ECHOES OF BATS AND MEN 



whichever medium conveyed it to the surface. That is, 

 airborne sound waves are reflected back ahnost totally 

 from the water, and underwater sound is equally well 

 reflected back downward from the surface. Even if we 

 dive beneath the water, we do not hear as well as fish can. 

 This helps to explain why the noises made by certain fish 

 and whales are so seldom noticed, though they have been 

 known for centuries to fishermen and whalers. Even 

 biologists have been slow to realize that fish can hear 

 underwater sounds. Nevertheless, all fish have inner ears 

 basically similar to our own, and while the sound waves 

 reach these auditory sense organs by different routes 

 (through the body itself rather than through air-filled 

 canals), they stimulate the auditory nerve in very nearly 

 the same way. 



At frequencies up to about 1000 cycles per second 

 (c.p.s.) the minimum amount of sound energy audible 

 to a catfish is below the minimum energy detectable by 

 the human ear. This includes the range of many musical 

 instruments and the fundamental pitch of the human 

 voice. At higher frequencies fish are less sensitive to 

 sounds than we are, but their hearing is not inferior to 

 that of land animals in any basic way. 



An ability to hear underwater sounds is still far re- 

 moved from the bUnd man's problem of learning how 

 to use echoes for obtaining more information about his 

 surroundings. With fish there is only suggestive evidence 

 that certain species may utilize echoes. But marine 

 mammals, the whales and porpoises, are not only more 

 closely related to ourselves but also have almost as highly 

 developed brains. Their cerebral hemispheres rival ours 

 in size and complexity. Porpoises, which are no larger 

 than a man, have extremely well developed inner ears 

 and equally prominent auditory areas within their brains. 

 Nor are they silent creatures. Once proper equipment 



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