Monaural and binaural aerial hearing thresholds in a California sea lion (Zalophus californianus)

Though studies have focused on some aspects of binaural hearing in otariid pinnipeds, none have directly measured monaural hearing thresholds. Recent auditory evoked potential (AEP) testing with a young male California sea lion suggested that hearing threshold differences potentially existed between the subject's two ears. To further investigate these findings, aerial psychophysical audiograms were collected for this sea lion. Using headphones to deliver stimuli, monaural and binaural thresholds were measured at seven frequencies from 0.5 to 32 kHz in one-octave steps. The binaural and two monaural (left and right ears) psychophysical audiograms were all similar in that they had the typical mammalian U-shape, with peak sensitivity from 8 to 16 kHz. The right ear showed generally elevated psychophysical thresholds when compared to the left, although all differences were less than 7 dB. The psychophysical and AEP audiograms were similar in terms of shape; however, the directions of monaural threshold differences determined with AEP methods were not always consistent with those obtained using psychophysical methods. [Work supported by ONR.]