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A mammalian order comprising approximately 90 living species of whales, dolphins, and porpoises and their fossil relatives. Like all other mammals but unlike all fish, cetaceans nurse their young with milk produced by the mother, are endothermic (warm-blooded), breathe air, have a lower jaw that consists of a single bony element (the dentary), and have three small bones (hammer, anvil, and stirrup) subserving sound transmission within the ear.
Living cetaceans are aquatic animals that cannot live on land. They have streamlined bodies, with the nasal opening (blowhole) on top of the head (see illustration). Their forelimbs are modified into flippers, they lack external hindlimbs, and their tail forms a flat horizontal fluke. Modern cetaceans lack hair except for whiskers in the young of some species. Many of these features are common in aquatic vertebrates, and they have evolved convergently as adaptations for life in water.
The brain of cetaceans is large and highly developed, and they are thought to be very intelligent. The cetacean sense of smell is nearly or totally absent. In most species the nerve that carries olfactory information to the brain is absent, which is very unusual among mammals. The eyes of most species are well developed. The ear is the most important sense organ. Toothed whales (Odontoceti) echolocate, emitting high-frequency sounds and using the echoes to determine shapes and distances in their surroundings. Odontocetes do not emit sounds with their voice box (larynx) like other mammals, but have modified nasal passages through which bursts of air are forced. Mysticetes (baleen whales) do not echolocate; rather, they produce low-frequency sounds with their larynx. These sounds can travel through the ocean for hundreds of miles and are used for communication.
The two extant suborders of cetaceans, odontocetes and mysticetes, have different dietary specializations. Most odontocetes have simple, pronglike teeth which are used to grab and hold, but not chew, large prey items. Prey includes a variety of fish of all sizes, crustaceans, and squid and other mollusks. Some of the larger odontocetes, such as the killer whale (Orcinus orca), eat large prey, including sea lions and dolphins. A pod of killer whales will also hunt together, attacking much larger prey such as gray whales. Modern mysticetes do not have teeth and are filter feeders, straining water filled with clouds of marine organisms (krill) through a network of baleen. Baleen is a keratinlike substance that hangs down in plates from the upper jaws of the whale.
All modern cetaceans swim by swinging their horizontal tail fluke through the water, while their forelimbs are used for steering and navigating. The flippers of modern cetaceans resemble flat oars, although the bones for five fingers are present internally. The dorsal fin stabilizes the body during swimming. Under the skin of cetaceans is a layer of blubber, a fatty tissue that serves to insulate the animal and affects its buoyancy and streamlining. Some species are capable of diving to great depths [commonly more than 5000 ft (1500 m) in the sperm whale], yet all cetaceans must come to the surface to breathe. Whales have a number of adaptations for diving and staying underwater for long periods of time. They exhale before they dive, allowing them to submerge faster. They store oxygen in the muscle (myoglobin) and not in the lungs or blood (hemoglobin) and change circulation patterns of blood to save oxygen. Their chests can easily collapse under increasing pressure (with depth) without causing permanent damage. See Hemoglobin
A variety of social structures are found among cetaceans. Two examples are marine dolphins and sperm whales. Most marine dolphins live in schools that may contain dozens of animals, sometimes composed of multiple species. Herds of sperm whales consist of related females and juveniles. Clusters of young males form bachelor groups, and adult males, much larger than the females, are solitary.
Cetaceans are found in all oceans and seas. Some species are restricted to coastal environments (such as bottlenosed dolphins), whereas others live only in the open sea (such as sperm whales). Some species live in all seas and oceans of the world (such as killer whales). Many mysticetes and sperm whales are migratory. A number of dolphin species have left the sea and live permanently in rivers.
Cetaceans originated from a four-footed terrestrial ancestor. This predecessor, a mesonychian, may have resembled a wolf or a hyena and lived approximately 55 million years ago. Modern odontocetes are diverse, ranging from the enormous sperm whales [up to 20 m (66 ft) long and 52,000 kg (114,500 lb)] to the tiny porpoises [Phocaenidae, smallest around 9 kg (20 lb), length 1.5 m (5 ft)]. Dolphins (Delphinidae, which includes the killer whale), porpoises (Phocaenidae), and fresh-water dolphins (Iniidae, Pontoporidae, Platanistidae) are the smallest odontocetes. The largest animal ever to live on Earth is the blue whale (Balaenoptera musculus), a baleen whale that is 25 m (82 ft) long and weighs 130,000 kg (286,340 lb). Other examples of mysticetes are humpback whales, right whales, gray whales, and minke whales. See Mammalia