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The genus ''Berardius'' encompasses two species of beaked whale which have an antitropical distribution; Arnoux’s beaked whale ''Berardius arnuxii'' in cold Southern Hemisphere waters and Baird’s beaked whale ''Berardius bairdii'' in the cold temperate waters of the North Pacific. There has been some debate over whether these two forms represent distinct species or whether they are simply geographic variants. Several morphological characters have been suggested to distinguish them, but the validity of each has been disputed; currently, it seems that there are no significant skeletal or external differences between the two forms, except for the smaller size of the southern specimens known to date. ''Berardius'' spp. are the largest of the beaked whales, growing up to 10–12 m in length. They are sometimes referred to as 'four-toothed whales' or 'giant beaked whales', but are most commonly known by their genus name, ''Berardius''. The morphological similarity of these forms gave rise to the hypothesis that the northern and southern populations were sympatric as recently as the last Pleistocene Ice Age, approximately 15,000 years ago,〔 but recent genetic analyses suggest otherwise. Phylogenetic analyses of the mitochondrial DNA control region (D-loop) revealed that Baird's and Arnoux's beaked whales were reciprocally monophyletic—lineages from each of the species grouped together to the exclusion of lineages from the other species. Diagnostic DNA substitutions were also found. These results are consistent with the current classification of Baird's and Arnoux's beaked whales as distinct species. Further, the degree of differentiation between the northern and southern forms of Berardius suggest that the species may already have been separated for several million years.〔 Arnoux's beaked whale was described by Georges Louis Duvernoy in 1851. The genus name honors admiral Auguste Bérard (1796-1852), who was captain of the French corvette ''Le Rhin'' (1842-1846), which brought back the type specimen to France where Duvernoy analyzed it; the specific name honors Dr. Maurice Arnoux, the ship's surgeon who found the skull of the type specimen on a beach near Akaroa, New Zealand.〔Beolens, Bo, Michael Watkins, and Michael Grayson. 2009. ''The eponym dictionary of mammals''. Baltimore: Johns Hopkins University Press, p. 38, 54.〕 Baird's beaked whale was first described by Leonhard Hess Stejneger in 1883 from a four-toothed skull he had found on Bering Island the previous year. The species is named for Spencer Fullerton Baird, a past Secretary of the Smithsonian Institution.〔''Sharks and Whales'' (Carwardine et al. 2002), p. 356.〕 ==Physical description== The two species have very similar features and would be indistinguishable at sea if they did not exist in disjoint locations. Arnoux's is generally smaller. Estimated lengths of live Arnoux's at sea have been up to , but all dead specimens have been considerably smaller. The Baird's, on the other hand, have been confirmed to grow to . The weight is up to . Both whales have a very long prominent beak, even by beaked whale standards. The lower jaw is longer than the upper and the front teeth are visible even when the mouth is fully closed. The melon is particularly bulbous. The body shape is slender—the girth is only 50% of length. The body is uniformly coloured and a particular individual's colour may be anything from light grey through to black. The flippers are small, rounded and set towards the front of the body. The dorsal fin similarly is small and rounded and set about three-quarters of the way along the back. Adult males and females of both species pick up numerous white linear scars all over the body as they age and may be a rough indicator of age. There is little sexual dimorphism in either species. 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「giant beaked whale」の詳細全文を読む スポンサード リンク
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