Least Weasel (Mustela nivalis): Smallest Carnivore Facts

Appearance

The least weasel has a long, thin, extremely flexible body, short legs, a small flat head no wider than its neck, small rounded ears and dark, bead-like eyes. Its build lets it slip into any hole a mouse can use. The feet have sharp claws and furry soles.

In summer the upper parts are brown, from rich chocolate to pale sandy brown depending on the region, and the underparts are white. The line between the two colors is usually fairly straight. The short tail is brown to the tip and never black, which is the easiest way to tell it from the larger stoat; see stoat vs weasel for all the differences.

In the northern part of the range and high in mountains, least weasels molt into a pure white winter coat. The species name nivalis comes from the Latin word for snow. In milder places such as Britain and southern Europe, they stay brown all year, and in some areas they turn only partly white.

How big is a least weasel?

Few mammals vary in size as much across their range. The smallest least weasels, in North America and Siberia, can weigh as little as 25–60 g (0.9–2 oz), about as much as two or three mice. The largest, in North Africa, Central Asia and parts of southern Europe, can weigh about four times as much.

Males are clearly larger than females, often about half as heavy again or more. A typical British male weighs around 100–130 g (3.5–4.6 oz), a female about 55–70 g (1.9–2.5 oz). This tiny size is why the least weasel is generally regarded as the smallest member of the order Carnivora, explained in detail in what is the smallest carnivore?.

Habitat and range

The least weasel has one of the widest ranges of any carnivore. It lives across almost all of Europe, including Great Britain but not Ireland, and on most large Mediterranean islands. It reaches North Africa in Morocco, Algeria, Tunisia and Egypt, and spreads across Asia from Turkey and the Middle East through Siberia to Japan.

In North America it lives in Alaska, most of Canada and the northern United States, south through the northern Great Plains and along the Appalachian Mountains. People introduced it to New Zealand in the 1880s in a failed attempt to control rabbits, and it also lives on Malta, Crete and the Azores.

Least weasels can live almost anywhere with enough small rodents: meadows, farmland, hedgerows, field margins, young plantations, open woodland, marshes, sand dunes, steppe, tundra and mountain slopes up to about 3,000 m (9,800 ft). They avoid dense forest and open ground with no cover, because they need tall grass, stone walls, log piles or snow to hide from predators.

What do least weasels eat?

Least weasels are specialist hunters of small rodents. Voles, mice and lemmings make up most of the diet, and in Britain the field vole alone can form half of it in good vole years. They also eat shrews, young rats, moles, birds, eggs and chicks, and occasionally lizards, frogs and insects. Males, being bigger, take young rabbits more often than females do.

The weasel’s slim body lets it follow voles and mice into their burrows and into the tunnels they make under snow, where few other predators can reach. It kills small prey with a quick bite to the back of the skull or neck. Because it loses heat fast and cannot store much fat, it must eat about a third of its body weight every day, and it caches surplus kills in its den. The full story is in what do weasels eat?.

Behavior

Least weasels are solitary and active by both day and night, hunting in short bursts separated by rest. They move in quick bounds, often stopping to stand upright on their hind legs to look around. They climb and swim well, and can get through gaps of about 2.5 cm (1 in).

Each adult has a home range, typically from about 1 to 25 hectares (2.5 to 60 acres), much smaller where rodents are abundant. A male’s range usually overlaps those of one or more females. Weasels mark their ranges with droppings, urine and scent from their anal glands and skin glands, which they rub on the ground.

They do not dig their own homes. Instead, a weasel takes over the burrow of a vole, mole or other rodent, often the one whose owner it has just eaten, and lines the nest chamber with dry grass and the fur of its prey. When threatened, a least weasel hisses, gives sharp barking shrieks and releases a strong-smelling musk.

Reproduction and life cycle

Least weasels breed mainly from spring to late summer. Unlike the stoat, the least weasel has no delayed implantation: pregnancy lasts only about 34–37 days. A litter usually has 4–8 kits, with an average of about six. In years when voles are very common, females may raise two litters, and young females born early in the year can breed in their first summer.

Newborn kits weigh only about 1.5–4.5 g (0.05–0.16 oz) and are blind, deaf and pink. They grow a fine white down within days. Their eyes open at 3–4 weeks, they can kill prey by about 8 weeks, and the family breaks up at around 9–12 weeks. The mother raises the kits alone.

This fast breeding lets weasel numbers rise quickly when rodents boom and crash when they become scarce. In poor years many females fail to breed at all.

How least weasels survive winter

Winter is the hardest time for an animal this small. A least weasel’s long, thin body has a large surface area for its weight, so it loses heat much faster than a rounder animal of the same mass, such as a vole. It also has little room to store fat. To cope, it spends much of the coldest weather inside a nest of dry grass and fur, and hunts in short bursts.

Snow, surprisingly, helps. Under a thick snow layer there is a sheltered space next to the ground, called the subnivean zone, where the temperature stays close to freezing even when the air above is far colder. Voles and lemmings stay active there all winter, and the least weasel is one of the few predators slim enough to follow them through their snow tunnels. In the far north, the weasel’s white winter coat hides it from owls and foxes on the few occasions it comes to the surface.

Lifespan, predators and threats

Least weasels live fast and die young. Most wild weasels live only one or two years, and few reach four. In captivity, individuals have lived up to about 10 years.

Their small size makes them prey for many larger animals: owls such as barn owls, tawny owls, eagle-owls and great horned owls; hawks and buzzards; red foxes; stoats; and some snakes. Domestic cats also kill them. Cold winters and crashes in vole numbers cause many deaths through starvation.

Human threats are mostly indirect. Intensive farming that removes hedgerows and field margins reduces rodent numbers and cover, and rodent poisons can harm weasels that eat poisoned prey. Roads also kill some.

Conservation status

The IUCN Red List classifies the least weasel as Least Concern. It has an enormous range across three continents, adapts to farmland and is common in many areas. Local numbers can vary greatly from year to year with rodent populations, which makes long-term trends hard to measure. In New Zealand, where it was introduced, it is considered an invasive predator of native wildlife, although it is much less widespread there than the stoat.

Myths and folklore

Because it is small and fierce, the least weasel appears in many legends. In parts of Europe it was believed to be the only animal that could kill the mythical basilisk. Old stories also claimed weasels gave birth through their ears or mouths, which is untrue.

A common modern idea is that weasels suck the blood of their prey. They do not; they kill with bites to the head and neck and then eat the flesh. The famous “weasel war dance” of leaps and twists is real, but there is no evidence that it hypnotizes prey.

Relationship with people

Farmers have long valued weasels as natural controllers of voles and mice, although they were also persecuted in the past for taking eggs and chicks from poultry yards and game bird rearing pens. Today least weasels often live close to people on farms, in parks and large gardens, where they are rarely noticed. A glimpse of one is usually just a brown streak crossing a path or a small head peering out of a stone wall.