SECTION II 



UNDERWATER HEARING AND SOUND PRODUCTION OF MARINE MAMMALS 

 Anatomy 



The anatomy and function of the auditory and associated neural structure 

 for several species of odontocete cetaceans has been reviewed by several 

 authors (Fraser and Purves, 1960; Morgane and Jacobs, 1972; Bullock, Grinnell, 

 Ikezono, Kameda, Katsuki, Nonoto, Sato and Yanagisawa, 1968; McCormick, Wever, 

 Palin and Ridgway, 1970; Wever, McCormick, Palin and Ridgway, 1971). The 

 sound path to the inner ear is not well understood. One theory (Fraser and 

 Purves, 1960) concludes that the dolphin receives sound via the external 

 auditory meatus, while Norris (1969) suggests that the sound is received via 

 bone conduction through the fat layer of the lower jaw. Electrophysiological 

 recordings and cochlear microphonic measurements (McCormick, Wever, Palin and 

 Ridgway, 1970) has demonstrated that sound passing through the lower jaw 

 excites cochlear and mid-brain with greater intensity than those sounds 

 presented in the area of the external auditory meatus. These finding support 

 the theory presented by Norris (1969). 



The hearing capabilities of large whales are difficult to establish. 

 Anecdotal evidence is available suggesting that mysticetes respond to ship 

 noises, sonar pings and low-flying aircraft (Norris and Reeves, 1978). The 

 anatomical structure of the mysticete auditory structure has been reviewed by 

 several authors (Reysenbach de Haan, 1957; Dudok van Heel, 1962; Purves, 

 1966). Fleischer (1976) compared cochlear morphometries in extinct and extant 

 cetaceans, and concluded that mysticete cochlea have structurally evolved for 

 sensitivity to low frequency sounds as compared to odontocete high-frequency 

 sensitivity, although mysticete hearing for high frequency is probably \jery 

 good. 



The outer ear structure of pinnipeds has undergone considerable 

 modification. The external ear of the sea lion is very reduced and may serve 

 to close off the ear canal during diving; true seals have no external ears. 

 The middle ear of pinnipeds has undergone additional modification in order to 

 function in an aquatic environment; yet despite these modifications the 

 pinniped ear must also function in air. One theory suggests that with 



A- 7 



