How Echolocation Works: Seeing With Sound
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A bat flying through a pitch-dark cave at speed is not guessing. It is shouting, at pitches too high for us to hear, and listening to what comes back. From the timing, loudness and pitch of each echo it builds a running picture of the space around it precise enough to catch a mosquito in mid-air. Dolphins do the same thing underwater, and a handful of blind people have taught themselves a version of it with clicks of the tongue.
Distance from delay, direction from the ears
The basic principle is the one behind a ship's sonar. Sound travels at a known speed, about 343 metres per second in air, so if a call goes out and its echo returns two thousandths of a second later, the object is about 34 centimetres away. Bats send out a call, wait for the echo, and time it.
Direction comes from having two ears. An echo from the right reaches the right ear a fraction earlier and slightly louder, and the folds of the outer ear change the sound depending on whether it comes from above or below. A bat's brain combines all of that in real time, and it has to, because a hunting bat calls up to two hundred times a second in the final approach to an insect, a burst researchers call the feeding buzz.
Why the calls are so high
Most bat calls lie between 20 and 200 kilohertz, above the 20 kilohertz ceiling of human hearing. The reason is physics. A sound wave only reflects cleanly from an object roughly as large as its wavelength or bigger. A 100 kilohertz call in air has a wavelength of about 3.4 millimetres, small enough to bounce off a midge; a call at a pitch we could hear would wash straight around it.
The cost is range, because high frequencies fade quickly in air, so most bats only sense objects within a few metres. Different species solve this differently. Some use short, downward-sweeping calls that give precise distance. Horseshoe bats use a long call at one steady pitch and listen for the tiny Doppler shift that a moving insect's wingbeats put on the echo, and they even lower their own call pitch in flight to keep the returning echo in the sweet spot of their hearing.
Dolphins, and sound under water
Sound travels more than four times faster in water than in air and carries much further, which is why toothed whales rely on it. A dolphin makes clicks not with its larynx but by forcing air past a pair of structures in its nasal passages called phonic lips. The clicks pass through the melon, a lens-shaped mass of fat in the forehead that focuses them into a beam, and the returning echoes are picked up through fat-filled channels in the lower jaw that lead to the inner ear.
The result is a sonar that can detect a small fish at over a hundred metres and tell a hollow object from a solid one. Sperm whales use the same system, with the largest sound-producing organ of any animal, to hunt squid a kilometre down in total darkness.
An arms race with the prey
Echolocation is loud from the target's point of view, and insects have evolved to exploit that. Many moths have ears tuned to bat frequencies and drop or spiral away when they hear an approaching call. Tiger moths go further and produce ultrasonic clicks of their own, which either warn the bat that the moth is foul-tasting or, in some species, actively jam the bat's sonar with a burst of noise timed to the feeding buzz. Some bats have responded by calling at pitches outside the moths' hearing range.
Beyond bats and whales, echolocation has appeared independently several times: in oilbirds and cave swiftlets that nest in the dark, in some shrews and tenrecs, and in humans. Daniel Kish, blind since infancy, navigates by tongue clicks and has taught the technique to others; brain scans of skilled human echolocators show the visual cortex responding to the echoes.
Reading an echo
What a bat or dolphin extracts from a single returning sound is a small list of measurements, and it is worth knowing them because every sonar, medical ultrasound scanner and depth sounder uses the same ones:
- •Delay between call and echo gives distance
- •Difference between the two ears gives direction
- •Loudness of the echo gives size and hardness
- •Change in pitch gives the target's speed towards or away
- •The spread of echoes over time gives texture and shape
How scientists worked out what bats were doing
For a long time nobody could explain how bats avoided obstacles in darkness. In the 1790s the Italian naturalist Lazzaro Spallanzani found that bats deprived of sight still flew through rooms strung with obstacles without crashing, which suggested they were not using their eyes. The Swiss surgeon Louis Jurine then showed that plugging a bat's ears made it collide with things, so hearing was clearly involved. But the bats seemed silent, and without any way to detect their calls the idea that they were listening to something remained puzzling, and many scientists doubted it for more than a century.
The answer came in the late 1930s, when the American student Donald Griffin brought bats to the physicist George Pierce, who had built a device that could pick up ultrasound. The bats turned out to be calling constantly at pitches far above human hearing. Working with Robert Galambos, Griffin showed that bats needed both to call and to hear the echoes to steer, and he coined the word echolocation in the 1940s. The story is a good lesson in how science works: a phenomenon can be real and well observed for generations while remaining unexplained until the right measuring instrument exists.
Not blind, and not deafened
The phrase blind as a bat is wrong. Every bat species has working eyes, and many echolocating bats use vision for longer-range navigation, such as homing on landmarks, switching to sound for close work. The large fruit bats and flying foxes of the Old World tropics mostly do not echolocate at all; they find fruit and flowers by sight and smell, and their eyes are large and sensitive. One group of fruit bats, the rousettes, has evolved a simple sonar of its own using clicks of the tongue rather than calls from the larynx.
Their own calls pose a different problem. Measured close to the mouth, the calls of some species exceed 120 decibels, a level that would be painful to human ears if it fell within our hearing range, while the returning echoes can be thousands of times fainter. A bat avoids deafening itself with tiny muscles in the middle ear that contract just as each call goes out, damping the sound reaching the inner ear, then relax in time to catch the echo. As they close in on a target, many bats also lower the volume of each call, which keeps the returning echo at a fairly steady loudness.
Plants that want to be heard
Echolocation has shaped the bats' surroundings as well as the bats. In the rainforests of Central and South America and the Caribbean, many plants rely on nectar-feeding bats for pollination, and some have evolved parts that work as acoustic signposts. A Cuban vine, Marcgravia evecta, grows a concave, dish-shaped leaf above its flowers. Like a reflector on a bicycle, it returns a strong, distinctive echo from a wide range of angles, so a bat sweeping its calls across a cluttered forest picks it out easily. In experiments, bats found flowers carrying the dish markedly faster than flowers with it removed.
A pitcher plant in Borneo has gone further. Like its relatives, Nepenthes hemsleyana can trap insects in its fluid-filled pitchers, but it catches relatively few. Instead, small woolly bats roost inside the pitchers by day, and their droppings supply the plant with nitrogen. Researchers found that the back wall of each pitcher is shaped to reflect the bats' calls strongly, and that bats found the pitchers more easily when this structure was intact. Cases like these show that echoes, like colours and scents, can become part of the signalling between species, with plants evolving to stand out to an animal that perceives the world through sound.
The takeaway
Echolocation is the use of self-made sounds and their echoes to sense surroundings: delay gives distance, the two ears give direction, and pitch shift gives speed. Bats and toothed whales have refined it into a sense as detailed as sight, and a few humans have shown that the same brain can learn to read echoes too.