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Dolphins are a widely distributed and diverse group of fully aquatic marine mammals. They are an informal grouping within the order Cetacea, excluding whales and porpoises, so to zoologists the grouping is paraphyletic. The dolphins comprise the extant families Delphinidae (the oceanic dolphins), Platanistidae (the Indian river dolphins), Iniidae (the new world river dolphins), and Pontoporiidae (the brackish dolphins). There are 40 extant species of dolphins. Dolphins, alongside 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. Dolphins range in size from the long and Maui's dolphin to the and killer whale. Several species exhibit sexual dimorphism, in that the males are larger than females. They have streamlined bodies and two limbs that are modified into flippers. Though not quite as flexible as seals, some dolphins can travel at . Dolphins use their conical shaped teeth to capture fast moving prey. They have well-developed hearing − their hearing, which is adapted for both air and water, is so well developed 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. Although dolphins are widespread, most species prefer the warmer waters of the tropic zones, but some, like the right whale dolphin, prefer colder climates. Dolphins feed largely on fish and squid, but a few, like the killer whale, feed on large mammals, like seals. Male dolphins 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. Dolphins produce a variety of vocalizations, usually in the form of clicks and whistles. Dolphins are sometimes hunted in places like Japan, in an activity known as dolphin drive hunting. Besides drive hunting, they also face threats from bycatch, habitat loss, and marine pollution. Dolphins have been depicted in various cultures worldwide. Dolphins occasionally feature in literature and film, as in the Warner Bros film Free Willy. Dolphins are sometimes kept in captivity and trained to perform tricks, but breeding success has been poor and the animals often die within a few months of capture. The most common dolphins kept are the killer whales and bottlenose dolphins. ==Etymology== The name is originally from Greek (''delphís''), "dolphin",〔(δελφίς ), Henry George Liddell, Robert Scott, ''A Greek-English Lexicon'', on Perseus Digital Library〕 which was related to the Greek (''delphus''), "womb".〔(δελφύς ), Henry George Liddell, Robert Scott, ''A Greek-English Lexicon'', on Perseus Digital Library〕 The animal's name can therefore be interpreted as meaning "a 'fish' with a womb". The name was transmitted via the Latin ''delphinus''〔(delphinus ), Charlton T. Lewis, Charles Short, ''A Latin Dictionary'', on Perseus Digital Library〕 (the romanization of the later Greek δελφῖνος – ''delphinos''〔(δελφίν, -ῖνος ), Henry George Liddell, Robert Scott, ''A Greek-English Lexicon'', on Perseus Digital Library〕), which in Medieval Latin became ''dolfinus'' and in Old French ''daulphin'', which reintroduced the ''ph'' into the word. The term mereswine (that is, "sea pig") has also historically been used.〔(【引用サイトリンク】title=mereswine - definition and meaning of mereswine at Wordnik.com )〕 The term 'dolphin' can be used to refer to, under the suborder Odontoceti, all the species in the family Delphinidae (marine dolphins including killer and pilot whales) and the river dolphin families Iniidae (South American river dolphins), Pontoporiidae (La Plata dolphin), Lipotidae (Yangtze river dolphin) and Platanistidae (Ganges river dolphin and Indus river dolphin).〔 This term has often been misused in the US, mainly in the fishing industry, where all small cetaceans (dolphins and porpoises) are considered porpoises, while the fish ''dorado'' is called dolphin fish. In common usage the term 'whale' is used only for the larger cetacean species, while the smaller ones with a beaked or longer nose are considered 'dolphins'. The name 'dolphin' is used casually as a synonym for bottlenose dolphin, the most common and familiar species of dolphin. Killer whales also belong to the family Delphinidae and therefore qualify as dolphins. Though the terms 'dolphin' and 'porpoise' are sometimes used interchangeably, porpoises are not considered dolphins and have different physical features such as a shorter beak and spade-shaped teeth; they also differ in their behavior. Porpoises belong to the family Phocoenidae and share a common ancestry with the Delphinidae.〔 A group of dolphins is called a "school" or a "pod". Male dolphins are called "bulls", females "cows" and young dolphins are called "calves".〔(【引用サイトリンク】url=http://dictionary.reference.com/writing/styleguide/animal.html )〕 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「Dolphin」の詳細全文を読む スポンサード リンク
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