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Discovery of Sound in the Sea
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Hearing in Cetaceans and Sirenians, the Fully Aquatic Ear

The most drastic changes in auditory systems between terrestrial mammals (see Hearing in Land 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) (see Hearing in Pinnipeds). 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?

Photo of a killer whale head.
Whale, like this killer whale, do not have external ears. Photo courtesy of Jo Guerrero.
 

 
Photo of a Beluga whales.
The ear canals of whales, such as these beluga whales, are blocked by debris and wax. Photo ┬ęTom Kieckhefer.


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.

Diagram of a dolphin head showing air sacs and the lower jaw.
Dolphins produce sounds by passing air through air sacs in their head. They receive sound through their lower jaw. The core of the lower jaw is filled with fats that enter the ear (tympanic bulla) at a thin bony area at the rear of the lower jaw (mandible) known as the pan bone. The pan bone acts as an acoustic window. Adapted from Dolphin Acoustical Structure (1991) Scheifele, P.M. NUSC TR3080

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.

The table that follows summarizes the differences between the fully aquatic ears of cetaceans and sirenians and the aerial ears of terrestrial mammals. As you can see, the ears of cetaceans and sirenians have changed a lot to be able to hear sound well underwater!

Cetacean versus Human Ears and Hearing
  Cetaceans
(Fully Aquatic Ears)
Human
(Aerial Ears)
Outer Ear Temporal bone is not part of skull Temporal bone is part of the skull
No pinnae Pinnae
Auditory meatus is plugged Air-filled auditory meatus
Middle Ear Middle ear is filled with air and soft tissue Middle ear completely air-filled
Inner Ear Basilar membrane thin and broad at apex of odontocetes Basilar membrane thick and narrow at the basal end
Mysticetes basilar membrane thinner and wider than odontocetes and humans  
Strong support of basilar membrane for odontocetes, less for mysticetes Little support of basilar membrane
Long basilar membrane length Short basilar membrane length
Semi-circular canals are small Semi-circular canals are average to large
Large number of auditory nerve fibers Average number of auditory nerve fibers