common pipistrelle of Europe calls with most energy near 45 kHz, while the very similar soprano pipistrelle calls near 55 kHz. This difference in “voice” was the clue that led scientists to recognize them as two separate species in 1999. The little brown bat of North America uses calls that sweep downward from about 80 to 40 kHz in a few milliseconds.

Bats use two main types of call:

• Frequency-modulated (FM) calls: short sweeps from high to low pitch, good for pinpointing prey among leaves and branches.
• Constant-frequency (CF) calls: long, steady tones, used by horseshoe bats to detect the flutter of insect wings from a distance.

Calls are also extremely loud. Bats that hunt in open air can produce about 130–140 decibels close to the mouth. To avoid deafening themselves, bats tighten tiny muscles in the middle ear as each call goes out.

From search to feeding buzz

A hunting bat changes its calls as it closes in on prey. While searching, it calls slowly, about 10–20 times a second, often timing each call with a wingbeat and breath to save energy. Long gaps between calls let echoes return from objects many meters away.

Once it detects an insect, the bat calls faster and shorter, updating its picture of the target. In the last fraction of a second, the rate rises to as much as 150–200 calls per second, a rapid burst called the feeding buzz. With a bat detector, the feeding buzz sounds like a short raspberry or zip, a sign that a bat has just attacked an insect.

Bats catch prey with their mouths or scoop it with a wing or the tail membrane, sometimes doing a quick somersault in flight. These skills let small bats eat large numbers of insects; see how many insects bats eat.

Do all bats echolocate?

Not all of them. Most of the world’s more than 1,400 bat species echolocate with the larynx, including all the insect-eating bats of Europe and North America. The main exception is the Old World fruit bat family, which includes flying foxes such as the large flying fox. These bats have big eyes and a keen sense of smell and find fruit and flowers mostly by sight and scent.

A few rousette fruit bats are a twist on the rule: they echolocate with simple clicks of the tongue, which helps them navigate in dark caves. Some birds, including oilbirds and cave swiftlets, use audible clicks in the same way, and toothed whales such as dolphins echolocate underwater.

Echolocation does not mean bats cannot see. Every bat species has functional eyes, and many insect-eating bats also use vision for long-distance navigation at dusk.

Moths that fight back

Bats and moths have been in an evolutionary arms race for tens of millions of years. Many moths have ears tuned to ultrasound. 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. They produce their own ultrasonic clicks, which can warn bats that the moth tastes bad. In 2009, researchers showed that the tiger moth Bertholdia trigona clicks rapidly in the final moments of an attack and jams the bat’s sonar, causing the bat to miss. Some large moths have long tails on their hind wings that twist in flight and draw bat attacks away from the body.

Bats have countermeasures too. The so-called whispering bats call very quietly so that moths hear them too late, and the spotted bat uses calls low enough that most moths cannot detect them.

Listening to bats with a bat detector

People can eavesdrop on echolocation with a bat detector, a device that converts ultrasound into audible clicks or records calls to show on a screen. Bat groups across Europe and North America use detectors on evening walks and in surveys, and researchers build “call libraries” to identify species.

Not every species can be identified this way, because some call types overlap, but detectors have revealed bats in places where they were never seen. Listening is completely harmless to bats, unlike handling or disturbing them in their roosts.