224 WHALES 



obstacles in much the same way. Let us see Avhat evidence there is to 

 support this hypothesis. 



First of all, there is the pitch of the notes which they emit and to which 

 they are known to respond. Secondly, Kellogg, Kohier and Morris, all 

 American zoologists, discovered in 1953 that, as far as Bottlenose Dolphins 

 are concerned, sounds somewhere between 7 and 1 5 kilocycles are emitted 

 with continuously changing pitch, while sounds between 20 and 1 70 kilo- 

 cycles (i.e. the noises resembling" a squeaking door) consist of a series 

 of very short blasts of variable duration. Both effects, i.e. frequency and 

 pulse modulation, are also used in radar; and pulse modulation, in 

 particular, is used in echo-sounding and in asdic. 



Further evidence is the fact that whales can be trapped in nets - a time- 

 honoured method used in Japan and elsewhere. The animals never try to 

 break through this fragile barrier and healthy Bovitus, porpoises, and 

 River Dolphins of the genus Sotalia are known to be capable of avoiding 

 every kind of net. Thus McBride reports that Bottlenose Dolphins give a 

 wide berth to all nets of fine mesh, even when the sea is turbulent or the 

 water muddy. They simply jump out of the water to clear the obstacle, 

 and only when the mesh is ten inches square or more do they ignore its 

 presence and allow themselves to be caught. All this fits in with our asdic 

 hypothesis, just as do Schevill and Lawrence's experiments on the way in 

 which Bottlenose Dolphins find their prey in the dark. At first Schevill and 

 Lawrence failed to discover any form of ultrasonic sound emission used 

 in echo-locating, but during their latest experiments (1956) in a very quiet 

 pool near Woods Hole they did in fact note that, under special conditions, 

 the animals emitted very weak sounds, not normally detectable. Thus, 

 whenever they came near to a fish that was being offered them, they 

 emitted clicking noises with a frequency of between 100 and 200 kilo- 

 cycles. Even more convincing are experiments carried out by Kellogg in 

 1958 and 1959 in a specially constructed echo-free pool on the Florida 

 coast. The pool contained turbid water to exclude the visual factor, and 

 the experiments were therefore carried out 'in the dark'. Even so, the 

 Bottlenose Dolphins steered clear of a host of obstacles and produced 

 characteristic clicks or squeaks with frequencies above 100 kilocycles the 

 moment a new obstacle was put into the tank. Worthington and Schevill 

 noted similar sounds being made by a Sperm Whale off" Cape Cod. Very 

 convincing experiments have been made by Norris (Marineland of the 

 Pacific) and Wood (Marineland, Florida). They blindfolded Bottlenose 

 Dolphins with rubber suction cups and observed that the animals swam 

 about the tank without any indication of uncertainty. When a fish was 

 thrown into the water the animals emitted their sofar sounds and swam 

 unerringly to the food. We may therefore safely conclude that all the 



