ECHOES OF BATS AND MEN 



density is such that one of these small bats would have 

 to fly all night before its mouth encountered a single 

 mosquito purely by chance. Yet the actual rate of capture 

 is one every few seconds. 



Photographs of the bat pursuing an insect show that 

 they sometimes begin their maneuvers when 2 or 3 feet 

 from a mosquito. The pattern of the orientation sounds 

 begins to change a fraction of a second before the bat 

 turns toward its victim. The implications of these obser- 

 vations can be understood after a brief explanation of 

 the rate at which a bat's frequency-modulated chirps are 

 repeated during various types of flight. When a Uttle 

 brown bat is flying fairly straight and is not close to any- 

 thing of immediate concern, it repeats its 1- to 2-milli- 

 second chirps at rates of 10 to 20 per second. But when- 

 ever it approaches any small obstacles, such as wires 

 stretched across a laboratory room to test its skill, many 

 more chirps are emitted in a given interval of time. For 

 brief periods the repetition rate may rise as high as 250 

 per second. When this happens, the individual chirps be- 

 come shorter, usually less than 1 millisecond, so that 

 silent intervals still exist between chirps. When the high- 

 frequency sounds of these bats are studied under natural 

 conditions, a clear distinction between straight and level 

 flight and the active pursuit of flying insects becomes ap- 

 parent. Such eavesdropping is only possible, of course, 

 when we have apparatus which will detect the bat sounds 

 that are inaudible. In one convenient form this apparatus 

 "translates" each of the bat's high-frequency sounds into 

 audible clicks in earphones or a small loudspeaker. This 

 makes it possible to watch the bat while at the same 

 time "listening" to its orientation sounds in this trans- 

 posed form. Most of the details, such as the octave of 

 frequency modulation, are lost, but there is one cUck 



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