Echolocation

Abstract

Echolocation, which the Medical Subject Headings vocabulary classifies under animal communication, is the perceptual system by which an animal emits sounds and reconstructs the world from their returning echoes. This article explains how bats and toothed whales turn the delay, pitch, and structure of an echo into a perceived scene: how the call-echo delay fixes a target's range, how frequency-modulated and constant-frequency calls are matched to different sensing problems, how horseshoe bats compensate for Doppler shift to hold the echo in an auditory fovea, and how the search-approach-buzz foraging sequence tracks a closing target. It closes with trained human click-based echolocation. A worked example converts echo delay into distance, and three interactive demonstrations let the reader range a target, tune Doppler compensation, and drive a foraging sequence.

Keywords: echolocation, biosonar, call design, Doppler-shift compensation, range perception

Most animals perceive the world from energy that arrives on its own: light from the sun, sound made by other things. An echolocating animal does something stranger. It manufactures the very signal it will sense, broadcasts it into the dark, and then listens for the altered copy that bounces back, reading the delay, the pitch, and the fine structure of that echo as a picture of the objects around it. A hunting bat can intercept a moth it has never seen, in total darkness, guided by nothing but the echoes of its own cries.

This article treats echolocation as an active sense: a perceptual loop in which the animal controls its own stimulus. It begins with what the system is and which animals evolved it, turns to the core computation—recovering distance from the time an echo takes to return—and then to the design of the calls themselves, the Doppler tricks of bats that hunt in clutter, and the stereotyped sequence of calls that unfolds as a bat closes on prey. It ends with the surprising fact that some blind humans learn to do a version of the same thing.

What Echolocation Is

Echolocation, also called biosonar, is the perception of objects by emitting sounds and extracting information from the echoes that return. It is an active sense, and that is its defining feature: unlike vision or ordinary hearing, which process whatever energy happens to arrive, an echolocating animal produces its own probing signal and then interprets the reflected copy. The returning echo differs from the emitted call in ways that are lawful and therefore readable—it is delayed by the round trip to the target, reduced in amplitude, shifted in frequency if the target is moving, and reshaped by the target's size, texture, and motion. The animal's nervous system treats these differences as the raw data of a perceived world. The term itself was coined by Donald Griffin, who with Robert Galambos first showed that bats avoid obstacles by hearing the echoes of their own ultrasonic cries, and who drew the explicit analogy to the radar and sonar being developed at the time (Griffin, 1944).

Two great groups of mammals evolved sophisticated biosonar independently. Microbats (the echolocating bats, order Chiroptera) emit calls through the mouth or nose and hear the echoes with ears tuned to ultrasound. Toothed whales—the odontocetes, including dolphins and porpoises—generate clicks in nasal structures and focus them forward through a fatty forehead organ, the melon, that acts as an acoustic lens; returning echoes are received through the lower jaw and conducted to the ear (Au, 1993). That two lineages separated by the whole span of mammalian evolution arrived at functionally matched solutions—a short probing pulse, a delay-to-range computation, a sensitivity to Doppler shift—makes echolocation a striking case of convergent evolution, in which the shared physics of sound in a medium constrains both designs toward the same answers (Madsen & Surlykke, 2013). Within bats the question of how many times echolocation arose, and whether it was present in the common ancestor and later lost in the fruit bats, remains an active problem in phylogenetics (Jones & Teeling, 2006).

How Biosonar Measures Distance

The central computation of echolocation is turning time into distance. When an animal emits a call and the echo returns, the call-echo delay is simply the time sound takes to travel out to the target and back. Because the speed of sound in the medium is effectively constant, the target's range is half the delay multiplied by that speed: the factor of one half corrects for the two-way travel, out and back. In air, where sound moves at about 343 metres per second, an echo returning 10 milliseconds after the call places the target at roughly 1.7 metres; in water, where sound travels more than four times faster, the same delay would mean a target over seven metres away. The animal does not consciously compute this, but its auditory system is organized so that a given delay reliably evokes the perception of a given distance.

