14. SOUND PRODUCTION BY MARINE ANIMALS 



W. E. ScHEViLL, R. H. Backus, and J. B. Hersey 



1. Introduction 



Because sound is rapidly and efficiently transmitted by water, hearing is 

 often of greater utility than the other senses to active, wide-ranging marine 

 animals. Although the transfer of information by light is practically instan- 

 taneous, vision is useful only at ranges up to a few tens of meters in the clearest 

 of surface waters because light is rapidly attentuated in water. Besides, it is 

 night half the time. By using the chemical senses (taste and smell), exceedingly 

 low concentrations of various substances are detected by many aquatic 

 organisms, but the rate of transmittal between source and sensor in these 

 systems is dependent on mixing processes, which in the ocean proceed at 

 velocities of a few meters per second at most. The lateral line system of the 

 elasmobranchs and bony fishes appears to be a turbulence detector which, 

 while highly directional, operates at very short range. It is by hearing that 

 marine animals get information from great range and with little time delay. ^ 



Given effective hearing, the development of sound-producing mechanisms 

 seems a logical way in which to extend the usefulness of this sense. That is, 

 while the simple awareness by a species of the sounds of predators and prey is 

 of survival value, there are other ways in which sound may be put to use if it 

 can be purposefully self -generated. 



It is the intention in this chapter to consider the sounds produced by marine 

 animals — what these sounds are like, how they regulate the lives of the animals 

 which make them, how these sounds have been and might be studied, and how 

 they can be used by oceanographers to elucidate other problems. We are mainly 

 restricting ourselves to the use of sound by marine animals in their natural 

 environment, with only occasional allusions to laboratory studies of captives. 

 This is not to underrate the contributions that such investigations may make 

 to our understanding of the field problems, but is because we are not yet 

 certain how to separate the artificial influences of captivity. 



2. History 



As has often been pointed out, it was well known to the ancients that 

 certain animals made sounds under water. The writings of Aristotle (fourth 

 century B.C.), and subsequently those of Athenaeus and Pliny, describe sounds 



1 The "absorption coefficient" of underwater acoustics is analogous to the ''extinction 

 coefficient" of underwater optics, IJIq = e~^K where Iq is the intensity at a point of observa- 

 tion, I is the intensity at a second point of observation further removed from the source 

 by the distance I (by one meter in our examples), e is the base of the natural logarithm and 

 k is the coefficient. Optical extinction coefficients vary from about 7 x 10~2 (clear oceanic 

 water) to about 3.5 x 10"^ (turbid coastal water) for that part of the spectrum best trans- 

 mitted. The absorption coefficient for sound of five kilocycles per second is about 6 x 10~5. 

 Thus the rate of absorption of light by sea-water is about 1000 to 5000 times greater than 

 that of sound of mid-frequency. 



[MS received Septetnber, 1960] 540 



