Many of us have heard the term ‘as blind as a bat’ but bats are not blind at all, most have eyesight that is on a par with our own. However, for nocturnal species, that eyesight is fairly redundant in the dark so bats use sound to help them see, it is an acoustic process known as echolocation.
Animals that use echolocation use sound, rather than vision to map out their surroundings. They rely on their voices, their ears and echoes.
Sound is a vibration that travels away from its source in waves. When an animal uses echolocation it creates those sound waves with its voice, often by creating clicking or chirping noises.
These waves of sound move out from the animal in all directions; when they hit an object, they bounce off it and return to the animal as an echo. The animal uses its ears to pick up the varying echoes that return and its brain then interprets these sounds and builds up a mental map of the surrounding space.
Of course, if sound is bouncing off everything in the area then that is a lot of information being fed back to the animal. How does it filter out the information it does not need
and focus in on what it does need to know, like the exact location of a nearby meal?
If we take the example of bats, they will overcome this problem by changing the type of chirping sounds they create. They will emit chirps of different frequencies, starting with lower frequencies and then moving up to higher frequency chirps. These different chirps will bounce off objects in different ways, depending on their size, shape, density and distance from the bat, allowing the bat map the area and dismiss what details it does not require.
Bat chirps can be very loud (up to 140 decibels) so, to protect their own hearing, they turn off their middle ear just before emitting these sounds. They restore their hearing a fraction of a second later, to listen for echoes.
Apart from bats, it seems a small number of other animals can use echolocation as well. They include whales, dolphins, some species of birds (such as swiftlets), shrews and small mammals called Tenrecs, found only in Madagascar.
Echolocation is not something we associate with humans but some people have actually trained themselves to map out their surroundings based on the echoes they receive back from clicking sounds they make. These people rely on sound instead of sight because they are blind. Studies have shown that the visual part of their brains is active while they echolocate and they describe being able to create a fairly accurate 3D map of their surroundings, including object position, depth perception and even a sense of density and texture.
Moths make a tasty meal for a hungry bat. Just like some insects will announce that they are poisonous with colour, some moths will do the same with sound. Certain tiger moths, for example, announce that they are poisonous to bats by emitting their own short bursts of clicks. Even more interesting though is one species of tiger moth (Bertholdia trigona) that emits clicks at very high speed, not to mimic the warning sound of their poisonous cousins, but to scramble the bat’s echolocation process. They effectively surround themselves in a veil of sound, confusing the bat and preventing detection.