Abstract

Temporary Threshold Shift (TTS) is a brief, reversible change in hearing sensitivity due to exposure to a loud sound, and is a phenomenon that has been investigated and observed in both humans and some terrestrial animals. TTS is an important sign that can indicate when a sound is beginning to exceed the ear's ability to respond. With the intent to show that TTS can occur in marine mammals, we investigated the effects of louder sounds on the hearing thresholds of five dolphins, Tursiops truncatus, and two white whales, Delphinapterus leucas. Following the collection of baseline hearing thresholds, animals were exposed to 1-s tones at 3, 10, 20, and 75 kHz at gradually increasing levels up to 202 dB re: 1 µPa at a rate of one per day. Masking noise was used to create a floor effect, thereby eliminating between-session threshold variability due to an ever-changing ambient noise environment in San Diego Bay. Both dolphins and whales began exhibiting alterations in their behavior at levels around 180 dB. TTS was determined to have occurred when there was at least a 6-dB increase in hearing thresholds from baseline testing. TTS was exhibited when tone levels increased to 194-201 dB at 3 kHz, 192-196 dB at 10 kHz, 193-196 dB at 20 kHz, and 192-194 dB at 75 kHz. In all cases, hearing thresholds were back to baseline levels by the day's end. The studies have been conducted without any harm to animal health, day-to-day behavior, or long-term change in baseline masked hearing thresholds. Evidence of behavioral alteration and TTS in marine mammals will be used to establish criteria for safe noise levels around marine mammals both in the open ocean, and in zoos and aquaria.