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The least weasel (''Mustela nivalis''), or simply weasel in the UK, is the smallest member of the genus ''Mustela'' and of the family Mustelidae (as well as the smallest of the Carnivora), native to Eurasia, North America and North Africa, though it has been introduced to New Zealand, Australia, Malta, Crete, Bermuda, Madeira Island, Azores, Canary Islands, Sao Tome, the Falkland Islands, Argentina and Chile.〔LONG JL 2003. Introduced Mammals of the World: Their History, Distribution and Influence (Cabi Publishing) by John L. Long (ISBN 9780851997483)〕 It is classed as being of least concern by the IUCN, due to its wide distribution and presumed large population. Least weasels from various parts of its range vary greatly in size. The body is slender and elongated and the legs and tail are relatively short. The colour varies geographically, as does the pelage type and length of tail. The dorsal surface, flanks, limbs and tail of the animal are usually some shade of brown while the underparts are white. The line delineating the boundary between the two colours is usually straight. At high altitudes and in the northern part of its range, the coat becomes pure white in winter. Eighteen subspecies are recognised. Small rodents form the largest part of the least weasel's diet, but it also kills and eats rabbits and other mammals, and occasionally birds, birds' eggs, fish and frogs. Males mark their territories with olfactory signals and have exclusive home ranges which may intersect with or include several female ranges. Least weasels use pre-existing holes to sleep, store food and raise their young. Breeding takes place in the spring and summer, and there is a single litter of about six kits which are reared exclusively by the female. Due to its small size, fierce nature and cunning behaviour, the least weasel plays an important part in the mythology and legend of various cultures. ==Taxonomy and evolution== The least weasel was given its scientific name ''Mustela nivalis'' by Carl Linnaeus in his 12th edition of Systema Naturae in 1766. The type locality was Westrobothnia in Sweden. As an animal with a very wide distribution, the morphology of the least weasel varies geographically. The species was reviewed by Reichstein in 1957 and again by van Zyll de Jong in 1992 and Reig in 1997. Youngman (1982) placed it in the subgenus ''Mustela'' while Abramov (1999) considered it should be included in the subgenus ''Gale''. Based on skull characteristics, Reig (1997) proposed that the taxon should be split into four species, ''M. subpalmata'', ''M. rixosa'', ''M. vulgaris'' and ''M. eskimo''. Abrimov and Baryshinikov (2000) disagreed, recognising only ''M. subpalmata'' as a separate species. Within the genus ''Mustela'', the least weasel is a relatively unspecialised form, as evidenced by its pedomorphic skull, which occurs even in large subspecies. Its direct ancestor was ''Mustela praenivalis'', which lived in Europe during the Middle Pleistocene and Villafranchian. ''M. praenivalis'' itself was probably preceded by ''M. pliocaenica'' of the Pliocene. The modern species probably arose during the Late Pleistocene. The least weasel is the product of a process begun 5–7 million years ago, when northern forests were replaced by open grassland, thus prompting an explosive evolution of small, burrowing rodents. The weasel's ancestors were larger than the current form, and underwent a reduction in size to exploit the new food source. The least weasel throve during the Ice Age, as its small size and long body allowed it to easily operate beneath snow, as well as hunt in burrows. It probably crossed to North America through the Bering land bridge 200,000 years ago. 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「least weasel」の詳細全文を読む スポンサード リンク
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