Knowledge of the hearing abilities and other acoustic features of marine animals is important when measuring the effects of sound on marine animals. (For more information see How do marine animals hear sounds?). If an animal is unable to detect a sound due to limitations in hearing range or loudness, it is unlikely the animal will be affected by the sound. Most hearing studies are performed on animals in captivity, so the hearing information that is available tends to be for the smaller marine mammals such as pinnipeds (seals, sea lions, and walruses), sirenians (manatees and dugongs), and many odontocetes (toothed whales). Very few, if any, hearing studies have been done with the mysticetes (baleen whales) because they are not kept in captivity, and it is very difficult to perform hearing tests with these animals in the wild. Hearing studies provide information that may be used to predict how sound sources and levels may affect animals in the wild. Hearing studies on marine mammals are conducted in three different ways: behavioral studies, electro-physiological studies, and anatomical studies.
Behavioral studies are conducted to determine the softest sounds that an animal can hear at different frequencies. This is called the hearing threshold. These studies are often performed in captivity with trained animals. The animal is trained to station underwater while a sound is played. If the animal hears the sound, it is trained to respond in a particular way. If the animal doesn't hear the sound (or if no sound is played), it responds in a different way. bottlenose dolphins were trained to push a paddle if they heard a tone and to remain stationary if they don’t hear anything. In this way the scientists could determine what frequencies and sound levels the bottlenose dolphins could hear. This information is presented in the form of a hearing threshold curve.
Behavioral studies have been performed on several toothed whale species including dolphins, beluga whales and harbor porpoises, and a number of pinniped species. All tested species of toothed whales (Odontocetes) hear best in the high frequency range (10,000 to 50,000+ Hz). Pinnipeds (seals and sea lions) hear best at frequencies lower than most Odontocetes. (For more information about marine animals’ perception of sound, go to the sections on Marine Animal Sound Production and Marine Animal Sound Reception.)
Electro-physiological studies have also been used to determine the threshold of hearing in many animals. The response of the nervous system to sound can be recorded from the change of electric charge, or voltage, in nerve cells. During these non-invasive studies, small electrodes placed on the surface of the animal's head record the voltages produced by nerve cells in the central auditory nervous system. The results are plotted as a graph. The auditory brainstem response (ABR) is the voltage produced by the brainstem in response to a sound stimulus. The ABR test is powerful because it can be done rather quickly compared to behavioral hearing methods and because it can be performed with untrained or stranded animals. Studies have compared the results of behavioral responses and ABR tests (conducted on the same individuals) to better understand marine mammal hearing sensitivity.
Behavioral studies and/or ABR studies can also be used to study how an animal's hearing can change after being exposed to specific levels of sound (For more information see Hearing Loss.)
The hearing capabilities of marine mammals are also studied by conducting anatomical examinations of dead animals. Scientists are able to learn about hearing capabilities from the dissection of the animal body and ear. By examining the air-filled middle ear and fluid-filled inner ear, researchers have been able to estimate the range of frequencies that an animal may be able to hear. Much of our knowledge of mysticete hearing has come from these anatomical studies.
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- Miksis, J.M., Grund, M.D., Nowacek, D.P., Solow, A.R., Connor, R.C., and Tyack, P.L. 2001, "Cardiac responses to acoustic playback experiments in the captive bottlenose dolphin (Tursiops truncatus)" Journal of Comparative Psychology 115: 227-232.
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- Office of Naval Research. 2001, "Final Environmental Impact Statement for the North Pacific Acoustic Laboratory, May 2001." Office of Naval Research, Arlington, VA. (Link)
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- "Testing the Hearing of Whales and Dolphins." (Link)