SENSES OF BATS — VESEY-FITZGERALD 319 



The supersonic tone is not the only sound produced by bats. In 

 addition to the supersonic squeak, which cannot be heard by human 

 beings, they produce three other kinds of sound: (1) The ordinary 

 voice, which is the flight call, and which Hartridge has named the 

 signaling tone; (2) a buzz, which is audible if one is quite close to the 

 animal; and (3) a click, which is usually audible at a distance of 

 several feet. I am not competent to say whether the buzz differs in 

 any way with the species ; it sounds exactly the same to me whether 

 emitted by a serotine (Eptesicus serotinus) or a pipistrelle {Pipistrel- 

 lics pipistrellus) , nor can I distinguish any difference between the clicks 

 emitted by the different species. The flight calls, however, are quite 

 distinct. It is possible to distinguish between the species on the wing 

 by means of their flight calls (it is also possible with practice to dis- 

 tinguish between the types of flight), and I have elsewhere attempted 

 to translate the differences between the calls of the various species to 

 paper. It has been shown that the buzz and the click are accompanied 

 by the supersonic tone. In the case of the click there is a single short 

 burst of supersonic energy, but in the case of the buzz there is a con- 

 tinuous evolution of the interrupted supersonic tone. The flight call, 

 however, may be produced by itself or it may be accompanied by the 

 supersonic tone. Some bats, notably the lesser horseshoe {Rhino- 

 lophus hipposideros) ^ have a considerable vocabulary of ordinary 

 sounds, whereas others, notably the whiskered {Myotis mystacinus) , 

 scarcely ever make a sound of this sort. I have never heard the whisk- 

 ered utter a sound on the wing, and of the many I have kept in captivity 

 only a few have uttered faint grunts, and those very occasionally, when 

 at rest. 



How these four different types of sound are produced is not yet 

 clear, but examination of the vocal organs of bats shows that they are 

 very different from those of other animals. The larynx, which in 

 most animals is built of cartilage, in bats is made of bone and is com- 

 paratively a massive structure with large and powerful muscles. The 

 strength of the bat's larynx and its small size are obviously well 

 adapted to the production of supersonic sounds, for there must be 

 tremendous energy in the supersonic squeaks as compared with the 

 audible squeaks. The higher the note and the greater the frequency, 

 the greater the energy. 



Though it is not yet known exactly how the supersonic vibrations 

 are produced, it is known how the bat avoids hearing the squeak. It 

 is, of course, essential that only the echo should be heard by the bat. 

 The same difficulty had to be overcome in radar transmission. In 

 order to ensure that only the echo is picked up by the receiver, this is 

 suppressed while the transmitter sends. Something of the same kind 

 happens in the bat when emitting the supersonic tone. Griffin and 

 Galambos found that while the squeak is being made a muscle in the 



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