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 membrane and 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.
To understand the differences in human and marine mammal hearing it is important to mention a few more items. First, detecting sound is only half the battle. Detecting sound means knowing that there is sound being produced in the environment. Localizing the sound source, or determining which direction the sound is coming from, is also important.Terrestrial mammals are able to localize sounds because our ears can tell us if sound arrives at each ear at different times or with a different loudness. For example, the ear farther away from the sound hears it later and softer. The differences in time of arrival and loudness allow humans and other mammals to localize sounds.
Second, 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 more narrow and stiff the basilar membrane is, the better the ear responds to high frequency sounds. The more massive and less stiff the basilar membrane is, the better the ear responds to low frequency sounds. The greater the range of stiffness of the basilar membrane, the greater the frequency range of hearing the ear has. Animals like bats and dolphins have supports that also add stiffness to the basilar membrane which allows them to hear frequencies far above what humans can hear (ultrasonics). Some animals like mole rats, elephants, and baleen whales have membranes that are very thin and flexible which lets them hear exceptionally low frequencies (infrasonics). The comparison between human and marine mammal hearing is discussed in further detail in Hearing by Cetaceans and Sirenians.
- Au, W.W.L., Popper, A.N., and Fay R.R. (eds). 2000, "Hearing in Whales and Dolphins." Springer-Verlag, Inc., New York, NY.
- Yost, W.A. 1994, "Fundamentals of Hearing: An Introduction." 3rd ed. Academic Press, New York, NY.
- Computerized Scanning and Imaging Facility at Woods Hole Oceanographic Institution, "2 dimensional and 3 dimensional CT scan images of cetaceans and other animals." (Link)
- Andrew Wright, "Animation of the Human Ear." (Link)
- "Beltone - Anatomy of the Ear." (Link)
- "Hyperphysics - The Ear and Hearing." (Link)
- "The Physics Classroom: The Human Ear." (Link)