Abstract
Abstract : We obtained underwater hearing thresholds from a male California sea lion at frequencies of 2.5, 6, 10, and 35 kHz, and at depths of 10, 50, and 100 m. Results showed systematic changes in response bias with changes in depth; false alarm probabilities decreased significantly with depth, indicating that the sea lion adopted a more conservative response criterion in deeper water. Sensitivity at frequencies below 35 kHz decreased with depth, while sensitivity at 35 %Hz increased with depth. Increasing pressure with depth probably affects the size of the middle ear air space due to the expansion of cavernous tissue into the middle ear cavity. Our findings are consistent with the idea that the middle ear impedance changes with depth, altering the frequency response and sensitivity of the ear. Thus, the sea lion middle ear plays a functional role in underwater sound detection. However, the presence of cavernous tissue in the sea lion middle ear does not appear to enhance sensitivity at depth except at high frequencies.
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