How do marine mammals hear?
Hearing is the detection of sound. Both modern land mammals, including humans, and marine mammals evolved from ancestors that had air-adapted ears. So, many of the structures of the ear in both land and marine mammals are similar. Some marine mammals that live exclusively in water, like whales and manatees, hear very well in water but hear poorly, if at all, in air. Marine mammals that live on land at least part of the time, such as seals, sea lions and walruses (pinnipeds), otters and polar bears, have ears that are amphibious and can hear in both air and water.
The basics of hearing are the same in both land and marine mammals. Hearing is the result of the combined activity of the ear’s three basic divisions: (1) the outer ear collects and directs sound, (2) the middle ear filters and amplifies the acoustic energy to the inner ear, and (3) the inner ear transforms the acoustic energy to electrical signals (neural impulses) that are processed by the brain.
In order to understand how marine mammals hear sound, it is helpful to understand the mechanisms by which terrestrial mammals hear sound on land. There are many similarities in the basic hearing processes in marine mammals and terrestrial mammals. We will use the human ear as a model for terrestrial ears. Although there are differences among the ears of different species, the basic processes of hearing are the same.
The human ear, which is located in the skull, is divided into three sections: (1) the outer ear collects and directs sound, (2) the middle ear filters and amplifies the acoustic energy to the inner ear, and (3) the inner ear transforms the acoustic energy to electrical signals (neural impulses) that are processed by the brain. The outer ear includes the ear flap (pinna) and outer ear canal (external auditory meatus). The pinna funnels sound into the outer ear canal. The canal ends in the eardrum, or tympanic membrane, which separates the outer and middle ear. The middle ear in land mammals is an air-filled space that contains a series of three small bones or ossicles called the incus (anvil), malleus (hammer), and stapes (stirrup). These bones connect the tympanic membrane and the oval window, which is the opening to the inner ear.
Sound pressure waves entering from the outer ear cause the tympanic membrane to vibrate. The vibrations are transferred via the ossicles to the oval window. The lever action of the ossicles amplifies the sound energy that can enter the inner ear.
The function of the inner ear is to change the sound intensity into electrical signals that the brain processes. The inner ear consists of organs for both hearing (cochlea) and balance (vestibular system).
The cochlea is a fluid-filled, spiral labyrinth that houses many structures related to hearing, including the basilar membraneand the organ of Corti. Sound causes the stapes to move which causes the inner ear fluid to move, resulting in vibrations along the basilar membrane. When the basilar membrane vibrates, tiny hair cells (organ of Corti) on top of the basilar membrane bend and trigger the release of chemicals that create an electrical signal or a neural impulse. The neural impulse is carried from the organ of Corti by auditory nerve fibers via the auditory ganglion cells to the brain.
The hearing range of any mammal depends mostly on the characteristics of the basilar membrane. The end of the basilar membrane that is closest to the oval window (base of the cochlear spiral) is narrow, thick, and stiff. The membrane becomes broader, thinner, and more elastic as you move farther from the oval window (apex of the cochlear spiral). High frequencies cause the basilar membrane to vibrate more near the base, while lower frequencies cause the membrane to vibrate most towards the apex.
The most drastic changes in auditory systems between terrestrial mammals and marine mammals can be found in the cetaceans (whales, dolphins and porpoises) and the sirenians (manatees and dugongs). There are even significant differences between the ears of cetaceans and those of pinnipeds (seals, sea lions, and walruses). Like some pinnipeds, cetaceans have no external pinnae. But, unlike pinnipeds, the ear canals of cetaceans are not thought to be functional. In odontocetes (toothed whales) the ear canal is narrow and plugged with debris and dense wax. Additionally, the ear canals do not attach to the tympanic membrane (ear drum). In mysticetes (baleen whales), the narrow ear canal is terminated by a waxy cap. If the ear canals do not attach to the tympanic membrane, how is sound getting to the middle ear?
In odontocetes, the lower jaw is surrounded by fats that, along with a thin bony area called the pan bone, are thought to conduct sound to the middle ear. Unlike land mammals that have ears attached to the skull, the middle and inner ears of cetaceans are encased in bones that are located in a cavity outside the skull. In odontocetes, these bones are attached to the skull by ligaments. In mysticetes and sirenians, the earbones have bony connections to the skull. The exact mechanism that mysticetes use for hearing is still being researched.
The inner ear of cetaceans works in the same way as that of terrestrial mammals. The differences lie in the inner ear characteristics. The major differences are the number of auditory ganglion cells, the ratio of the number of ganglion cells to the number of hair cells, the size of the auditory nerve, the size of the basilar membrane, and the support of the basilar membrane. Toothed whales have more ganglion cells associated with hearing than terrestrial mammals. Baleen whales have fewer nerve cells associated with hearing compared to toothed whales, but more than terrestrial mammals. Cetaceans also have a lot more ganglion cells associated with each hair cell than do humans and a much larger auditory nerve. All of these adaptations mean that cetaceans may be able to do more complex auditory processing.
The thickness and width of cetacean basilar membranes are closely linked to the unique hearing capacities of toothed and baleen whales. The thicker and stiffer the basilar membrane, the more tuned an ear will be for higher frequency hearing. Toothed whales have evolved additional adaptations that increase the stiffness of the basilar membrane. Bony supports are present in toothed whale cochlea to increase stiffness. The thickness of the membrane is also larger compared to terrestrial mammals of the same body size. These adaptations contribute to the exceptionally high hearing range in toothed whales. Baleen whales, on the other hand, have exceptionally broad, thin, and elastic basilar membranes. These characteristics contribute the low frequency hearing range in baleen whales.
What do marine mammals hear?
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 mammals 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.
Tutorial Sections: Determine if a sound affects a marine animal
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- Szymanski, M.D., Bain, D.E., Kiehl, K., Pennington, S., Wong, S., and Henry, K.R. 1999, “Killer whale (Orcinus orca) hearing: Auditory brainstem response and behavioral audiograms.” Journal of the Acoustical Society of America 106(2): 1134-1141.