What makes bat biosonar extraordinary is the precision of this timing. In psychophysical experiments James Simmons trained big brown bats to discriminate between echoes at slightly different delays and found that they resolve differences of a few microseconds, corresponding to a range resolution on the order of a fraction of a millimetre—finer than the width of the targets themselves (Simmons, 1973). The auditory system achieves this not by any single clock but by a neural architecture that behaves like a bank of matched filters, each tuned to a particular delay, so that the population response encodes echo delay—and therefore target range—as a place on a neural map. The demonstration below makes the delay-to-range relationship concrete: set a target's distance and medium and watch the echo return after the corresponding two-way delay.

Call Design

An echolocating animal cannot sense anything its call does not interrogate, so the design of the call is itself a perceptual decision, and different acoustic structures solve different sensing problems (Schnitzler & Kalko, 2001). Two broad call types anchor the range of designs. A frequency-modulated (FM) call is brief and sweeps rapidly downward through a wide band of frequencies; because it is broadband and short, it pins down a target's position and range with great accuracy and resolves fine detail, which is why FM sweeps dominate when a bat must localize a target precisely or discriminate its structure. A constant-frequency (CF) call is a long tone held at a single frequency; it is poor at pinpointing range but excellent at detecting the characteristic flutter of insect wings, which imposes tiny periodic Doppler shifts on the returning echo. Many bats combine the two, opening with a CF component for detection and closing with an FM sweep for localization.

The bats that rely most heavily on long CF calls, the horseshoe bats, face a problem created by their own flight: as the bat moves toward a target, the echo is Doppler-shifted upward in frequency, and the faster it flies the larger the shift. These bats solve it with Doppler-shift compensation, described by Hans-Ulrich Schnitzler: the bat lowers the frequency of its emitted call by just enough that the returning echo arrives back at a fixed reference frequency, regardless of flight speed (Schnitzler, Moss, & Denzinger, 2003). That reference frequency is special because the bat's cochlea and auditory cortex devote a hugely disproportionate share of their tissue to a narrow band around it—an auditory fovea, directly analogous to the dense patch of receptors at the centre of the retina—so compensation keeps the information-bearing echo parked on the most finely resolved part of the hearing system. The demonstration lets the reader fly the bat at different speeds and watch it adjust its emitted frequency to hold the echo on the fovea.

The Foraging Sequence

Watching a bat hunt reveals that echolocation is not a steady stream of identical calls but a structured sequence that changes continuously as the bat gathers information and closes on prey. Ethologists divide this foraging sequence into three phases (Schnitzler, Moss, & Denzinger, 2003). In the search phase the bat cruises with relatively long, widely spaced calls—perhaps ten per second—scanning a large volume of air for any echo at all. Once an echo of interest appears, the approach phase begins: the bat orients toward the target and steadily shortens both the interval between calls and the duration of each call, sampling the target more often as it draws near and needs faster updates. The sequence culminates in the terminal buzz, a rapid burst of very short calls fired off at well over a hundred per second in the final fraction of a second before capture, giving the bat an almost continuous stream of range updates as it intercepts the prey.

The logic of this progression is that the bat trades temporal detail for update rate exactly when it matters. Spacing calls widely in the search phase avoids confusion between the echo of one call and the next from distant clutter; packing them tightly in the buzz maximizes information just when the target is moving fastest relative to the bat. The calls are also loud—free-flying bats emit search calls at intensities that would be painful to a human ear, a necessity given how little of the emitted energy returns as a usable echo (Surlykke & Kalko, 2008). The demonstration below lets the reader drive a bat through the three phases and watch the call interval and duration collapse into the terminal buzz.

Worked Example: From Echo Delay to Distance

The delay-to-range computation at the heart of echolocation can be written in one line. If an animal emits a call and the echo returns after a delay Δt, the distance to the target is

d = (c × Δt) / 2,

where c is the speed of sound in the medium and the division by two accounts for the two-way travel of the pulse out to the target and back. The equation is exact; all the biological interest lies in how finely an animal can measure Δt and how the constant c differs between air and water. Table 1 evaluates it for a range of echo delays in both media.

