ECHO-LOCATION 185 



all the time. The frequency of the vibrations given forth 

 varies, but is most commonly about 50,000 a second ; at this 

 pitch each squeak lasts for about two-hundredths of a second 

 or less. When bats are at rest they make ultrasonic squeaks 

 about ten times a second, but as soon as they start flying the 

 squeak rate goes up to about thirty a second ; the faster the 

 squeak rate the more information about the position of 

 surrounding objects is obtained in a given time — and a bat 

 in flight needs the information much more urgently than one 

 at rest. If a bat flies through a narrow aperture the squeak 

 rate becomes faster and faster as the bat approaches it, and 

 then returns to normal when it is safely through. (See 

 Plate 12.) 



Bats are often gregarious creatures and in some places they 

 roost in enormous numbers huddled close together in the 

 roofs of caves. When they take to the wing and start giving 

 out ultrasonic sounds there might appear to be a danger that 

 this babel of high-frequency sound would cause complete 

 confusion and put the sonar out of action. Two things prevent 

 this occurring: first, ultrasonic waves do not travel far in 

 air, but quickly die away just as ordinary sound gets fainter 

 and finally fades out as distance increases. The distance from 

 which the ultrasonic squeaks of bats are able to return a 

 useful echo is only about five yards, so that unless the bats 

 are close together they do not interfere with each other's 

 sonar. This probably also explains the amazingly quick 

 swerves and aerobatics that bats indulge in, for they have to 

 be very quick to turn when flying at full speed if they get 

 warning of an impending collision when they are only five 

 yards or less from an object. The second thing that prevents 

 confusion is that the frequency of the vibrations made by 

 any two bats is not necessarily the same, in fact it is extremely 

 unlikely that they will be of exactly the same pitch ; a dif- 

 ference of very few vibrations a second is probably enough 

 to distinguish them, so that if many bats are flying together 

 there is no danger of confusion because each will recognize 

 its own voice. 



The matter is not, however, quite as simple as it may 

 appear because the squeak made by each bat is not generally 

 of constant pitch or frequency, and so a bat cannot recognize 



