Abstract
Cochlear microphonic (CM) potentials were recorded from the round window of the monotreme Ornithorhynchus anatinus (platypus) in response to pure tones between 500 Hz and 20 kHz. A 1.0-μV threshold curve showed best sensitivity for the platypus at 5 kHz with a high-frequency roll-off of approximately 20 dB/octave and a low-frequency rolloff of approximately 15 dB/octave. Intensity functions obtained at 2, 6, and 10 kHz were essentially parallel over the range of intensities studied. The maximum cochlear microphonic amplitude, even at the most sensitive frequency, did not exceed 100μV. The intensity function obtained at 6 kHz, close to the best frequency of the animal, was linear over a 40-dB range, and passed through a maximum at approximately 90 dB SPL. Click responses showed a cochlear microphonic potential followed by two neural potentials analogous to the N1, N2 frequently reported in other species. This study lends support to the notion that the auditory system of the platypus is intermediate between that of reptiles and mammals.
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