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SUMMARYTemporary threshold shift (TTS) after loud noise exposure was investigated in a male and a female beluga whale (Delphinapterus leucas). The thresholds were evaluated using the evoked-potential technique, which allowed for threshold tracing with a resolution of ~1min. The fatiguing noise had a 0.5octave bandwidth, with center frequencies ranging from 11.2 to 90kHz, a level of 165dBre.1μPa and exposure durations from 1 to 30min. The effects of the noise were tested at probe frequencies ranging from -0.5 to +1.5octaves relative to the noise center frequency. The effect was estimated in terms of both immediate (1.5min) postexposure TTS and recovery duration. The highest TTS with the longest recovery duration was produced by noises of lower frequencies (11.2 and 22.5kHz) and appeared at a test frequency of +0.5octave. At higher noise frequencies (45 and 90kHz), the TTS decreased. The TTS effect gradually increased with prolonged exposures ranging from 1 to 30min. There was a considerable TTS difference between the two subjects.

In a bottlenose dolphin positions of sound receiving areas on the head surface were determined by comparing the acoustic delays from different sound-source positions. For this investigation, auditory brainstem responses (ABRs) to short tone pips were recorded and their latencies were measured at different sound source positions. After correction for the latency dependence on response amplitude, the difference in ABR latencies was adopted as being the difference of the acoustic delays. These delay differences were used to calculate the position of the sound-receiving point. Measurements were conducted at sound frequencies from 16 to 128 kHz, in half-octave steps. At probe frequencies of 16 and 22.5 kHz, the receiving area was located 21.7-26 cm caudal of the melon tip, which is near the bulla and auditory meatus. At higher probe frequencies, from 32 to 128 kHz, the receiving area was located from 9.3 to 13.1 cm caudal of the melon tip, which corresponds to a proximal part of the lower jaw. Thus, at least two sound-receiving areas (acoustic windows) with different frequency sensitivity were identified.

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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.]