204 WHALES 



Boltlenose Dolphins has been hunted by a particular boat, they subse- 

 quently avoid it like the plague, though other boats do not bother them. 

 This delicate differentiation between sounds is often found in cattle, 

 which show clear signs of nervousness Avhen they hear the approach of the 

 vet's car. Cetaceans, however, have a still more sensitive ear, which is 

 best seen from the fact that porpoises. Sperm Whales and at least some 

 of the other Cetaceans apparently react to asdic gear, a kind of under- 

 water radar used for depth sounding and also for locating solid obstacles 

 in the ship's path. Radar and asdic are both used for detecting reflected 

 waves, but \vhile radar emits and receives short radio wave-lengths, asdic 

 emits and receives ultrasonic vibrations. There is nothing mysterious about 

 such vibrations, except that they are too high in pitch to be audible to the 

 human ear. The pitch of a note depends on the number of vibrations its 

 source emits in unit time, and is usually measured in kilocycles (i,ooo 

 vibrations) per second. Now the limit of human hearing is between 15 and 

 20 kilocycles, while monkeys respond to notes up to 33, cats up to 50, 

 mice and rats up to 90, and bats even up to 175 kilocycles per second. 

 These animals can therefore hear a great many sounds that escape us 

 altogether, and would be able to react to asdic vibrations \vhich have a 

 frequency of between 20 and 40 kilocycles. 



The hearing of Cetaceans is second only to that of bats. Naturally, their 

 upper auditory limit is very difficult to determine, and all we know of the 

 bigger wdiales is that some species respond to asdic. However, much more 

 is known about the Bottlenose Dolphins in Marineland Aquarium. In 

 1953 Schevill and Lawrence taught one of these animals to come up for 

 food in response to a sound signal. The experiment cost 1,200 fish, but, 

 at the end, the two investigators knew that the dolphins could respond to 

 notes up to 153 kilocycles, though their response fell off at 120 kilocycles. 

 Kellogg and Kohier had noticed even earlier that the Bottlenose Dolphins 

 in the big tank were frightened by sounds between 100 and 400 cycles 

 (roughly the range between our lower c and upper a), but that they 

 merely swam a little more quickly when they heard sounds between 400 

 cycles and 50 kilocycles. 



This acute sense of hearing in Cetaceans is not surprising when we 

 consider how badly developed their other senses are. All the evidence 

 seems to point to the fact that they not only locate their prey by sound, 

 but that sound is also their chief means of communication. We shall 

 return to this subject in greater detail in the next chapter; here we shall 

 merely consider what they can hear and how their auditory organs work, 

 i.e. how sound vibrations reach their inner ear, and how auditory stimuli 

 are transmitted from it to the brain. 



Since whales are aquatic animals, we shall begin by comparing their 



