What is echolocation?
Echolocation, also referred to as biosonar, is a method used by some animals as a means of sensing their environment. These animals are able to make specialised noises – usually high-pitched clicks or chirrups – that bounce off any objects around them and create an echo. The sound of this echo tells the animal about the size, shape, location, and movement of that object. Echolocating animals use this information to navigate, identify potential threats, and find their prey in situations where their vision is restricted.
What animals have echolocation?
The animals most renowned for their biosonar are odontocetes – the group of cetaceans known as toothed whales – and bats. Echolocation evolved independently in each of these taxa, as an adaptation for hunting in low-visibility environments. Bats are nocturnal predators, and have to locate small prey, such as insects, in environments crowded with obstacles, such as dense woodland, in complete darkness.
Toothed whales don’t necessarily hunt at night, but they do hunt underwater. Light is absorbed much more quickly in water than in air, which means that even in the best possible conditions, visibility in the ocean is never more than about 100 metres – most of the time, it’s not even that. Lots of factors can severely reduce visibility; for example, depth, sediment and algae in the water, and the mixing of fresh and seawater. Because of this, toothed whales cannot always rely on their vision when they are trying to locate and pinpoint their prey.
Echolocation has been well-researched in these two groups, but they are not the only animals to use biosonar. Some shrews and cave-dwelling birds such as swiftlets and oilbirds can also echolocate.
How does echolocation work?
Toothed whales produce echolocation signals by forcing pressurised air through the ‘phonic lips’: a set of two tissue complexes in the nasal passage just below the blowhole. Most odontocetes can operate each set of lips independently, allowing the animal to produce two different signals simultaneously.
The sound signal is then aimed forward through the forehead, which is full of connective tissues and the bulbous, lipid-filled ‘melon’. Air-filled sacs behind the phonic lips direct the sound and the melon focuses the beam as it is transferred into the surrounding water.
Odontocete echolocation clicks can be characterised as frequency-modulated (FM) sweeps, broadband high frequency (BBHF) clicks, narrowband high frequency (NBHF) clicks, or low frequency clicks, which are used only by sperm whales.
BBHF clicks are the most common type of echolocation, produced by many families of toothed whale including the dolphins. ‘Broadband’ in this context means that the sound signal is made up of a wide range of frequencies. BBHF clicks also tend to be much shorter than the other types of signal.
Beaked whales produce FM clicks. The frequency of the sound changes from low to high in what is called an ‘upsweep’. Beaked whale echolocation is lower in pitch than the clicks made by dolphins, but not as low as sperm whales.
Beaked whales are the most extreme divers of all cetaceans, often spending several hours at depths of up to 3km below the surface. They dive in pairs: swimming in synchrony during ascent and descent, parting once they reach a certain depth in order to hunt separately, then using echolocation to reunite for the end of the dive.
They do not echolocate through the whole dive, only in the latter part of their descent and when actively searching for food. It is thought that this helps the beaked whales to avoid being overheard by killer whales! Killer whales are, for most beaked whales, a major predator – however, they are not capable of diving to the same extremes. If beaked whales echolocate while ascending or while at the surface, that could give away their location to eavesdropping killer whales. So, they avoid making any noises where killer whales could hear them, and only use their biosonar once they are deep enough that they are out of their predators’ reach.
NBHF clicks are produced only by porpoises, dolphins in the genus Cephalorhynchus, one river dolphin (the La Plata dolphin), and the pygmy and dwarf sperm whales. These groups are not closely related, which means that this form of echolocation arose independently four separate times within the odontocetes!
The driving force behind this particular sound signal is most likely, once again, the threat of killer whales. NBHF clicks are so high-pitched that they are outwith killer whale’s hearing range, thus allowing these small toothed whales to use their echolocation without fear of being overheard by predators.
Sperm whales produce sound in a slightly different way to other toothed whales. Their blowhole and the phonic lips – the organ in the nasal passage that creates sound for echolocation – is positioned near the front of the head. The initial pulse of sound propagates backward through the spermaceti organ and bounces off an air sac in front of the skull. The reflected sound is then directed forward through the so-called ‘junk’ and beamed into the water. The spermaceti organ and the junk both help to keep the signal focused.
Sperm whale echolocation clicks are incredibly loud – some of the loudest sounds produced in nature!
Sperm whales click for almost the entirety of their descent. During the initial pulse, a lot of low-frequency noise enters the water and travels backwards. Because low-frequency sounds travel very long distances in water, this backwards pulse will hit the surface of the water and create an audible echo even when the sperm whale is several hundred metres below it. So, by clicking constantly, they can keep track of how far away the surface is – or, to put it another way, sperm whales have a depth gauge built in to their echolocation!
How do they hear the echoes?
Regardless of the type of signal produced, all echolocating animals face the same problem of differentiating between the emitted sound and the returning echoes. This can be solved in one of two ways: by separating call and return either by time, or by frequency.
A handful of bat species use a change in frequency to pick out the returning echoes. This is known as a high duty cycle (HDC) and occurs when the reflected signal shifts to a higher frequency, allowing the bat to identify the echoes as it closes in on its prey. For toothed whales, however, a HDC is simply not possible: their echolocation clicks are not narrow enough to show the same shift in pitch between signal and return. Instead, toothed whales operate on a low duty cycle (LDC). They emit a much shorter signal which is then followed by a longer silent interval, where the returning echo can be heard.
Cetacean hearing is slightly different to our own because they live in an aquatic environment, and, like light, sound travels in a different way through water and through air. In a land mammal ear, sound is collected by the pinna (the bits that stick out from our head!) and guided through the auditory canal to the eardrum which begins to vibrate. These vibrations pass into the fluid-filled inner ear, where they stimulate the nerve impulses to our brain.
Most of the structures of the outer and middle ear are designed to transport soundwaves through air into fluid without losing too much of the signal. But in the ocean, sound travels through a medium which is the same density as the inner ear. Odontocetes have completely lost their external pinna, and the ear canal is narrow and non-functional. Many components of the middle ear have also lost their function, as there is just no need for these intermediary structures to transfer sound energy form air to water. Instead, odontocetes hear through their jaws! The lower jaw is hollow and filled with fat; sound waves are transferred through this fat-filled cavity and pass directly into the fluid of the inner ear. Despite having lost the opening to their ear canal, cetaceans still have ear wax – but since the wax has nowhere to go, it builds up in layers over the course of the whale's whole life, forming a huge, waxy plug! Imagine the size of the cotton bud you'd need to clean that out!
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Jensen, F.H., Johnson, M., Ladegaard, M., Wisniewska, D.M. and Madsen, P.T., 2018. Narrow acoustic field of view drives frequency scaling in toothed whale biosonar. Current Biology, 28(23), pp.3878-3885.
Cholewiak, D., DeAngelis, A.I., Palka, D., Corkeron, P.J. and Van Parijs, S.M., 2017. Beaked whales demonstrate a marked acoustic response to the use of shipboard echosounders. Royal Society open science, 4(12), p.170940.