ECHOES OF BATS AND MEN 



a complete mystery. But once this simple fact became 

 known, all seemed clear, at least in its broad outlines. 



As a matter of fact, the sounds with which bats guide 

 their flight are not totally inaudible. While more than 

 99.9 per cent of the sound energy emitted by bats that 

 have been studied most thoroughly is at frequencies 

 above the human range, there is also a faint audible 

 component. It is so faint that one is likely to suspect the 

 sounds come from the fluttering of the wings, and, in 

 fact, they were overlooked by Spallanzani. Whenever a 

 bat emits a burst of very high-frequency sound which 

 can be detected by suitable electronic apparatus, one 

 can also hear a faint tick accompanying each of them. 

 Perhaps some readers may have an opportunity to watch 

 and listen to bats on a warm evening. The bats foimd 

 in temperate climates often roost in crevices in buildings 

 and fly out every evening between sunset and complete 

 darkness. If one stands close to where they fly (1 to 2 

 meters), and if it is really quiet and you can refrain 

 from squealing, you can hear these ticking sounds. The 

 younger you are, the more easily you can hear them, 

 for even the audible component has a frequency of 

 roughly 5000 to 10,000 cycles per second. There are 

 also a few kinds of tropical, fruit-eating bats which make 

 clearly audible ticks when they fly in dark caves. Where 

 there is any light they use their eyes, which are much 

 larger than those of the insectivorous bats. Two kinds of 

 cave-dwelling birds also click loudly when flying in com- 

 plete darkness but rely on vision at other times. 



The faint ticking component of a bat's orientation 

 sound is of very short duration, not imlike the ticking 

 of a lady's wrist watch. But, unlike a watch, the bat's 

 ticks will vary markedly in their tempo. If it is flying 

 straight at some obstacles from a distance, there may be 

 from five to twenty ticks per second. But if the bat is 



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