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Cetacea is a widely distributed and diverse infraorder of fully aquatic marine mammals. There are 88 extant species of cetaceans. The two suborders of cetaceans, Mysticeti and Odontoceti, are thought to have split up around 34 million years ago. Whales and dolphins, the paraphyletic groups of Cetacea, as well as porpoises, belong to the clade Cetartiodactyla with even-toed ungulates; their closest living relatives are the hippopotamuses which diverged about 40 million years ago. Cetaceans range in size from the and vaquita to the and blue whale, which is also the largest creature on earth. Several species of mysticetes exhibit sexual dimorphism, in that the females are larger than males. They have streamlined bodies and two limbs that are modified into flippers. Some cetaceans can travel at up to 20 knots. Balaenopterids use throat pleats to expand their mouths in order to take in gulps of water. Balaenids have heads that can make up 40% of their body mass. Odontocetes have conical teeth designed for catching fish or squid. Mysticetes have a well developed sense of "smell", whereas odontocetes have well-developed hearing − so well adapted for both air and water that some can survive even if they are blind. Some species are well adapted for diving to great depths. They have a layer of fat, or blubber, under the skin to keep warm in the cold water. Cetaceans are widespread, but some, as with the mysticetes, specialize in certain environments. Most mysticetes prefer the colder waters of the Northern and Southern Hemispheres, and migrate to the equator to give birth. Odontocetes feed largely on fish and squid, but a few, like the killer whale, feed on mammals, such as pinnipeds. Grey whales are specialized for feeding on bottom-dwelling mollusks. Males typically mate with multiple females every year, but females only mate every two to three years. Calves are typically born in the spring and summer months and females bear all the responsibility for raising them. Mothers of some species fast and nurse their young for a relatively long period of time. Some whales produce a variety of vocalizations, notably the songs of the humpback whale. Many species, mainly dolphins, are highly sociable, with some pods reaching over a thousand individuals. Once relentlessly hunted for their products, whales are now protected by international law. Some species are attributed with high levels of intelligence. At the 2012 meeting of the American Association for the Advancement of Science, support was reiterated for a cetacean bill of rights, listing cetaceans as non-human persons. The North Atlantic right whales nearly became extinct in the twentieth century, with a population low of 450, and is considered functionally extinct by cetologists, and the Baiji is also considered functionally extinct by the IUCN with the last sighting in 2004. Besides whaling, they also face threats from bycatch and marine pollution. The meat, blubber and baleen of whales have traditionally been used by indigenous peoples of the Arctic. Whales occasionally feature in literature and film, as in the great white whale of Herman Melville's ''Moby Dick''. Small cetaceans, mainly dolphins, are kept in captivity and trained to perform tricks, but breeding success has been poor. Whale watching has become a form of tourism around the world. ==Biology== Cetaceans are fully aquatic marine mammals. They are not able to survive on land. If cetaceans are stranded, their body weight compresses their lungs or breaks their ribs, as they sometimes can weigh . Smaller whales can die of heatstroke because of their well-developed thermal insulation. The build of cetaceans is well adapted to their habitat, yet they still share essential characteristics with all other higher mammals (Eutheria): *Cetaceans have lungs, meaning they're air-breathers. The amount of time an individual can last without a breath varies from a few minutes to over two hours depending on the species. *Cetaceans have especially powerful hearts. Likewise, the oxygen in the blood is distributed very effectively throughout the body. *Cetaceans are warm-blooded animals, i.e., they hold, as opposed to the cold-blooded animals, a nearly constant, independent body temperature. *Cetaceans give birth to fully developed calves and nurse them with an extremely high-fat milk from specific mammary glands. The embryonic development takes place in the body of the mother. During this time, the embryo is fed by a special nutritive tissue, the placenta. Cetaceans, mainly whales, include the largest animals that ever lived. The blue whale (''Balaenoptera musculus'') can grow to a length of and weigh up to , making it the largest known creature ever. The sperm whale (''Physeter macrocephalus'') is the largest predatory animal on Earth. By contrast, the smallest of cetacean species can only reach a maximum body length of about , such as the La Plata dolphin, Hector's dolphin and the vacquita. Among cetaceans, whales are also distinguished by an unusual longevity compared to other higher mammals. Some species, such as the bowhead whale (''Balaena mysticetus''), can reach an age of over 200 years. Based on the annual rings of the bony otic capsule, the age of the oldest known specimen is a male determined to be 211 years at the time of death. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「cetacea」の詳細全文を読む スポンサード リンク
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