Temporary shift in masked hearing thresholds of bottlenose dolphins, Tursiops truncatus, and white whales, Delphinapterus leucas, after exposure to intense tones

A behavioral response paradigm was used to measure masked underwater hearin g thresholds in five bottlenose dolphins and two white whales before and im mediately after exposure to intense 1-s tones at 0.4, 3, 10, 20, and 75 kHz . The resulting levels of fatiguing stimuli necessary to induce 6 dB or lar ger masked temporary threshold shifts (MTTSs) were generally between 192 an d 201 dB re: 1 mu Pa. The exceptions occurred at 75 kHz, where one dolphin exhibited an MTTS after exposure at 182 dB re: 1 mu Pa and the other dolphi n did not show any shift after exposure to maximum levels of 193 dB re: 1 m u Pa, and at 0.4 kHz, where no subjects exhibited shifts at levels up to 19 3 dB re: 1 mu Pa. The shifts occurred most often at frequencies above the f atiguing stimulus. Dolphins began to exhibit altered behavior at levels of 178-193 dB re: 1 mu Pa and above; white whales displayed altered behavior a t 180-196 dB re: 1 mu Pa and above. At the conclusion of the study all thre sholds were at baseline values. These data confirm that cetaceans are susce ptible to temporary threshold shifts (TTS) and that small levels of TTS may be fully recovered. [S0001-4966(00)00106-5].