Least Weasel (Mustela nivalis)

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Author of the description: 
Sallai Tibor
Mustela nivalis

Name: Least Weasel (Mustela nivalis)

Scientific classification:

Kingdom: Animalia

Phylum: Chordata

Class: Mammalia

Order: Carnivora

Family: Mustelidae

Genus: Mustela

Species: Mustela nivalis

The least weasel has a circumboreal, Holarctic distribution, encompassing much of Europe and North Africa, Asia and northern North America, though it has been introduced in New Zealand, Malta, Crete, the Azore Islands and also Sao Tome off west Africa. It is found throughout Europe and on many islands, including the Azores, Britain (but not Ireland), and all major Mediterranean islands. It also occurs on Honshu and Hokkaido islands in Japan and on Kunashir, Iturup, and Sakhalin Islands in Russia. 

Physical 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 large, bulging and dark coloured. The legs and tail are relatively short, the latter constituting less than half its body length. The feet are armed with sharp, dark 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. It usually has 4 pairs of nipples, but these are only visible in females. The baculum is short (16–20 mm), 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 x 5 mm, 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 18–35 cm.

Dimensions vary geographically, to an extent rarely found among other mammals. Least weasels of the vulgaris 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 from 13−30% of the length of the body. Average body length in males is 130–260 mm, while females average 114–204 mm. The tail measures 12–87 mm in males and 17–60 mm in females. Males weigh 36−250 grams, while females weigh 29.5−117 grams.

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. The dividing line between the dark upper and light lower parts is straight, but sometimes forms an irregular line. In winter, the fur is pure white, and only exhibits black hairs in rare circumstances.


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. Rabbits are commonly taken, but are usually young specimens. Rabbits become an important food source during the spring, when small rodents are scarce and rabbit kits 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 foxes and birds of prey. It is adapted for pursuing its prey down tunnels, though it may also bolt prey from their burrows and kill it in the open. It kills small prey, such as voles, with a bite to the occiputal region of the skull or the neck, dislocating the cervical vertebrae. Large prey typically dies of blood loss or shock. When food is abundant, only a small portion of the prey is eaten, usually the brain. The average daily food intake is 35 grams, which is equivalent to 30−35% of its body weight.

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 its range during spring or during food shortages. Its scent marking behaviour is similar to the stoat's; 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 dens, but nest in the abandoned burrows of other species such as moles and rats. The burrow entrance measures about 2.5 cm across and leads to the nest chamber located up to 15 cm below-ground. The nest chamber (which is used for sleeping, rearing kits and storing food) measures 10 cm in diametre, 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 species' 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.


The least weasel mates in April–July, with a 34−37 day gestation period. In the northern hemisphere, the average litter size consists of 6 kits, which reach sexual maturity in 3–4 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 Septmember-February.

The female raises its kits alone, which are 1.5−4.5 grams in weight when born. 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 2–3 weeks of age, at which point they are weaned, though lactation can last 12 weeks. The eyes and ears open at 3–4 weeks of age, and by 8 weeks, killing behaviour is developed. The family breaks up after 9–12 weeks.

Non-endangered species, protected in Hungary. Notional value of 10,000 Ft