Least weasel

The least weasel (Mustela nivalis), little weasel, common weasel, or simply weasel is the smallest member of the genus Mustela, family Mustelidae and order Carnivora. It is native to Eurasia, North America and North Africa, and has been introduced to New Zealand, Malta, Crete, the Azores, and São Tomé. It is classified as least concern by the IUCN, due to its wide distribution and large population throughout the Northern Hemisphere. Eighteen subspecies are recognised.

The least weasel varies greatly in size over its range. 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.

Small rodents form the largest part of the least weasel's diet, but it also kills and eats rabbits, 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 and fierce nature, the least weasel plays an important part in the mythology and legend of various cultures.

The least weasel was given its scientific name Mustela nivalis by Carl Linnaeus in his Twelfth edition of Systema Naturae in 1766; its epithet nivalis comes from the Latin word nix meaning "snow" because it grows a white coat during the winter season. The type locality was Västerbotten 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 (the Egyptian weasel) as a separate species. However, Rodrigues et al. (2016) recognized M. subpalmata as a distinct population of nivalis rather than a distinct 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 thrived 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.

The least weasel has a high geographic variation, a fact which has historically led to numerous disagreements among biologists studying its systematics. The least weasel's subspecies are divided into three categories:

The least weasel has a thin, greatly elongated and extremely flexible body with a small, yet elongated, blunt-muzzled head which is no thicker than the neck. The eyes are small in relation to their head size and are bulging and dark colored. The legs and tail are relatively short, the latter constituting less than half the body length. The feet have sharp, dark-coloured claws, and the soles are heavily haired. The skull, especially that of the small rixosa group, has an infantile appearance when compared with that of other members of the genus Mustela (in particular, the stoat and kolonok). This is expressed in the relatively large size of the cranium and shortened facial region. The skull is, overall, similar to that of the stoat, but smaller, though the skulls of large male weasels tend to overlap in size with those of small female stoats. There are usually four pairs of nipples but these are only visible in females.

The baculum is short, 16 to 20 mm (0.63 to 0.79 in), with a thick, straight shaft. Fat is deposited along the spine, kidneys, gut mesentries and around the limbs. The least weasel has muscular anal glands under the tail, which measure 7 by 5 mm (0.28 by 0.20 in), and contain sulphurous volatiles, including thietanes and dithiacyclopentanes. The smell and chemical composition of these chemicals are distinct from those of the stoat. The least weasel moves by jumping, the distance between the tracks of the fore and hind limbs being 18 to 35 cm (7.1 to 13.8 in).