Abstract: In odontocetes, underwater audiograms have been obtained mostly in one or two individuals in a species. A representative number of animals should be investigated to document variability. In the present study, an attempt has been made to estimate the audiogram mean and scatter among normal bottlenose dolphins (Tursiops truncatus). Measurements were made in dolphins captured in the wild and kept in captivity for three to five months, using auditory evoked potential (AEP) technique (envelope-following response [EFR]) to measure underwater hearing thresholds. Fourteen subjects, 11 males and 3 females, provisionally from 3 to 15 years old, were investigated. Hearing thresholds were measured at frequencies from 8 to 152 kHz with 1⁄⁄4-octave steps. All the subjects had qualitatively similar audiograms, except one. The averaged audiogram featured the best sensitivity (the threshold below 50 dB re 1 μPa) at 45 kHz. Thresholds rose slowly to lower frequencies (up to 65 dB at 8 kHz) and steeply at higher frequencies (up to 97 dB at 152 kHz). Inter-individual standard deviations varied, depending on frequency, from 4.4 to 11.7 dB, mostly not more than 10 dB. One animal featured a significant hearing loss with increased thresholds at frequencies above 54 kHz. An analytical formula for a standard audiogram is suggested based on these data.

Key Words: odontocetes, bottlenose dolphin, Tursiops truncatus, audiogram, auditory evoked potentials, hearing loss

Document Type: research article

DOI: 10.1578/AM.33.1.2007.24

Page Numbers: 24-33