The basic mechanism of sound detection is the same in all mammals, whether they live on land or in the sea. Sound waves are transformed to neural impulses in the inner ear, which are interpreted by the brain as sounds (see How do marine mammals hear sounds?). Exposure to loud sounds can interfere with this process and cause hearing impairment or loss.

Researchers use various techniques to evaluate the effect of sound on marine mammal hearing. Hearing sensitivity can be measured using auditory testing methods similar to those used on humans and other terrestrial mammals. For more information see Hearing Sensitivity Studies.

The dolphin's hearing is being measured using an auditory brainstem response (ABR) test. The probes, attached to the animal's head and back using suction cups, measure small electrical voltages produced by the brain in response to an acoustic stimulation (1, 2). Photo courtesy of Paul E. Nachtigall, Hawaii Institute of Marine Biology.

Hearing loss in humans is relatively well understood. Data regarding marine mammal hearing loss are just beginning to emerge. We can use what we know about hearing loss in humans and other land mammals to help understand hearing loss in marine mammals because the basic mechanism of sound detection is the same in all mammals. We know that hearing loss in mammals depends on the intensity of the sound, the frequency of the sound, and how long the animal is exposed to the sound (duration). Humans exposed to extremely loud sounds for short periods of time (e.g. rock concerts, explosions) experience temporary or permanent hearing impairment. Human hearing damage can also be caused by exposure to moderate levels of noise over long periods of time, as in a noisy work environment (3). Hearing impairment does not occur if the frequency of the sound to which the animal is exposed is outside the range that the animal can hear.

Hearing threshold curve for the beluga whale. A hearing threshold curve shows the sound intensity required for an animal to just detect sounds across the frequency band that it can hear. The lowest point on the curve indicates the frequency where the animals hear best, about 40,000 Hz for beluga whales. The vertical y-axis is relative (threshold) intensity in dB re 1 µPa. The horizontal x-axis is the frequency of a sound on a logarithmic scale in Hz. (4, 5).

The softest sound that an animal can hear at a specific frequency is called the hearing threshold at that frequency. If an animal is exposed to sound below the threshold of hearing, the animal cannot hear the sound. The animal can accommodate sounds that are above the threshold of hearing until a certain combination of intensity and duration is reached. Above this limit, the animal's hearing threshold will be elevated. This means that the softest sound that the animal can hear is louder than it was before exposure to the sound, or they have lost the ability to hear the softest sounds they could hear before. The shift will be temporary or reversible, and the threshold will return to near normal levels. The loss of sensitivity is called a temporary threshold shift or TTS. As the sound exposure increases yet further, a higher level will eventually be reached at which the threshold shift will be permanent, and the effect is called a permanent threshold shift or PTS. PTS can occur as a result of repeated occurrences of TTS, or it can occur catastrophically as a result of a single exposure to a very intense sound.

Temporary threshold shift (TTS) studies have been conducted with two species of odontocetes (bottlenose dolphins and beluga whales) (6, 7, 8, 9, 10) and three species of pinnipeds (harbor seals, California sea lions, and Northern elephant seals) (11). The hearing thresholds of these captive animals are determined using both behavioral and auditory brainstem response (ABR) methods (for more information see Hearing Sensitivity Studies). The TTS investigations introduce sounds at varying frequencies, intensities, and durations to determine sound exposures that cause temporary threshold shifts. (The marine mammals were not exposed to high enough levels to cause permanent threshold shifts (PTS) and completely recovered their hearing.)

One study exposed bottlenose dolphins and beluga whales to signals at frequencies of 400, 3,000, 10,000, 20,000, and 75,000 Hz that lasted for one second (12). The received levels (estimated at the position of the animals ear) required to cause TTS were generally between 192 and 201 dB re 1 µPa. The data from this study are plotted in the following figure. The highest receive level that was tested at the 400 Hz frequency (193 dB re 1 µPa) did not produce threshold shifts in any of the animals - indicated by the dashed line.

Minimum received levels required to cause TTS in bottlenose dolphins and beluga whales for 1-second tones at a number of frequencies. No TTS was observed after exposure to 400 Hz tones for the highest received level achieved, 193 dB re 1 µPa. Reprinted with permission from Schlundt, C.E., Finneran, J.J., Carder, D.A. and Ridgway, S.H. 2000. Temporary shift in masked hearing thresholds of bottlenose dolphins, Tursiops truncatus, and white whale, Delphinapterus leucas, after exposure to intense tones. Journal of the Acoustical Society of America 107(6): 3496-3508. Copyright 2000, Acoustical Society of America.

The data from all of these TTS studies are being used to model the effects of sound exposure on hearing loss. They show that longer exposures to quieter sounds have the same effect as shorter exposure to louder sounds (13).

The amount of data on hearing loss in marine mammals is limited. While TTS studies have been done, they have been limited to a few individuals of a few marine mammal species. No research has been conducted on PTS in marine mammals. Estimates of the intensity levels that might cause PTS must be extrapolated from terrestrial laboratory mammals such as chinchillas, though it is not certain that such an extrapolation is appropriate (14). The majority of data on marine mammal hearing has come from studies of the anatomy of the ear of different marine mammal species.

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