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?
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.
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!