Hearing and acoustic orientation in marine animals 4| 5 



about one to ten metres. Most of these sounds last for at least one second, so that in 

 the immediate vicinity of the fish there is ample time for interference and reinforce- 

 ment between successive waves to be set up, especially if it is close to the bottom or 

 to other solid objects, as is very often the case. At first thought one would expect 

 the principal nodes of such standing waves to be separated by distances equal to the 

 wavelength in water of the particular frequency involved; for 500 c.p.s. this would 

 mean nodes roughly three metres apart. Yet when I have arranged apparatus to 

 generate sounds of a few hundred c.p.s. in small tanks, and measured the resulting 

 sound levels with a hydrophone, there have always been wide fluctuations in the 

 sound pressure over distances that were only a very small fraction of the wave- 

 length. (See also Poggendorf, 1952, and Kleerekoper and Cmagnon, 1954, for 

 similar observations.) The physical basis for these variations in sound level is no 

 doubt somewhat complex, perhaps involving interactions between sound waves in 

 the water and in the materials of which the tank is constructed. Comparable con- 

 ditions would not ordinarily prevail in the open ocean, but they must often occur 

 near the bottom, rocks, or other hard objects. 



Since fish have keen hearing, they must experience fluctuations in the loudness 

 of whatever sounds are present as they swim about in the proximity of any major 

 acoustic discontinuity, and indeed their own movements would alter the standing 

 wave patterns. Since such changes in sound level bear some relation to the geometry 

 of the fish's environment, it is possible that they could learn to use them for orienta- 

 tion (for evidence that fish can easily learn to react to sounds see Bull, 1928, and 

 Haralson and Bitterman, 1950). Presumably such standing wave patterns would 

 be simpler, and hence more readily interpreted by fish, if they were caused by relat- 

 ively pure tones. Dijkgraaf (1933, 1947) and Kramer (1933) have described in fish 

 and amphibians respectively a type of orientation based on very low frequency 

 sounds or vibrations (or even perhaps static pressure). The sense organs involved 

 are those in the skin or lateral fine, rather than the ear. This type of orientation 

 which Dijkgraaf calls " Ferntastsinn " seems limited to distances of much less 

 than one metre. At higher frequencies the speciahzed inner ear and accessory struc- 

 tures provide a much greater sensitivity, and hence a potentially greater range of 

 acoustic orientation. It therefore seems desirable to devote some future research to 

 testing the possibihty that fish or cetaceans orient themselves by reacting to the 

 complex standing wave patterns set up in water near solid objects. 



Whether such a type of acoustic orientation would be based on variations in sound 

 fields generated by the fish itself or those from other sources, if it occurs at all, am 

 only be learned by further investigation. As stimuli for such investigations the 

 attention of interested readers is called to the papers of Supa, Cotzin, and Dallen- . 

 BACH (1944), Cotzin and Dallenbach (1950), and Twerskv (1951) tor convincing 

 evidence that blind men detect obstacles by acoustic orientation based on a variety 

 of continuous sounds, including pure tones. Furthermore Lissmann has reported 

 that certain fish orient themselves by means o{ electrical fields ol their own making 

 apparently sensing in some manner yet to be explained the changes in electrical 

 field due to the proximity of objects differing from water in dielectric properties. 

 (See Lissmann, 1951, and Gray, 1953, for preliminary accounts of these remarkable 

 findings concerning which no complete report has yet been published). In view of 

 the e4tence of such modes of orientation we should be prepared to lind cases of 



