Echolocation is a technique used by some animals to locate objects at a distance when the view is not or hardly practicable (at night, in dark areas such as caves or underwater). It is notably used by all toothed or odontocetes cetaceans (dolphins, beaked whales and sperm whales).
The physical principle is the same as that of sonar: a short and intense acoustic wave is emitted, part of which is reflected by the obstacle and is picked up in return by the animal. The time between transmission and reception of the signal gives information on the distance between the transmitter and the target. The shape and strength of the return signal gives further indication of the texture and size of the target. Finally, the direction is determined by two complementary devices:
- on the one hand, at emission, by focusing the wave, i.e. the emitted energy is concentrated in a beam of small angular dimension. This system allows the animal to essentially “scan” the space in front of it.
- on the other hand, on reception, by acoustic sensors whose geometry is strongly non-symmetrical. Thus, it is thought that the bones (of the jaw, of the skull) of dolphins play an important role in the detection of echoes.
Video 1. Animation describing the principle of echolocation of an odontocete, the bottlenose dolphin (Tursiops truncatus). [Source: Animation Franck Malige and Julie Patris]
The organ associated with the focusing of the acoustic emission is called the “melon”, which gives the wide rounded forehead characteristic of dolphins. For most toothed cetaceans, the impulse is emitted by the phonic lips at the back of the skull. For sperm whales on the other hand, the respiratory vent being located at the front of the head, the phonic lips emit towards the inside of the skull, through the fatty organ called spermaceti. The wave is then reflected on the air sac present at the back of the melon before being focused again through the fatty masses of the muzzle. Because of the losses at each stage of this complex amplification, sperm whale clicks have a multiple structure characteristic of the species.
Video 2. Animations describing the principle of echolocation in another odontocete, the sperm whale (Physeter macrocephalus). [Source: Animation Franck Malige and Julie Patris]
The emitted signals are generally short, and at high frequency. Indeed, the time between transmission and reception is short: a round-trip distance of 15 metres is covered by the sound wave in 15 / 1500 = 1/100th of a second. It is necessary to emit signals shorter than the estimated round trip time, generally shorter than a few milliseconds. The signals emitted by toothed cetaceans are called “clicks” because of their acoustic properties (short and wide frequency range).
Video 3. Clicks emitted by a sperm whale (Physeter macrocephalus). Sounds recorded by the CNRS DYNI team in the Mediterranean, near Toulon. [Source: Animation Franck Malige and Julie Patris]
We can see that the frequency ranges of these clicks vary greatly from one species to another. Sperm whales, immense inhabitants of the open spaces, emit medium frequency clicks, of the order of 5 kHz, audible to the human ear (Video 3). At the other extreme, river dolphins, which live in a complex environment, very diversified on a small scale (tree roots, frequent obstacles), emit very high frequency clicks (more than 50 kHz, inaudible by most other animals), probably allowing them a high precision (Video 4).
Video 4. Clicks emitted by an Amazonian pink dolphin (Inia geoffrensis). To be audible, the frequency of the sounds has been divided by 5. Sounds recorded by the CNRS DYNI team in Peru, near Iquitos. [Source: Animation Franck Malige and Julie Patris]