Audiogram variability in normal bottlenose dolphins

Abstract

Audiograms have been obtained in about a dozen of odontocete species, but mostly in one or two individuals each. However, some inter-individual difference in hearing sensitivity is inevitable. Therefore, a representative number of animals should be investigated to get a normal audiogram standard. In the present study, an attempt has been made to estimate the audiogram scatter among normal bottlenose dolphins. Measurements were made in dolphins captured in wild and kept in captivity 3 to 5 months, using auditory evoked potential technique (envelope following response) to measure hearing thresholds in far acoustic field. Seven subjects, 5 males and 2 females, provisionally from 3 to 15 years old, were investigated during the summer season of 2004. Hearing thresholds were measured at frequencies from 8 to 152 kHz with quarter-octave steps. All the subjects had qualitatively similar audiograms. The best sensitivity was from 38.9 dB re 1 uPa (at 32 kHz) to 51.9 dB (at 16 kHz), with a minimum of the averaged audiogram of 47.1 dB at 45 kHz. High-frequency cut-off was 152 kHz at a level of 40 dB above the lowest threshold. Standard deviation of threshold was from 4.1 to 10.3 dB. [Work supported by RFBR and Russian President Grants.]