ECHOES OF BATS AND MEN 



the boat, and the porpoise had to decide at more than 

 that distance which side of the net to choose in swim- 

 ming up to the boat where food might be expected. He 

 was fed irregularly, sometimes at one end of the boat, 

 sometimes at the other, but in cruising past under water 

 on a dark night he would almost never turn in closer 

 to the bank than the end of the net unless a fish was 

 being held beneath the surface. If the porpoise was not 

 "creaking" as he swam past, he did not swim toward 

 the boat even when a fish was offered. 



For the most significant experiments Schevill and 

 Lawrence sat at opposite ends of the anchored boat, 

 each holding a fish at arm's length as the hungry por- 

 poise swam past through the dark and murky water. 

 Sometimes one and sometimes the other would gently 

 and silently sHp a 15-centimeter fish just below the sur- 

 face, and if the hungry porpoise was "creaking" as he 

 passed by, he would usually swim in to pick up his fish. 

 In about three quarters of the tests he would choose the 

 correct side of the net, even though he had been ac- 

 customed to pick up his food about equally often at 

 either end of the anchored skiff. 



This and other experiments show that porpoises 

 can do more than simply detect isolated echoes of 

 their creaking sounds from objects as small as 15- 

 centimeter fish. More impressive still, they can discrimi- 

 nate such echoes from all the other echoes that are re- 

 turning from the bottom of the pond, the surface of the 

 water, the bank, the net, the bottom of the skiff, to say 

 nothing of the rocks and aquatic vegetation. 



If a porpoise can echolocate a 15-centimeter fish, 

 what prevents a blind man from hearing echoes from 

 objects of similar size that He on the floor or on a table? 

 Actually there are several factors which work to the 

 disadvantage of the porpoise. Sound travels about four 



24 



