Sam H. Ridgway, DVM, PhD
U.S. Navy Marine Mammal Program, San Diego, CA, USA; Veterinary Medical Center, Department of Pathology, School of Medicine, San Diego, University of California, La Jolla, CA, USA
Abstract
The first detailed audiogram of the bottlenose dolphin showed good underwater hearing with a threshold of around 42 dB re 1 µPa (10–14 W m2) at 60 kHz. Studies also showed dolphin and white whale sensitivity to sound frequencies from about 60 Hz to 150 kHz, almost eight times the frequency span of human hearing (humans are slightly more sensitive to sound pressure in air but human frequency range is limited to about 20 kHz).
The audiogram of both species has been replicated many times and it is clear that sensitive ears connected to a massive auditory central nervous system are fundamental to the dolphin’s echolocation and communication. It is reasonable to ask how the animal, with such excellent hearing, avoids damaging its own ears with the loud sounds it produces during echolocation. The dolphin ear, anatomically only a few centimeters away from its sound production mechanism, processes high-frequency echolocation pulses up to 230 dB re 1 µPa in peak-to-peak amplitude. Using intense pulses and sensitive ears, dolphins can detect echoes (as quiet as a human whisper) from small objects at l00 m and more. Because the dolphin’s pulses are very brief (on the order of 40 µs, and 25,000 of these would equal one second of sound) the total energy within each pulse is miniscule. Anatomic structures, including highly reflective air sinuses that attenuate sound, probably help the animal avoid damaging its own ears. Humans introduce loud sound in the sea for purposes of improved sonar, construction, oil exploration, and acoustic communication. So how do we determine if human generated sound in the sea poses a serious threat to marine life?
My presentation will cover experience with simulated sonar pings and impulsive devices that have been employed in open water hearing tests with trained dolphins and white whales. Simulated one-second pings of 140 to 202 dB re 1 µPa at frequencies of 0.4, 3.0, 10.0, 20.0, and 75.0 kHz were presented on many different days to seven different cetaceans without ill effect. These pings did produce a temporary shift in hearing threshold at an average level of 195 db re 1 µPa in the mid-frequency range.1 Single impulses from an underwater seismic water gun up to peak pressures of 160 kPa (226 dB re 1 µPa) were tested with a white whale and 207 kPa (228 dB re 1µPa) with a bottlenose dolphin.2 No short- or long-term injury was observed, although some hormonal indicators of mild immune-system stress were observed in blood specimens collected after exposure.3
Literature Cited
1. CE Schlundt, Finneran JJ, Carder DA, Ridgway SH. Temporary shift in masked-hearing thresholds (MTTS) of bottlenose dolphins, Tursiops truncatus, and white whales, Delphinapterus leucas, after exposure to intense tones. J Acoust Soc Am. 2000;107(4):3496–3508.
2. Finneran JJ, Schlundt CE, Dear R, Carder DA, Ridgway SH. Temporary shift in masked hearing thresholds in odontocetes after exposure to single underwater impulses from a seismic water gun. J Acoust Soc Am. 2002;111:2929–2940.
3. Romano T, Keogh M, Kelly C, Feng P, Berk L, Carder D, et al. Anthropogenic sound and marine mammal health: measures of the nervous and immune systems before and after intense sound exposure. Canadian Journal of Fisheries and Aquatic Sciences (in press). 2004.