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Porpoises are a group of fully aquatic marine mammals, that are sometimes referred to as mereswine, all of which are classified under the family Phocoenidae, parvorder Odontoceti (toothed whales). There are 8 extant species of porpoise. They are small toothed whales that are very closely related to oceanic dolphins. The most obvious visible difference between the two groups is that porpoises have shorter beaks and flattened, spade-shaped teeth distinct from the conical teeth of dolphins. Porpoises, and other cetaceans, belong to the clade Cetartiodactyla with even-toed ungulates, and their closest living relatives are the hippopotamuses, having diverged about 40 million years ago. Porpoises range in size from the and Vaquita, the smallest cetacean to be discovered, to the and Dall's porpoise. Several species exhibit sexual dimorphism, in that the females are larger than males. They have streamlined bodies and two limbs that are modified into flippers. Dall's porpoise is one of the fastest cetaceans discovered, with the ability to travel 40 knots. Porpoises have the ability to produce biosonar and it is their primary sensory system. 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. Porpoises are not very widespread, with all specialising near the polar regions, usually near the coast. Porpoises feed largely on fish and squid, much like the rest of the odontocetes. 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. Some porpoises produce a variety of clicks and whistles, which are thought to be primarily for social purposes. A few species, like the harbour porpoise, are highly sociable, but pods, generally, don't exceed ten individuals for most species. Porpoises were, and still are, hunted by some countries by means of drive hunting. 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 Vacquita nearly became extinct in the twentieth century, with a predicted population less than 100 individuals, and, with the extinction of the Baiji, is considered the most endangered cetacean. Besides drive hunting, they also face threats from bycatch, competition (from humans), and marine pollution. Porpoises are sometimes kept in captivity and trained to perform tricks, but breeding success has been poor. ==Taxonomy and evolution== Porpoises, along with whales and dolphins, are descendants of land-living ungulates (hoofed animals) that first entered the oceans around 50 million years ago (Mya). During the Miocene (23 to 5 Mya), mammals were fairly modern. The cetaceans diversified, and fossil evidence suggests porpoises and dolphins diverged from their last common ancestor around 15 Mya. The oldest fossils are known from the shallow seas around the North Pacific, with animals spreading to the European coasts and Sllllouthern Hemisphere only much later, during the Pliocene. *ORDER ARTIODACTYLA * *Infraorder Cetacea * * *Parvorder Odontoceti toothed whales * * * *Superfamily Delphinoidea * * * * *Family Phocoenidae – porpoises * * * * * *Genus †''Haborophocoena'' * * * * * * *''H. toyoshimai'' * * * * * *Genus ''Neophocaena'' * * * * * * *''N. phocaeniodes'' – finless porpoise * * * * * *Genus †''Numataphocoena'' * * * * * * *''N. yamashitai'' * * * * * *Genus ''Phocoena'' * * * * * * *''P. phocoena'' – harbour porpoise * * * * * * *''P. sinus'' – vaquita * * * * * * *''P. dioptrica'' – spectacled porpoise * * * * * * *''P. spinipinnis'' – Burmeister's porpoise * * * * * *Genus ''Phocoenoides'' * * * * * * *''P. dalli'' – Dall's porpoise * * * * * *Genus †''Septemriocetus'' * * * * * * *''S. bosselaersii'' * * * * * *Genus †''Piscolithax'' * * * * * * *''P. aenigmaticus'' * * * * * * *''P. longirostris'' * * * * * * *''P. boreios'' * * * * * * *''P. tedfordi'' Recently discovered hybrids between male harbour porpoises and female Dall's porpoises indicate the two species may actually be members of the same genus. 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「Porpoise」の詳細全文を読む スポンサード リンク
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