Using bone conduction brainstem auditory evoked response tests in gray seals (<i>Halichoerus grypus</i>)

Gray seals (Halichoerus grypus) outer ears close when sedated, requiring appropriate methods to test hearing, circumventing the outer and middle ear (air conduction). Anthropogenic noise exposure (e.g., offshore windfarms) in the marine environment is a risk to seals, requiring appropriate methods for testing hearing. Bone Conduction Brainstem Auditory Evoked Response (BC-BAER) was investigated in six gray seals to characterize responses elicited by different acoustic stimuli, evaluate the potential of using BC-BAER to estimate hearing sensitivity, and best placement of the bone transducer. Seals were tested in December 2023 and February 2024 during ongoing research at the Sea Mammal Research Unit, University of St. Andrews, Scotland. BC-BAER measurements in six seals using broadband click and gated tone bursts stimuli had responses from 250 to 8000 Hz, the limits of the RadioEar-B71 bone transducer. Visual identification of Wave V was used to estimate hearing thresholds. Average threshold values were 45.67 dB re 20 µPa for tone bursts at 8000 Hz, the maximum sensitivity of frequencies tested. Recording of BC-BAER is a viable means of studying hearing in gray seals with the best placement of the RadioEar-B71 bone transducer being the zygomatic process compared to the mastoid.