Underwater audiogram of the grey seal (Halichoerus grypus).

The hearing sensitivity of two grey seals (Halichoerus grypus) was measured using a psychophysical staircase method from 0.125 to 64 kHz. Both animals had best hearing sensitivity at 4 kHz (43-44 dB re 1 µPa). Below 4 kHz, hearing thresholds differed. These first underwater audiograms using psychophysical methods demonstrate that high-frequency hearing in grey seals is comparable to other phocids. However, they show better sensitivity below 8 kHz, indicating that low-frequency hearing may play a more important role in this species, potentially increasing its vulnerability to anthropogenic noise. Notably, hearing thresholds at low frequencies were lower than predicted by the standard audiogram used to generate phocid frequency weighting function, which is currently used to assess the impact of anthropogenic sound on marine mammal hearing. These functions may not adequately represent grey seal sensitivity at low frequencies. Our results may, if corroborated by further audiograms, motivate modifications of the phocid standard audiogram.