l86 THE SENSES OF ANIMALS 



its own voice as a definite pitch differing if only a little from 

 that of others. The squeak of most kinds of bats sweeps 

 through a very wide range of frequencies in a five-hundredth 

 of a second, falling from a high note to a lower one. This 

 frequency modulation probably helps the bats' power of 

 discriminating the echoes returned from objects at different 

 distances. 



As mentioned above, there is another way of locating 

 objects by means of reflected sound waves, provided the 

 object and the locator are moving with relation to each 

 other. If you are standing on the platform of a railway 

 station and an express train blowing its whistle dashes 

 through without stopping, the note you hear appears to rise 

 in pitch to a peak as the train approaches and then to fall 

 away again as it recedes. This is not just because the noise 

 is louder when it is nearer, for the note given out by the 

 whistle is constant in pitch. This "Doppler effect" is caused 

 by the source of the sound being in motion ; if the note 

 emitted has, say, a frequency of 5,000 a second, more than 

 5,000 vibrations a second reach your ear when the whistle 

 moves towards you and less when it moves away. The sonar 

 system of certain kinds of bats makes use of the Doppler 

 effect. These are the Horseshoe bats, so named from the 

 presence of a bare flap of skin, shaped something like a 

 horseshoe, on the face surrounding the nostrils. The sonar 

 squeaks of the Horseshoe bats, unlike those of the others we 

 have already discussed, are not frequency-modulated but 

 consist of practically a single frequency. The squeaks or 

 bursts of sound last much longer than those of the other bats 

 and may occupy as much as a tenth of a second instead of 

 one two-hundredth at most. There is, too, another difference : 

 the Horseshoe bats emit their squeaks through the nose, 

 whereas the others emit theirs through the mouth; and the 

 horseshoe of skin, or "nose-leaf", acts as a trumpet to con- 

 centrate the ultrasound into a narrow beam which can be 

 likened to that of a searchlight. The Horseshoe bat twists 

 and turns its head about, directing the beam in different 

 directions as it scans its surroundings, and by means of the 

 Doppler effect it is able to fill in the details of the information 

 it obtains by the direct reflection of echoes. (See Plate 13.) 