Two lessons fall straight out of the arithmetic. First, the same delay means a very different distance in water than in air—a dolphin's 10-millisecond echo comes from a target more than four times farther away than a bat's, because sound travels so much faster in water, which is one reason odontocete biosonar operates over much longer ranges than a bat's. Second, consider the precision. If a bat resolves echo delay to about one microsecond, as Simmons' jitter experiments imply, then its range resolution is d = (343 × 0.000001) / 2 ≈ 0.17 millimetres (Simmons, 1973). The animal is reading distance from time with a sharpness that human sonar engineering reached only with considerable effort, and it does so with a brain smaller than a thumbnail. The EchoRangingDemo above is built on exactly this equation: every distance it reports is c Δt / 2 for the medium selected.

Discussion

The unifying idea of echolocation is that perception and action are not separate stages but a single closed loop. In vision the stimulus is given and the observer's task is to interpret it; in echolocation the observer creates the stimulus, so every perceptual act is also a motor act, and the animal's choice of what to emit determines what it can perceive next. This is why the field studies call design, flight, and hearing together rather than in isolation: a bat that shortens its calls in the approach phase, lowers its frequency to compensate for Doppler shift, or sweeps a broadband FM pulse to resolve a target is adjusting its own sensory input in real time (Moss & Surlykke, 2010). The natural scene the bat perceives is therefore partly of its own making, assembled from a rapid sequence of self-generated probes rather than received whole.

Seeing echolocation this way also explains the deep similarity between such different animals. Because the physics of sound—its speed, its spreading, its Doppler behaviour—is the same for a bat in air and a dolphin in water, the problems the two must solve are the same, and natural selection converged on matching solutions: a short broadband probe, a delay-to-range computation, a graded foraging sequence, a sensitivity to the motion of prey (Madsen & Surlykke, 2013). Echolocation is thus a clean example of how a shared physical environment can channel independent evolutionary histories toward the same functional design, and of how studying the behaviour reveals the computation the nervous system must be performing.

Cognitive Implications

For cognitive psychology, echolocation is a working model of active perception—the principle that perceivers do not passively receive information but act to obtain it. The bat makes this principle unusually visible because its probing is literally audible and its sensory sampling is under moment-to-moment motor control. Recordings from the bat's auditory system show that neurons are tuned not just to sound frequency but to echo delay, so that target range is represented as an orderly map, and that this representation is updated as the animal adjusts its calls—a direct neural readout of a self-controlled sense (Ulanovsky & Moss, 2008). The bat must also solve problems that any perceiving system faces: separating a target from background clutter, holding an object in attention as it moves, and remembering the layout of a space it has already probed (Moss & Surlykke, 2010).

The most direct bridge to human cognition is that some blind people learn to echolocate. By making sharp mouth clicks and attending to the echoes, expert human echolocators can detect, localize, and even judge the size and material of silent objects, and the skill supports real mobility (Thaler & Goodale, 2016). This is not a faint analogue of the bat's ability but a genuine, learnable perceptual skill in a species with no specialized biosonar anatomy, which makes it a powerful case study of how far perceptual learning and neural plasticity can extend an existing sensory system into an entirely new mode of sensing.

Current Directions

One active frontier is how echolocation combines with the animal's other senses rather than replacing them. Bats are not blind, and work tracking free-flying animals shows that they integrate biosonar with vision and spatial memory, switching between or fusing these sources as conditions change, so that echolocation is one input to a multimodal model of space rather than a self-contained sense (Danilovich & Yovel, 2019). Studying this integration in animals navigating real, cluttered environments—rather than restrained in the laboratory—has become possible with miniature onboard recorders and is reshaping what counts as the natural unit of biosonar behaviour.

The human side has advanced just as quickly. Careful acoustic analysis has characterized the mouth clicks of expert blind echolocators precisely enough to synthesize them, specifying the signal the skill actually depends on (Thaler et al., 2017). Neuroimaging has shown that in blind experts the echoes recruit parts of the visual cortex, so that the brain repurposes deafferented visual areas to process acoustic space (Thaler, Arnott, & Goodale, 2011). And training studies have shown that click-based echolocation can be taught—to blind and sighted people, and across a wide age range—with measurable gains in everyday mobility over a few weeks, moving the skill from curiosity to candidate rehabilitation tool (Norman et al., 2021).

