British Bats
Echolocation Calls
The method that bats use to navigate in the dark is known as echolocation. A number of different animals have developed echolocation. But it has reached its evolutionary peak in the bats. All families of bats are able to echolocate. Echolocation is the building up a picture of your surroundings by using the echoes of sound waves. An echo is the sound produced when sound waves are reflected from an object. For example if you shout in a large building, after a few seconds the sound will hit the walls and be returned to you, and you will hear this as an echo. The sound returned to you, has different properties from the sound you originally emitted; it has been changed while travelling between you and the object. Bats are able to interpret this information and use it to make a picture of their surroundings. In effect bats see but with sound. For example bats can easily measure how far they are from an object by using echoes. There is a delay from the time the sound is produced, and the time it takes the sound waves to travel to the object, be bounced back, and return to the bats eras. Bats can measure this short period of time and use it to work out how far away the object is from them. Echolocation is similar to the method called sonar, which submarines use to detect enemy ships. The version that bats use is much more advanced though. They can work out the speed of things travelling towards and away from them, and their is even evidence that they can recognize different colours with the use of echolocation! The sounds bats emit for echolocation purposes are known as echolocation calls. These calls are very high-pitched ultrasounds, which are higher in frequency than that which humans can hear, which means they are inaudible to us. As we are unable to hear these echolocation calls, how bats navigate remained a mystery until the twentieth century, when modern ultrasonic sound detectors were developed. Why bats use ultrasonic sounds will be covered later, but first we will look at how echolocation by bats was first discovered and the steps leading up to its discovery.
The Italian Spallanzani
How bats navigated in the dark was a mystery that the Italian scientist Spallenzani tried to answer using an ingenious experiment in the 1790s. Spallenzani wanted to find out how bats could easily fly around a room without bumping into things even though the room was in pitch darkness. He wondered whether bats had some sort of super sensitive sight, which allowed them to see in conditions that appeared to human eyes as being completely dark. To test this idea Spallenzani blinded some bats to see whether this would affect their ability to navigate successfully while in flight. Although rather gruesome, this experiment showed that the blinded bats could still manoeuvre equally as well in the dark as their sighted companions. Spallenzani realized therefore that bats were using some other sense to 'see' their way around, and were not relying on their eyesight. He wondered whether hearing played a part. In his next experiment he poured wax into some bats ears, before the wax cooled and set he pushed small tubes through the wax. These tubes could either by closed or left open. If left open the bat could still hear, if closed the bat was deafened. Now, when he studied the bats again he found that the deafened bats could no longer fly without problems. Instead they kept bumping into things. But when the tubes were reopened they began to navigate excellently again. Spallenzani had proved that bats sense their surroundings during flight not be using sight, but with their sense of hearing. Bats 'see' with their ears. Although Spallenzani had shown the way in which bats found their way in the dark, he could still not explain how it worked. This would have to wait until the twentieth century.
The discovery that bats use ultrasound
In the 1930s scientists were becoming more advanced and research into sound was progressing. The first generation of ultrasonic sound detectors had been made, and although basic were functional. Donald Griffen borrowed an ultrasonic detector from his biology lab and discovered that bats use ultrasonic calls in flight. Echolocation had been discovered!