Bats are not blind. In fact, they see just as well as humans do, and like humans, their night vision is mediocre. But at night, insects abound and competing predators are fast asleep, creating the perfect opportunity for bats to feast. During seasons when they are not hibernating, clouds of carnivorous bats fly out from caves and trees to hunt for insects using a process called echolocation that allows them to navigate and locate prey with sound. While hunting, they can eat up to 1200 insects an hour, making them indispensable to farmers.
A little over half of bat species echolocate. Most of these echolocating bats fall into a group called “microbats.” Although they are some of the smallest bats out there (it is not uncommon for a microbat to be the size of a human thumb), microbats are also some of the ugliest. Unlike their cuter counterparts, megabats, who basically look like foxes with wings and often only eat fruit, microbats have beady little eyes, wrinkled pig noses, and oversized ears. Consider them the pugs of the bat world. It is partly these oversized ears, however, that allow them to distinguish between objects half a millimeter apart.
The process of bat echolocation starts in the throat. Bats emit a clicking sound, almost like a chirp, through their mouths into their surroundings. These sounds, referred to as “calls”, are mostly ultrasonic, meaning they are at frequencies that the human ear cannot detect. And this is fortunate for us, because many bats (like the Myotis Lucifugus) produce calls as loud as 120 decibels. For reference, this is louder than a fire alarm ten centimeters from your ear. Bats are also mostly deaf to their own calls. The bones that connect the ear drum to the inner ear detach from each other when the bat emits its call so that no vibration is sent to the inner ear.
The calls permeate the surrounding area, reflecting off of trees, vines, other bats, and prey as the bat hunts. The sound waves then return to the bat’s ears after being changed by the surroundings. It processes the discrepancies between ingoing and outgoing sound waves to construct a mental map of the area. Bats are only able to process one of these call-and-echo pairs at a time, rather than a continuous stream of sound.
Bat calls range between 0.2 to 100 milliseconds in duration. As the bat moves closer to its prey, the calls get shorter so the bat can more quickly process information about the prey’s location. The amount of time between successive calls also decreases as the bat approaches the prey. This is because the sound does not have to travel as far out and back, so the bat is able to update its mental map of the insect’s location more frequently.
There are two main strategies of calls that bats use when echolocating. The first is frequency modulated sweeps. This is when the bat’s outgoing signal covers a broad range of frequencies, usually starting from high frequencies and going to low all in one call. This is good for when the bat is in an area with a lot of noise, such as trees or other bats. There is more information going out, including different harmonics, and therefore more information can be gained from the resulting echoes.
The second call strategy is using constant frequency tones rather than changing the frequency within the call. This is used when a bat is flying in an open area without much interference. It requires less energy from the bat than frequency modulation does. Another advantage of constant frequency calls is the detection of doppler shift, or the shifting of the frequency of sound waves as the source of the sound moves relative to the observer. When the source is moving toward the observer, each wave is emitted at a distance closer to the observer than the previous wave. This causes the sound waves to reach the observer at a quicker and quicker rate (or frequency!). Think about when an ambulance drives past you on the sidewalk. The pitch of its siren is raised until it passes you, and then it gets lower.
Similarly, a bat call goes out at one frequency, and the echo from a moving insect comes back at a lower or higher frequency. From this change, the bat can not only tell that the insect is moving, but it also knows its speed. Not to mention that tiny vibrations from insect wings even allow bats to know if their prey is in flight.
Even cooler is that bats compensate for doppler shift by raising or lowering the frequency of their calls so that the echo comes back at the optimal range of frequencies it can hear. This phenomenon was discovered by strapping a bat to a swing, facing it toward a wall, and recording the way the bat lowered and raised its call frequency depending on if it was going toward or away from the wall. Interestingly, different bat species that live in the same areas have evolved to hear best at frequencies distinct from one another so as not to interfere with the others’ hunting. How sweet of natural selection.
Before writing this, my experience of bats consisted only of the scene in The Office where Meredith contracts rabies after Dwight traps a bat in a bag around her head. And although they do carry disease and can look unsettling, microbats are incredible and important animals that can teach humans a great deal about using sound to navigate. For example, check out this article about a blind human man that uses echolocation!
References and cool stuff:
- http://chargedmagazine.org/2013/12/badass-biology- bat-echolocation/
- https://www.scientificamerican.com/article/how-do- bats-echolocate-an/