Common Misconceptions

Echolocation means the animal is blind.

No. Most echolocating bats have functional eyes and use vision alongside biosonar, integrating the two sources depending on conditions; echolocation supplements sight in darkness rather than substituting for a missing sense (Danilovich & Yovel, 2019).

Echolocation and sonar are unrelated to human technology.

On the contrary, the connection was explicit from the start. Griffin coined the word by analogy to the radar and sonar being developed in the 1940s, and he titled his founding report on blind men, bats, and radar together, recognizing the shared principle of ranging by reflected signals (Griffin, 1944).

All echolocators use the same kind of call.

They do not. Calls are tuned to the task and the environment: broadband frequency-modulated sweeps for precise localization, long constant-frequency tones for detecting fluttering prey in clutter, and the brief broadband clicks of toothed whales for long-range underwater sensing (Schnitzler & Kalko, 2001; Au, 1993).

Only specialized animals can echolocate.

Not quite. While sophisticated biosonar requires evolved anatomy, some blind humans with no such specialization learn click-based echolocation well enough to detect and localize silent objects and navigate, showing the ability can be acquired through perceptual learning (Thaler & Goodale, 2016).

Glossary

Approach phase.

The middle phase of the foraging sequence, in which a bat orients to a detected target and progressively shortens the interval and duration of its calls as it closes in.

Auditory fovea.

A narrow frequency band given hugely disproportionate representation in a bat's cochlea and auditory cortex, analogous to the fovea of the retina, on which the Doppler-compensated echo is held.

Biosonar.

A synonym for echolocation, emphasizing its parallel with engineered sonar: sensing the environment from the echoes of a self-generated acoustic pulse.

Call-echo delay.

The elapsed time between emitting a call and receiving its echo, from which target range is recovered; the primary quantity biosonar measures.

Click-based echolocation.

The trained human skill of sensing silent objects by making sharp mouth clicks and attending to their echoes, documented most fully in blind experts.

Clutter.

Unwanted echoes from background objects, such as foliage, against which the echo of a target of interest must be detected.

Constant-frequency (CF) call.

A long call held at a single frequency, well suited to detecting the fluttering wings of insects through the small Doppler shifts they impose, but poor at pinpointing range.

Doppler-shift compensation.

The behaviour of horseshoe bats of lowering the emitted call frequency so that the upward Doppler shift of flight returns the echo at a fixed reference frequency on the auditory fovea.

Echolocation.

The perception of objects by emitting sounds and extracting information from their returning echoes; an active sense in which the animal controls its own stimulus.

Foraging sequence.

The structured progression of a bat's calls from search through approach to terminal buzz as it detects and captures prey.

Frequency-modulated (FM) call.

A brief call that sweeps downward through a broad band of frequencies, giving precise target localization and fine resolution of structure.

Melon.

The fatty forehead organ of toothed whales that acts as an acoustic lens, focusing emitted clicks into a forward beam.

Odontocete.

A toothed whale—the cetacean group, including dolphins and porpoises, that evolved biosonar independently of bats.

Range resolution.

The finest difference in target distance a biosonar system can distinguish, set by how precisely the animal resolves echo delay.

Search phase.

The first phase of the foraging sequence, in which a bat emits long, widely spaced calls to scan a large volume of air for any target.

Terminal buzz.

The rapid burst of very short calls, fired well over a hundred per second, produced in the final moment before prey capture.

Two-way travel.

The round trip of the pulse, out to the target and back as an echo, which is why target range is half the delay times the speed of sound.

