kHz. In the test, tones up to 250 kHz were played to a Tursiops truncatus ; 

 results above 100 kHz were not given by the author. With LC-10 hydrophone 

 used as a sound source, underwater responses were noted as high as 200 kHz in 

 a Tursiops truncatus . In a similar test the upper threshold was 100 kHz with 

 another Tursiops . Since the dB intensity reference was in terms of the dB 

 loss relative to the highest cochlear potential measured with each animal, 

 absolute dB intensity thresholds were not obtained. Thus, only the upper- 

 frequency thresholds and the comparative slope of the intensity thresholds are 

 available. This last term refers to a comparison of their audiogram with that 

 of Johnson's (1966) Tursiops behavioral audiogram (Figure 12). A comparison 

 of the relative intensity thresholds can be made, and in this case the 

 increase in sensitivity of the cochlear microphonics audiogram is similar to 

 that of the behavioral tests. 



Pinnipedia 



Using deep cortical probes on both alert and anesthetized animals, 

 Bullock, Ridgway and Suga (1971) obtained audiograms from two Zalophus 

 californianus and one Phoca vitulina . All results were from subjects tested 

 in the air; the results from both species were similar. Evoked potentials 

 were recorded in the harbor seal from 400 Hz - 20 kHz, with maximum 

 sensitivity of 54 dB at 4 kHz. In one sea lion potentials were recorded from 

 500 Hz - 35 kHz and in the other from 500 Hz - 20 kHz. Maximum sensitivity 

 for both was from 4-8 kHz, with a maximum sensitivity threshold of 52 dB. 

 The results for Zalophus were in close agreement with those of Schusterman 

 (1972); however, results for Phoca differed significantly from those of Mohl 

 (1968) and Terhune (1972; 1975). 



Using deep cortical probes on six gray seals ( Halichoerchus grypus ) 

 audiograms were obtained for hearing both in and out of the water (Ridgway and 

 Joyce, 1975). The animals were not restrained and the EEG signals were 

 transmitted by radio from the animals. Responses were noted from the lowest 

 used frequency of 1 kHz to 150 kHz in water and from 200 Hz - 30 kHz in air. 

 Maximum sensitivity in water was 60 dB at 30 kHz and in air 70 dB at 5 kHz. 



A-15 



