Bats fly and hunt in the pitch dark, catching tiny insects out of the air — without bumping into anything. Their secret is sound. A bat lets out a quick, high-pitched click, far too high for us to hear, then listens for the echo bouncing back off whatever is in front of it. Sound takes time to travel, so the longer the echo takes to return, the farther away the object is. By firing click after click, dozens every second as it closes in on a moth, the bat builds a kind of sound-picture of the dark around it. Press the button in the simulator, watch a click race out, bounce off the moth, and come home, then read off the distance.
Most people think bats have extraordinary eyes or some mysterious sixth sense for flying in the dark. In fact they are listening to the shape of the world: they shout an ultrasonic click and time the echo, the same principle as radar and sonar.
What's actually happening
A bat swooping through a black cave or a moonless night seems to be doing the impossible: flying fast, dodging branches, and snatching insects it plainly cannot see. For a long time people assumed bats simply had very good eyes or some mysterious sixth sense. The truth, pinned down in the twentieth century, is that they are listening to the shape of the world.
The bat shouts (a brief, intense click pitched far above human hearing) and then falls silent to listen. The sound spreads out, strikes whatever is ahead, and a faint echo returns. Because sound travels at a fixed, fairly slow speed, the bat can use the delay as a ruler: a quick echo means something close, a long one means something far. At 343 metres per second, an insect four metres away sends its echo back in about 23 thousandths of a second, and the bat's brain turns that tiny delay into a distance. One click gives one reading; the magic is in repetition. A hunting bat fires clicks several times a second while cruising, then ramps up dramatically as it homes in — a final burst sometimes called the terminal buzz, well over a hundred clicks a second, sampling the closing gap almost continuously.
From this stream the bat assembles far more than distance. The strength and texture of an echo hint at an object's size and even whether it is a tasty moth or a leaf. Some bats listen for the fluttering Doppler shift of beating insect wings, and a few moths fight back — jamming the bat's sonar with clicks of their own, or simply going silent and still. It is the same principle we later built into radar and ship sonar: send a pulse, time its return, read the world from the echoes. The bats just got there first, tens of millions of years ago.
A bat times the round trip of its own click, so a stream of echoes builds a sound-picture precise enough to catch a moth in total darkness.
- 1Stand facing a large flat wall from far across an empty hall or car park — the farther the better.
- 2Give a single sharp clap and listen for the echo bouncing back; the gap before you hear it is the sound's round trip to the wall and back.
- 3Step closer and clap again: the echo returns sooner. You have just used delay to judge distance, exactly as a bat does — only it does the maths dozens of times a second.
Common questions
Sound travels at about 343 metres per second, so the bat uses the round-trip time as a ruler. A target four metres away returns its echo in roughly 23 thousandths of a second, which the brain reads as a distance.
As a bat closes in on prey it fires clicks faster and faster. This final burst, sometimes exceeding 150 to 200 clicks a second, samples the shrinking gap almost continuously.
Yes. Some tiger moths answer with rapid clicks of their own that scramble the bat's echo reading, while others simply go silent and still, an arms race fought in ultrasound.