Cetaceans evolved from ancestors that had air-adapted ears. Their entire body, including their auditory systems, underwent significant adaptations related to living in water. These include the shape of the skull, the formation of a pathway for receiving sound energy in water, and some components of the inner ear. The following movie shows the anatomical adaptations of the skeleton of odontocetes, from the oldest known cetacean to modern-day odontocete.
The oldest whale is from the genus Pakicetus, known from a 52-million-year-old fossil from Pakistan. It had four legs but is classified as a whale because its enlarged auditory bulla, has an enlarged, distinct tympanic wall, which is characteristic of modern whales. The next step in the evolutionary chain is Ambulocetus, discovered in 1994, which could both walk on land and swim. The skull of Ambulocetus elongated and included a fat pad in the lower jaw, which in modern-day odontocetes is the pathway for sound energy to reach the middle ear. Remingtonocetids and protocetids, the next steps in the evolutionary chain, were the first cetacean ancestors with an ear like that of modern whales. The movie concludes with a generic odontocete skeleton, showing an animal fully adapted to an aquatic existence.
The aquatic ear is highly derived compared to the ear of land mammals. Cetaceans have no external pinna and the ear canal is narrow and plugged with thickened tissue, dead cells, and wax. Sound gets to the middle ear of odontocetes through fats that surround a thin, hollow area of the lower jaw, or mandible.
The adaptations of the inner ear for underwater sound reception are discussed in the section Hearing in Cetaceans and Sirenians, the Fully Aquatic Ear. Important differences between a terrestrial inner ear and the odontocete inner ear that are associated with echolocation in water 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 dimensions of the basilar membrane, and the support of the basilar membrane. These adaptations mean that odontocetes have a broad hearing range extending to 200 kHz, and they are able to do complex auditory processing.
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