Bats: Echo location
Today, most bats fly at night and it is likely that this was always the case since the birds had already laid claim to the day. To do so, however, the bat had to develop an efficient navigational system. It is based on ultra sound like those made by the shrews and other primitive insectivores. The bats use them for sonar, an extremely sophisticated method of echo location. This is similar in principle to radar, but radar employs radio waves whereas sonar uses sound waves. These are frequencies that lie a long way above the range of the human ear. Most of the sounds we hear have frequencies of around several hundred vibrations a second. Some of us, particularly when we are young, can with difficulty distinguish sounds with a frequency of 20 000 vibrations a second. A bat flying by sonar, uses sounds of between 50 000 and 200 000 vibrations a second. It sends out these sounds in short bursts, like clicks, twenty or thirty times every second and its hearing is so acute that from the echo each signal makes, the bat is able to judge the position not only of objects around it but of its prey which is also likely to be flying quite fast. Most bats wait to receive the echo of one signal before emitting the next. The closer the bat is to an object, the shorter the time taken for the echo to come back, so the bat can increase the number of signals it sends the closer it gets to its prey and thus track it with increasing accuracy as it closes in for the kill.