Psychophysical audiogram of a California sea lion listening for airborne tonal sounds in an acoustic chamber

Many species-typical audiograms for marine mammals are based on data from only one or a few individuals that are not always tested under ideal conditions. Here, we report auditory thresholds across the frequency range of hearing for a healthy, five-year-old female California sea lion identified as Ronan. Ronan was trained to enter a hemi-anechoic acoustic chamber to perform a go/no-go audiometric experiment. Auditory sensitivity was measured first by an adaptive staircase procedure and then by the method of constant stimuli. Minimum audible field measurements were obtained for 500 ms frequency-modulated tonal upsweeps with 10% bandwidth and 5% rise and fall times. Thresholds were measured at 13 frequencies: in one-octave frequency steps from 0.1 to 25.6 kHz, and additionally at 18.0, 22.0, 36.2, and 40.0 kHz. Sensitivity was greatest between 1 and 23 kHz, with best hearing of 0 dB re 20 µPa at 12.8 kHz. Hearing range, determined at the 60 dB re 20 µPa level, extended from approximately 0.2 kHz to 38 kHz. ...