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, sirenians, and many odontocetes. Very few, if any, hearing studies have been done with the large 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.

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. For example, manatees were trained to push a paddle on their left if they heard a tone and a paddle on their right if no tone was present (2). In this way the scientists could determine what frequencies and sound levels the manatees could hear. This information is presented in the form of a hearing threshold curve. Hearing threshold curves have been measured for numerous species of small cetaceans and pinnipeds using this behavioral technique.

Estimates of the hearing thresholds for odontocetes and pinnipeds. The y-axis (vertical) is relative intensity in underwater dB. The x-axis (horizontal) is the frequency of a sound on a logarithmic scale. Figure is adapted from (3). Thresholds are a composite of measured lowest thresholds for multiple species (4)

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 (5).

The false killer whale's hearing is being measured using an auditory brainstem response (ABR) test (6, 7). 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. Photo courtesy of Paul E. Nachtigall, Hawaii Institute of Marine Biology.

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 (8). Much of our knowledge of mysticete hearing has come from these anatomical studies.

Additional Resources
• Gelfand, S.A. 1998. Hearing: An Introduction to Psychological and Physiological Acoustics. 3rd ed. Marcel Dekker publishing, New York.
• Moller, A.R. 2000. Hearing: Its Physiology and Pathophysiology. Academic Press, San Diego, CA.
• Testing the Hearing of Whales and Dolphins
• Discovery Channel Canada: The One That Didn't Get Away

See Full References List for this page.