Key Researchers

Robert Galambos

(University of California, San Diego; 1914–2010). American neuroscientist who, as a graduate student with Donald Griffin, provided the first experimental demonstration that bats detect obstacles by hearing the echoes of their own ultrasonic cries. Wikipedia · Wikidata

Donald R. Griffin

(Rockefeller University; 1915–2003). American zoologist who, with Robert Galambos, demonstrated that bats orient by ultrasonic echoes and who coined the term echolocation; his later work founded the field of cognitive ethology. Wikipedia

Cynthia F. Moss

(Johns Hopkins University). American neuroscientist whose work on the echolocating big brown bat links adaptive sonar behaviour to the neural representation of auditory space, including attention and spatial memory in active sensing. ORCID · Wikipedia

Kenneth S. Norris

(University of California, Santa Cruz; 1924–1998). American marine biologist who established that toothed whales and dolphins echolocate and proposed that the odontocete melon focuses the biosonar beam, founding the study of cetacean sonar. Wikipedia · Wikidata

Hans-Ulrich Schnitzler

(University of Tübingen, Professor Emeritus). German behavioural physiologist who described Doppler-shift compensation in horseshoe bats and developed the framework linking call design to foraging habitat and the search-approach-capture sequence. Wikipedia

James A. Simmons

(Brown University). American neuroscientist whose psychophysical experiments established the bat's microsecond-scale resolution of echo delay and the auditory mechanisms by which biosonar reconstructs target range and fine structure. Wikipedia · Faculty

Lore Thaler

(Durham University). Psychologist whose work established the perceptual and neural basis of human click-based echolocation, showing that expert blind echolocators recruit visual cortex and that the skill can be trained. ORCID · Faculty

Yossi Yovel

(Tel Aviv University). Israeli neuroecologist who studies how free-flying bats integrate echolocation with vision and spatial memory to navigate and forage in the wild, using onboard sensors to record biosonar in natural scenes. ORCID · Scholar

Frequently Asked Questions

What is echolocation?

Echolocation, or biosonar, is the perception of objects by emitting sounds and reading the echoes that return. It is an active sense: the animal produces its own probing signal and interprets the reflected copy, extracting a target's distance, direction, size, and motion from the delay, intensity, and structure of the echo (Griffin, 1944).

Which animals echolocate?

The most sophisticated biosonar is found in two independent groups of mammals: the echolocating bats, which emit ultrasonic calls and hear their echoes, and the toothed whales or odontocetes—dolphins and porpoises—which emit clicks and focus them through the melon. Some other animals use cruder forms, and some blind humans learn a click-based version (Au, 1993; Madsen & Surlykke, 2013).

How does an animal judge distance from an echo?

It reads distance from time. The call-echo delay is the round-trip travel time of the sound, so the target range equals half that delay multiplied by the speed of sound, d = c × Δt / 2. Bats resolve this delay to microseconds, giving sub-millimetre range resolution (Simmons, 1973).

What is the difference between FM and CF calls?

A frequency-modulated (FM) call is a brief downward sweep through many frequencies and is excellent for localizing a target and resolving its fine structure. A constant-frequency (CF) call is a long single tone that is poor at ranging but ideal for detecting the flutter of insect wings in clutter. Many bats use both in combination (Schnitzler & Kalko, 2001).

What is Doppler-shift compensation?

As a horseshoe bat flies toward a target, the echo is shifted up in frequency by its own motion. The bat lowers the frequency of its emitted call by just enough to cancel that shift, so the echo returns at a fixed reference frequency that lands on its auditory fovea, the finely tuned patch of its hearing system (Schnitzler, Moss, & Denzinger, 2003).

What is the terminal buzz?

It is the final phase of the foraging sequence, a rapid burst of very short calls emitted well over a hundred times a second in the fraction of a second before a bat captures prey, giving it an almost continuous stream of range updates as it intercepts the target (Schnitzler, Moss, & Denzinger, 2003; Surlykke & Kalko, 2008).

Can humans echolocate?

Yes. Some blind people learn to make sharp mouth clicks and interpret the echoes to detect, localize, and judge the size and material of silent objects, supporting real mobility. In expert blind echolocators the echoes recruit visual cortex, and training studies show the skill can be taught to blind and sighted people across a wide age range (Thaler & Goodale, 2016; Norman et al., 2021).

Are echolocating bats blind?

No. Most bats have functional vision and use it together with biosonar, integrating the two depending on the situation. Echolocation extends perception into darkness and clutter where vision fails; it does not replace a missing visual sense (Danilovich & Yovel, 2019).

References