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bighorn sheep
The bighorn sheep (''Ovis canadensis'') is a species of sheep native to North America named for its large horns. These horns can weigh up to , while the sheep themselves weigh up to .〔(【引用サイトリンク】work=Ultimate Ungulate page )〕 Recent genetic testing indicates three distinct subspecies of ''Ovis canadensis'', one of which is endangered: ''O. c. sierrae''. Sheep originally crossed to North America over the Bering land bridge from Siberia: the population in North America peaked in the millions, and the bighorn sheep entered into the mythology of Native Americans. By 1900, the population had crashed to several thousand, due to diseases introduced through European livestock and overhunting. Conservation efforts (in part by the Boy Scouts) have restored the population.
==Taxonomy and genetics==
''Ovis canadensis'' is one of three species of mountain sheep in North America and Siberia; the other two species being ''Ovis dalli'', which includes Dall sheep and Stone's sheep, and the Siberian snow sheep ''Ovis nivicola''. Wild sheep crossed the Bering land bridge from Siberia during the Pleistocene (about 750,000 years ago) and subsequently spread through western North America as far south as Baja California and northwestern mainland Mexico.〔 Divergence from their closest Asian ancestor (snow sheep) occurred about 600,000 years ago.〔 In North America, wild sheep have diverged into two extant species—Dall sheep, which occupy Alaska and northwestern Canada, and bighorn sheep, which range from southern Canada to Mexico. However, the status of these species is questionable given that hybridization has occurred between them in their recent evolutionary history.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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