ECHOES OF BATS AND MEN 



into their tank— they soon learn to swim directly to the 

 splash from wherever they may be. The eager approach 

 of the hungry porpoise could be explained as a simple 

 localization of the "loud" splash which they had learned 

 meant food. But two careful experimenters, William 

 Schevill and Barbara Lawrence (Mrs. Schevill), work- 

 ing at the Woods Hole Oceanographic Institution, 

 Woods Hole, Massachusetts, in 1955, noticed that their 

 captive porpoise found small, silent bits of food by echo- 

 location. The porpoise spent much time searching the 

 pond for food, and in doing so he emitted faint creak- 

 ing noises which could be detected only with sensi- 

 tive underwater listening equipment. They were not au- 

 dible to a person listening from the bank of the pond 

 or to a swimmer with his head under water. The creak- 

 ing consisted of a series of clicks repeated at varying 

 rates, sometimes so fast as to become a grating rasp or 

 buzz. Suspecting that the animal might be listening for 

 echoes from fish, Schevill and Lawrence sought to learn 

 whether he could detect and recognize a small dead fish 

 by echolocation and, if so, at what approximate dis- 

 tance. To eliminate vision, they frequently worked on 

 dark nights, and in any case their experimental pond, 

 only about 20 meters (about 65.6 feet) in diameter, was 

 stirred into a muddy soup by the constant swimming of 

 the porpoise. Even a brightly painted piece of metal be- 

 came invisible in bright sunlight when immersed to a 

 depth of about 60 centimeters (about 23.6 inches). 



When a man sitting in a small boat tied to the shore 

 quietly held a dead fish a few centimeters under water, 

 the porpoise learned to swim toward it, "creaking" aU 

 the time, and seize the morsel. To make the test more 

 critical as far as the distance of detection was concerned, 

 a fish net was placed perpendicular to the bank, as 

 shown in Fig. 1. The net extended out 2.4 meters from 



22 



