Ever lost your way in the dark? Finding your way in total darkness is more than just difficult... It can be impossible! But, bats can fly in total darkness and detect something as fine as a human hair. Today we call this unique skill "echolocation", but it wasn't until the mid 1930's that we had any idea what it was all about. During the thirties the first scientific apparatus was developed to detect high frequency sounds and it was then that the mystery of how bats could "see in the dark" was finally solved.
When we talk, yell or sing our larynx is vibrating in our throat causing pressure variations in the air passing through it. These pressure variations are picked up by the ear drums of the listener and interpreted by the listener as sound. Although the range of sounds we can make is limited, the range we can hear is quite broad. For humans, the range of audible sounds runs from a low of 20 cycles (vibrations) per second to a high of 20,000 cycles (vibrations) per second. For centuries we thought these were the only sounds to be heard. Then, with the development of high frequency (ultrasonic) sound detection systems we stepped into the auditory world of bats, dolphins, and a number of other animals. Bats have probably developed the most sophisticated echolocation techniques of any animal on earth. They started with the same mechanisms we use for vocalization and listening and refined them into a highly sophisticated sonar system. As they fly about in total darkness searching for insects they are calling out at frequencies of 20,000 to 120,000 Hertz. No, a Hertz isn't something you rent on vacation. In this case its a unit of measure equal to one cycle per second. Scientists who study these things say that some species of bats making echolocation calls a few inches from your ear would sound as loud as a smoke detector going off the same distance away. So, as you can see, bats are really very noisy animals. But the good news (or bad news, depending on how you look at it) is that we can't hear them.
Over the years bats have learned how to interpret these sounds and use them to hunt insects and maneuver in total darkness. How do they do it? Well, the high frequency sounds are reflected back to the bat like light from a mirror and the bat gains information from these reflected signals. For example, the length of time it takes the sound to travel from the bat to the object and back again reveals how close the object is. Its much like counting off the seconds between when lightning strikes and thunder is heard. Since the sound traveled approximately 1100 feet for each second that passed, one can estimate how far away the lightning was when it struck. This, of course, is a crude description of what the bat is doing with the reflected sounds. Obviously, it is gaining much more information from what is being heard. It might be helpful to imagine yourself tracking a mosquito by yelling at it with your eyes closed. Today anyone can listen to bats foraging for insects. Bat detectors can be purchased which pick up their high frequency calls and produce an audible tick (that means we can hear it) for each call made. As a bat flies about searching for food it is usually making 10 to 50 calls a second. When its sonar picks up a nearby insect, things begin to happen rapidly. The bat adjusts its course towards the insect. The insect initiates evasive maneuvers. The bat increases the rate of calls to 200 or more per second until they sound like a buzz on the detector. The insect darts to the side. The bat loops back. The buzz ends abruptly. The bat continues on. There is one less insect in the world.
A hungry bat can devour up to 600 insects an hour, which makes them pretty useful animals to have around. So look toward the sky just after sunset and watch the bats chasing insects and think how amazing it is that they are doing it with their ears.