184 THE SENSES OF ANIMALS 



an india-rubber ball against a wall and catching it as it 

 bounces back ; the shorter and more cleanly cut off the bursts 

 the less risk there is of them becoming blurred and distorted 

 in the process. They must be repeated at intervals such that 

 the echo of one burst arrives back before the next burst is 

 given out, for if more than one is in the air at the same time 

 it would be impossible to know which echo belonged to 

 which emission. For this reason the nearer an object is the 

 more quickly the bursts can follow each other because the 

 distance the sound has to travel is less. 



In most sonar the frequency of the sound waves used is 

 very high, so high that they are inaudible to human ears, 

 though perfectly audible to the animals that use them. There 

 are several advantages in using high frequencies, for one 

 thing they are reflected better than lower ones from very 

 small objects. To us, whose main sense is that of sight, and 

 whose whole way of life is centred round sight, the accuracy 

 and efficiency of sonar is almost unimaginable. Bats can fly 

 in the dark among a forest of obstacles without colliding 

 with them, and can pursue and capture small insects in rapid 

 flight, twisting and dodging to catch them with astonishing 

 speed and accuracy. 



The story of bats' sonar has been elucidated chiefly by 

 Griffin and his colleagues and others in America. Their first 

 experiments were simple and confirmed what had long been 

 known : that blindfolded bats are not handicapped in flight. 

 They then showed that if a bat's ears are covered it is not 

 able to avoid collisions when flying, and is in fact very 

 reluctant to take to the wing at all. If only one ear is covered 

 the bat can fly with moderate success but makes some col- 

 lisions, showing that its efficiency in locating its surroundings 

 is impaired. These experiments proved that bats are made 

 aware of neighbouring objects that they cannot see by means 

 of sound waves reflected from them. When the experimenters 

 covered the mouth and nose of the bats, but left the ears 

 uncovered, they found the animals were unable to fly without 

 collisions, proving that the sound waves reflected from objects 

 are produced by the vocal apparatus of the bats themselves. 



On making a closer investigation the experimenters found 

 that bats make ultrasonic squeaks at frequent intervals nearly 



