ECHO-LOCATION 189 



for the gap thus produced and escape. They are able to do 

 this in turbid water and on moonless nights, and McBride 

 inferred that they were locating the obstructions by echo- 

 sounding, by listening to their own voices reflected from the 

 fine-meshed net or the corked line. Experiments to test this 

 suggestion have amply demonstrated its truth, and it has 

 been shown, particularly by Schevill and Lawrence in 

 America, that dolphins can quickly find a fish no larger than 

 a herring in water so turbid that light can penetrate less than 

 a yard. The sonar system of whales is analogous to radar, 

 and consists of a series of impulses whose echoes give infor- 

 mation about the range and bearing of the objects scanned. 

 The series of ultrasonic pulses is made at repetition rates of 

 less than ten to over four hundred a second ; although the 

 ultrasonic component is inaudible to human ears the series 

 of impulsive clicks produces a note that can be heard, the 

 slower rate sounding like knocks, the faster like snarls, whines 

 or the creaking of rusty hinges. The accuracy of direction- 

 finding is aided by the whale weaving the head from side to 

 side as it approaches an object, so that it can orientate itself 

 with the signals received by each ear exactly balanced ; 

 furthermore the pulse repetition frequency is increased as 

 the animal approaches an object and the time for the return 

 of the echo becomes less. 



The sonar of whales is surprisingly subdued, and it is often 

 difficult for underwater listening apparatus to distinguish it 

 from the background noise. But whales make other sounds 

 that are far from subdued — whistles, grunts, groans and the 

 liquid trills of the sea canaries. These sounds are communica- 

 tive, conversation if you will ; they are rarely uttered by 

 solitary captives, but two or three animals kept together are 

 very loquacious, and the hubbub made by a large school of 

 dolphins is astonishing. 



Nearly all the observations on whale voices have been 

 made on the toothed whales, because this division of the 

 Cetacea, although it includes the mighty sperm whale, con- 

 tains the smaller species that can be kept in captivity; the 

 whalebone whales that feed by filtering plankton from the 

 sea are so huge that no oceanarium is large enough to hold 

 them, even if they could be captured unharmed. Seldom 



