Cetacea 



Bullock et a1 . (1968) played frequencies from 5 - 150 kHz to 29 

 anesthesized test animals, including Stenella coeruleoalba , S. attenuata , 

 Steno bredanensis and Tursiops gilli . Interspecific sensitivities were not 

 significantly different and were in close agreement with the behavioral 

 audiograms for Tursiops of Johnson (1966) and Ljungblad (Pers. comm.). 

 Auditory evoked potentials were noted from 5 kHz to 120-140 kHz. Maximum 

 sensitivities were 35 dB at 60 kHz. High sensitivities were noted from 20 - 

 80 kHz. Sensitivity decreased by approximately 100 dB/octave above 100 kHz. 

 In 1972 these tests were repeated on alert Tursiops truncatus (Bullock and 

 Ridgway, 1972). Responses to 120 kHz were noted. Maximum sensitivities were 

 from 40 - 60 kHz. The intensity at maximum sensitivities was approximately 60 

 dB greater than at 10 kHz where the evoked potentials were barely determinable. 



Seeley, Ridgway and Flanigan (1976) tested seven alert Tursiops 

 truncatus at frequencies between 5-200 kHz. Probes were mounted to the skulls 

 of alert animals and using a special artifact inhibiting system an audiogram 

 was obtained. Thresholds were obtained throughout the test frequencies. 

 Maximum sensitivities were around 70 kHz at 54 dB for the most sensitive 

 animal . 



An electrophysiological audiogram was completed at the Naval Ocean 

 Systems Center on the same Tursiops truncatus gilli used by Ljungblad for a 

 behavioral audiogram. The results of the two auditory threshold measurement 

 techniques for this cetacean were within 10 kHz. 



Using a cochlear microphonics technique on Lagenorhynchus obliquidens 

 and Tursiops truncatus , McCormick (1968) obtained several audiograms. In this 

 measurement technique an electrode mounted to the round window of the cochlea 

 monitors electrical potentials produced by firings of the hair cells. By 

 changing the frequency and intensity of tones played to the test subject 

 audiograms are obtained. McCormick tested auditory thresholds for signals 

 transmitted to the cochlea by bone and ossicular chain conduction. Tests were 

 done with the test subject in air and underwater. Five audiograms were 

 produced. The air audiograms using bone conduction were more variable than 

 those taken underwater where marked notches were noted, particularly at 10 



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