Stand by a pond on a summer evening and you may see bats darting and swooping just above the water. They change direction in a split second and pluck insects out of the air in light so dim you can barely see your own hand. How do they pull it off?
Many bats use echolocation. They send out calls, mostly too high-pitched for people to hear, and listen for the echoes that bounce back off objects around them. By comparing the outgoing call with the returning echo, a bat can work out where things are, how far away they are, and even what they might be. In effect, the bat builds a picture of the world out of sound.
How a Bat Reads an Echo
A bat has one sound source and two receivers: its ears, set slightly apart. An echo from an object off to one side reaches one ear a tiny bit sooner and a bit louder than the other. Bats use those small differences in timing and loudness to judge direction and distance.
By scanning again and again, a bat can build an accurate image of its surroundings and of its prey. Echolocation can reveal not only where something is, but also how big it is and what kind of animal it might be.
Most echolocating bats make their calls with the larynx, sending the sound out through the mouth or the nose. Some groups have fleshy flaps on their noses, called nose-leaves, that help shape the sound.
Loud, High, and Speeding Up
Bat calls range from about 11 kilohertz to 212 kilohertz. Most fall above the range of human hearing. Some calls are astonishingly loud, reaching over 140 decibels, and certain bats can lower the volume mid-call as they near something that reflects sound strongly, so the echo doesn't deafen them.
The rhythm changes during a hunt. While searching, a bat may click about 10 to 20 times a second. Once it locks onto an insect, the clicks speed up, ending in what scientists call a terminal buzz of up to 200 clicks a second. The bat's calls are timed with its breathing and wingbeats, so all that noise costs it very little extra energy.
No, Bats Aren't Blind
The old saying "blind as a bat" is simply wrong. No bat species is truly blind. Echolocating bats tend to have small eyes, but they are still sensitive to light and may use their vision to travel between roosts and feeding areas, since echolocation only works over short distances.
Not all bats echolocate, either. Most of the large fruit bats, sometimes called flying foxes, don't use this kind of sonar. They generally have good eyesight and may even see some color, which helps them pick out ripe fruit.
An Arms Race in the Dark
Insects have fought back. Some moths can hear bat calls and react by flying in random, zigzag escapes. Certain tiger moths make their own ultrasonic clicks to warn bats that they taste bad, and some can even jam a bat's sonar.
Bats have countermoves too. One species that feeds on moths uses an unusually low call that moths can't hear. Others, nicknamed whispering bats, call so quietly that moths have a harder time noticing them coming.
People took a long time to figure all of this out. In the 1700s, the Italian scientist Lazzaro Spallanzani showed that bats rely on some sense besides vision at night. It wasn't until the 1940s that the American zoologist Donald Griffin, working with Robert Galambos, demonstrated echolocation and gave it its name.
So the next time a bat flickers past your porch light, picture the scene through its ears: a stream of rapid calls, a cloud of echoes, and a flying insect that just ran out of luck.
