Species of mammal
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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.
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. Eighteen subspecies are recognised.
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.
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; 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.
Subspecies.
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:
Description.
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, , 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 , 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 .
Dimensions vary geographically, to an extent rarely found among other mammals. Least weasels of the "boccamela" group, for example, may outweigh the smaller races by almost four times. In some large subspecies, the male may be 1.5 times longer than the female. Variations in tail length are also variable, constituting 13 to 30 percent of the length of the body. Average body length in males is , while females average . The tail measures in males and in females. Males weigh , while females weigh .
The winter fur is dense, but short and closely fitting. In northern subspecies, the fur is soft and silky, but coarse in southern forms. The summer fur is very short, sparser and rougher. The upper parts in the summer fur are dark, but vary geographically from dark-tawny or dark-chocolate to light pale tawny or sandy. The lower parts, including the lower jaw and inner sides of the legs, are white. There is often a brown spot at the corner of the mouth. The dividing line between the dark upper and light lower parts is usually straight but sometimes forms an irregular line. The tail is brown, and sometimes the tip is a little darker but it is never black. In the northern part of its range and at high altitudes, the least weasel changes colour in the winter, the coat becoming pure white and exhibiting a few black hairs in rare circumstances.
Distribution and habitat.
The least weasel has a circumboreal, Holarctic distribution, encompassing much of Europe and North Africa, Asia and parts of northern North America. It has been introduced to New Zealand, Malta, Crete, the Azore Islands and also São Tomé off West Africa. It occurs throughout Europe, except Ireland, and on all major Mediterranean islands. It also occurs on Honshu and Hokkaido Islands in Japan and on Kunashir, Iturup, and Sakhalin Islands in Russia.
The least weasel inhabits fields, open woodland, bushy and rocky areas, parks and gardens at elevations of up to about .
Fossilised remains of the least weasel are known from Denisova Cave.
Behaviour and ecology.
Reproduction and development.
The least weasel mates in April–July and there is a 34- to 37-day gestation period. In the Northern Hemisphere, the average litter size consists of 6 kits and these reach sexual maturity in 3 to 4 months. Males may mate during their first year of life, though this is usually unsuccessful. They are fecund in February–October, though the early stages of spermatogenesis do occur throughout the winter months. Anestrus in females lasts from September until February.
The female raises her kits without help from the male. They are in weight at birth. Newborn kits are born pink, naked, blind and deaf, but gain a white coat of downy fur at the age of 4 days. At 10 days, the margin between the dark upper parts and light under parts becomes visible. The milk teeth erupt at 2 to 3 weeks of age, at which point the young start to eat solid food, though lactation can last 12 weeks. The eyes and ears open at 3 to 4 weeks of age, and by 8 weeks, killing behaviour is developed. The family breaks up after 9 to 12 weeks. There is a single litter each year and least weasels can live for 7 or 8 years.
Territorial and social behaviours.
The least weasel has a typical mustelid territorial pattern, consisting of exclusive male ranges encompassing multiple female ranges. The population density of each territory depends greatly on food supply and reproductive success, thus the social structure and population density of any given territory is unstable and flexible. Like the stoat, the male least weasel extends his range during spring or during food shortages. Its scent marking behaviour is similar to that of the stoat; it uses faeces, urine and anal and dermal gland secretions, the latter two of which are deposited by anal dragging and body rubbing. The least weasel does not dig its own den, but nests in the abandoned burrow of another species such as a mole or rat. The burrow entrance measures about across and leads to the nest chamber located up to below ground. The nest chamber (which is used for sleeping, rearing kits and storing food) measures in diameter, and is lined with straw and the skins of the weasel's prey.
The least weasel has four basic vocalisations; a guttural hiss emitted when alarmed, which is interspersed with short screaming barks and shrieks when provoked. When defensive, it emits a shrill wail or squeal. During encounters between males and females or between a mother and kits, the least weasel emits a high-pitched trilling. The least weasel's way of expressing aggression is similar to that of the stoat. Dominant weasels exhibit lunges and shrieks during aggressive encounters, while subdominant weasels will emit submissive squeals.
Diet.
The least weasel feeds predominantly on mouse-like rodents, including mice, hamsters, gerbils and others. It usually does not attack adult hamsters and rats. Frogs, fish, small birds and bird eggs are rarely eaten. It can deal with adult pikas and gerbils, but usually cannot overcome brown rats and sousliks. Exceptional cases are known of least weasels killing prey far larger than themselves, such as capercaillie, hazel hen and hares. In England, a favoured prey item is the field vole ("Microtus agrestis"). These have fluctuations in population size, and in years of abundance may form up to 54% of the weasel's diet. In years of scarcity, birds form a greater proportion of the diet and female least weasels may fail to breed.
Despite its small size, the least weasel is a fierce hunter, capable of killing a rabbit five to 10 times its own weight. Although they are commonly taken, the rabbits are usually young specimens, and become an important food source during the spring, when small rodents are scarce and rabbit kits are plentiful. Male least weasels take a higher proportion of rabbits than females, as well as an overall greater variety of prey. This is linked to the fact that being larger, and having vaster territorial ranges than females, males have more opportunities to hunt a greater diversity of prey.
The least weasel forages undercover, to avoid being seen by foxes and birds of prey. It is adapted for pursuing its prey down tunnels, though it may also bolt prey from a burrow and kill it in the open. The least weasel kills small prey, such as voles, with a bite to the occipital region of the skull or the neck, dislocating the cervical vertebrae. Large prey typically dies of blood loss or circulatory shock. When food is abundant, only a small portion of the prey is eaten, usually the brain. The average daily food intake is , which is equivalent to 30–35% of the animal's body weight.
Predators and competitors.
The least weasel is small enough to be preyed upon by a range of other predators. Least weasel remains have been found in the excrement of red foxes, sables, steppe and forest polecat, stoats, eagle owls and buzzards. The owls most efficient at capturing least weasels are barn, barred, and great horned owls. Other birds of prey threatening to the least weasel include broad-winged and