Hearing and acoustic orientation in marine animals 4 1 3 



Sounds and hearing of marine mammals 



A variety of complex sounds are produced by several species of ihc smaller looihcd 

 whales, porpoises, and dolphins. As in the case of fish, a few observations of noises 

 from cetaceans had been reported many years ago, but only during and after the war 

 was it reahsed how vociferous some of them actually were. The white whale or beluga 

 Delphinapterus leucas had long been known to make noises, for these could be heard 

 in the air or through the hull of a ship; and Schevill and Lawrlnci. (1949) have 

 described and analyzed in some detail their large vocabulary that includes whistles, 

 squeals, chirps, clicks, and rasping noises. Several other species of porpoises and 

 dolphins have also been studied in captivity or under natural conditions, and all 

 appear capable of producing sounds (Schevill and Lawrence, 1949; Kritzlkr. 

 1952). The baleen whales, on the other hand, have not yet been definitely shown to 

 emit sound, although there have been reports from several observers of loud, low 

 pitched notes heard when humpback whales (Megaptera nodosa) were in the vicinity. 



Excellent hearing has been demonstrated in porpoises and other cetaceans ranging 

 in size up to the pilot whale or " blackfish " Glohicephala macrorhyncha (Ki,i.i.(KiG 

 and KoHLER, 1952; Schevill and Lawrence, 1953; Kritzler, 1954). While 

 auditory thresholds have not yet been measured, the observational evidence is 

 adequate at least for the bottlenosed porpoise Tursiops truncatus to demonstrate that 

 sensitivity of hearing is high. In the most carefully controlled of these studies, 

 Schevill and Lawrence trained a Tursiops to come for food when it heard pure 

 tones of short duration generated without switching transients or clicks. The inten- 

 sity level was of the order of 10"'- watt/cm- or 1 dyne/cm-, and this porpoise learned 

 to respond almost every time the tone w?is sounded in the frequency range from 150 

 c.p.s. to 120 kc, i.e. the animal's auditory threshold was below this level over a very 

 wide range of frequencies. Above 120 kc. the percentage of positive responses fell 

 rather rapidly to less than 20% at 150 kc. 



Since marine mammals are often active at night or in turbid waters, it is natural to 

 suggest, as Kellogg, Kohler, and Morris (1953) have done, that some of their 

 complex and impulsive sounds may be used for echolocation, either to maintain 

 orientation with respect to the bottom, surface, or large obstacles, or possibly to 

 locate fish or other prey in the water as bats appear to do in the air (Griffin. 1953 a). 

 Kellogg et ah have shown that certain of the noises emitted by these porpoises have 

 components of considerable amplitude as high in frequency as 120 kc. and some 

 energy as high as 170 kc; and when intense sounds of short duration are generated 

 in the water there will of course be echoes from any solid objects in the \icinity. 

 Since porpoises have a high degree of adaptability and intelligence as well as keen 

 hearing (McBride and Hebb, 1948; Wood. 1953; and Lawrence and Schevill, 

 1954) it is plausible to infer that they might take advantage of the possibilities of echo 

 ranging. No direct evidence has yet been forthcoming, however, to support this 

 inference; and indeed Schevill and Lawrence report that porpoises remained 

 silent, as far as could be discerned, even during long periods of swimming about 

 through turbid water where it would seem that echolocation would have been most 

 helpful. It may well be that porpoise sounds are used solely or primarily lor com- 

 munication, especially since solitary porpoises seem ordinarily to be s.IaU 

 Yet the observation of McBride (in press) that porpoises can avoid small mesh but 

 not large mesh nets in turbid waters points towards some type of acoustic orientation. 



