pinniped underwater hearing the sound enters the head and proceeds directly to 

 the organ of corti, whereas aerial sound transmission is apparently 

 accomplished in a typical mammalian pattern -- from the tympanic membrane via 

 the middle ear to the oval window (Reppening, 1972). 



Marine Mammal Auditory Threshold 



In the early 1950' s scientific observations began on the auditory 

 capabilities of cetaceans. These were prompted in part by the hypothesis of 

 McBride (1956) and others in the 1940 's that some cetaceans were capable of 

 echolocation. The experimental designs of these studies range from the 

 initial "naturalistic observations" of Kellogg and Kohler (1952) to the more 

 detailed experimental paradigms pioneered by Johnson in 1966. 



A wide variety of techniques has been devised to deterine the auditory 

 thresholds of a mammal (Francis, 1975). These can be broadly separated into 

 behavioral and electrophysiological measurement techniques. Both measurement 

 techniques have been used with marine manuals. The goal of each study is the 

 measurement of hearing sensitivity (in dB) as a function of frequency (Hz). 

 These may be referred to as audiograms (Stevens, 1951) or absolute auditory 

 threshold curves (Licklider, 1951). This report will use the term audiogram. 



Behavioral audiograms have been obtained by training marine mammals to 

 respond to the perception of a tone with operant or Pavlovian conditioning 

 techniques. Electrophysiological audiograms are obtained by a) monitoring 

 evoked potentials to auditory stimuli; and b) measuring the cochlear 

 microphonics at the round window of the cochlea. 



The auditory capabilities of the following cetaceans have been 

 behavi orally tested: Tursiops truncatus truncatus and Tursiops truncatus 

 gilli , Phocoena phocoena , Delphinus delphis , Orcinus orca , Inia geoffrensis 

 and Delphinapterus leucas . Data from these tests are presented in Table 1. 

 Studies giving frequency and intensity thresholds are graphed in Figures 7 and 

 8. 



The following pinnipeds have been behaviorally tested: Phoca vitulina , 

 Pagophilus groenlandicus , Pusa hispida and Zalophus californianus . Data from 

 these tests are presented in Table 2 and figures 9 and 10. 



The following marine mammals have been tested using electrophysiological 

 measurement techniques: Stenella coeruleoalba, Stenella attenuata , Steno 



A-8 



