What is echolocation in cetaceans?

Ocean water is, most of the time, opaque to sight. At a hundred metres down almost no light is left; at a thousand metres it is complete darkness. To hunt, navigate and communicate in this world without visibility, the toothed whales: the odontocetes, have developed one of the most sophisticated biological senses in evolution: echolocation, a living sonar that lets them literally “see” with sound.

Like a military submarine probing its surroundings by echo, the animal emits a brief, intense acoustic wave. That wave travels through the water, meets an obstacle, prey, rock, conspecific, then comes back as an echo. By analysing the delay of the return, the distortion of the signal and its intensity, the cetacean’s brain reconstructs in real time a three dimensional acoustic image of its surroundings. The precision reached goes far beyond our most advanced technologies: a common bottlenose dolphin can detect a steel ball the size of a golf ball several hundred metres away, and tell apart the material of targets identical in shape.

How the sound is born

The first instinct would be to think of vocal cords, and that would be a mistake. Cetaceans do not vocalise through the larynx but through the nasal complex, located just below the blowhole. There, muscular and fatty structures nicknamed the phonic lips: or, more vividly, the monkey muzzle because of their shape, vibrate at very high speed when the animal pushes air through them. Each vibration produces an ultra brief click, sometimes at frequencies reaching 100 kHz, well above the upper limit of human hearing (20 kHz).

But emitting an omnidirectional sound would serve no purpose: locating a target would then be impossible. This is where a remarkable structure, peculiar to odontocetes, comes in: the melon. This fatty mass, which forms the animal’s characteristic bulging forehead, is not an energy reserve. It is a living acoustic lens. Its lipid composition varies gradually from the centre towards the periphery, creating a gradient of sound speed that concentrates the vibrations into a narrow, directional beam, comparable to the cone of light from a torch.

A melon the beluga can reshape

In some species, control of this beam reaches an extreme refinement. The beluga is able to deform its melon at will, one can literally see the animal’s forehead swell and draw back, in order to adjust the direction and the width of the acoustic beam in real time. It is the biological equivalent of a steerable telescope, and a strong argument for the cognitive complexity of odontocetes.

The extreme case of the sperm whale

If echolocation reaches its summit anywhere, it is in the sperm whale. Its enormous head, which can account for up to a third of the body length, contains the spermaceti organ: a reservoir filled with several hundred litres of clear oil, together with another fatty mass called the junk. These two structures combine to produce and focus the most intense sounds ever measured in the animal kingdom: up to 230 decibels at source for some clicks, the underwater acoustic equivalent of a rocket launch.

This acoustic power allows it to probe the ocean over several kilometres and to hunt squid at more than 1,500 metres down, in total darkness. When the sperm whale closes in on its prey, the rate of its clicks goes from a few per second to several hundred: this final burst is called the buzz, the acoustic equivalent of a gradual zoom as the target draws nearer.

A precision that defies imagination

Beyond simple location, echolocation conveys a wealth of information about targets: size, shape, density, and even internal composition. Research carried out in particular by Whitlow Au in Hawaii has shown that common bottlenose dolphins in captivity can tell apart, blindfolded, cylinders of copper, brass, steel or aluminium of the same dimensions, the acoustic signature of the materials being subtly different. More astonishing still, they recognise an object learned visually from its echo alone, evidence of a genuine mental representation shared between the senses.

This biological system, indispensable to their survival in the depths, makes odontocetes particularly vulnerable to noise pollution of human origin: shipping traffic, seismic surveys, military sonar. When the ocean becomes too noisy, it is their very capacity to perceive their surroundings that is compromised.

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