How do bats use echolocation? Echolocation works on the same principle as sonar in a submarine, but bats have special adaptations to be able to use it.
The principle behind echolocation and sonar is that you send a sound wave out and then measure how long it takes to come back to you and the strength of the returning sound. If you can do that, you can work out the distance to objects and even what objects they are. A submarine uses sonar to work out what is around it. A submarine has active and passive sonar. With active sonar, a sound is transmitted. If there is nothing near the submarine, the sound wave won’t come back. If there are rocks or another submarine nearby, the sound wave will be reflected back and picked up by the detector on the transmitting submarine. The onboard computer knows that the speed of sound in the ocean is 1,481 m/s. If the sound took 2 seconds to return, the onboard computer can work out how far away the sound was. As well as that, different materials will absorb or reflect different amounts of sound, and the computer can use the strength of the returning signal to work out what kind of object the sonar is picking up. The submarine also has passive sonar because it can’t always be transmitting sound, sound that an enemy submarine could pick up. Passive sonar only listens for soun,d and it can tell the onboard computer what is nearby, but it cannot tell how far away it is because it didn’t transmit the sound itself.
Bats do all of this, but without a complex onboard computer. They only have their mouths, their ears, and their brains. Bats can produce sounds that are much higher than we can, and their ears can hear frequencies that are much higher than we can. Our ears can pick up frequencies between 20 Hz and 20 kHz, depending on our age. The older we get, the fewer high-frequency sounds we can hear. Bats produce a sound that is between 9 kHz and 200 kHz. That means we can hear some of the lower frequency sounds bats make, which is why you can hear them squeaking at night if you listen carefully. But, the majority of their sounds we cannot hear.
To make these very high-frequency sounds, bats have to have a special kind of larynx. Their larynx differs from ours because they have 6 micrometer thick vocal membranes. That is much thinner than a human hair. These very thin membranes can vibrate independently of the rest of the larynx, and because they are so thin, they can vibrate incredibly quickly, producing the high-frequency sounds. These frequencies are called ultrasound, and they don’t travel very far. That means the bats have to produce them at very high volume so the sound wave will have enough energy to hit an object and return to the bat. They use the skin folds in their snouts to amplify the sound as well. If you could hear them, they would be screaming.
They need to make high frequency sounds because the length of a wave is directly proportional to its frequency. Low frequency waves have a long wavelength and high frequency waves have a short wavelength. Bats need to have short wavelengths because insects are very small, and a longer wavelength would pass right by the insect.
Sending out ultrasound is one thing, but, of course, bats need to be able to pick up the sound again as well. To do this, their ears have adaptations. Firstly, their ears are very large in proportion to their heads. They can also turn their ears independently and change their shape to act like a funnel for sound. They also have more nerves in their ears devoted to high frequency sounds than we do, so they can detect even tiny variations in frequency. This allows them to instantly work out the distance to their prey, and they can analyze the strength of the signal into each ear to work out which way the prey is as well.
Bats have to produce such a loud sound and their ears are so sensitive, that they could theoretically deafen themselves. To avoid this, they automatically close their ears when they emit the sound and then open them again straight away to receive the returning sound. They can open and close their ears ten times a second. They close their ears by separating the tiny bones in the inner ear.
Bats have far more mobility than birds and can turn extremely quickly. To avoid them, several insects have interestingly evolved defense mechanisms. Some moths have an organ called a tympanum, which is triggered by the ultrasound and automatically makes the moth’s muscles twitch, so its flight path becomes erratic and it is harder to catch. Tiger moths can release their own ultrasound, which mixes with the bat’s and sends back a confusing signal. And some poisonous moths send out an ultrasound warning that bats can pick up. And this is what I learned today.
Sources
https://www.thoughtco.com/how-bat-echolocation-works-4152159
https://www.nps.gov/subjects/bats/echolocation.htm
https://dnr.maryland.gov/wildlife/Pages/plants_wildlife/bats/batelocu.aspx
https://www.usff.navy.mil/Community-Outreach/US-Navy-Stewards-of-the-Sea/Science-of-Sound
https://en.wikipedia.org/wiki/Animal_echolocation
https://www.livescience.com/bat-conservation-echolocation.html
https://pmc.ncbi.nlm.nih.gov/articles/PMC9707786
Photo by Petr Ganaj: https://www.pexels.com/photo/bat-flying-low-above-water-17818819/