The dimensions of the stoat are variable and, unusually among the Carnivora, the size of stoats tends to decrease proportionally with latitude, in contradiction to Bergman's Rule. There is pronounced sexual dimrphism in size, with males weighing 1.5-2.0 times as much as females. On average, males measure 19–33 cm in body length, and females measure 17–27cm. The tail measures 7.5–12 cm in males and 6.5–11 mm in females. The height at the ear measures 14-23 cm. On average, males weigh 258 grams, while females weigh less than 180 grams.
In summer, the fur is sandy-brown on the back and head and a white below. The division between the dark back and the light belly is usually straight, though this trait is only present in 13.5% of Irish stoats. The stoat moults twice a year. In spring, the moult is slow, starting from the forehead across the back toward the belly. In autumn, the moult is quicker, progressing in reverse direction.
The stoat (or: ermine) occupies a wide range of habitats. These animals are often found in successional or forest-edge habitats, in scrub, alpine meadows, marshes, riparian woodlands, hedgerows, and riverbanks that have high densities of small mammals, especially Microtus and Arvicola species. Coniferous and mixed woodlands are preferred, but many other habitats are used including tundra and the summits of fells and mountains. Dense forests and deserts are avoided. In Mongolia in particular, they inhabit taiga, forest-steppe and rocky parts of the semi-desert.
This species is a specialist predator of small mammals, but will occasionally feed on fruit, earthworms, insects, eggs, and birds. Its local distribution is typically related to that of small rodents and lagomorphs. The species is nocturnal but is often seen in daylight hours. Estimates for home range size range from 4 to 200 hectares for males, most often falling between 10 to 40 hectares.
Stoats can live up to 7 years and are not monogamous, with litters often being of mixed paternity.
Total adult population size is unknown but it certainly exceeds 100,000. The density and structure of populations of this species are unstable, due to short life spans and high reproductive capacity. Populations are greatly influenced by fluctuations in prey supply (especially small mammals). Population fluctuations of stoats and their prey tend to increase in magnitude at more northerly latitudes, although fluctuations have also been recorded in Spain. In France, the species was declining, but now has stabilized as a result of full protection. In Spain it has been speculated that the population may be declining as a results of declines in the southern water vole, Arvicola sapidus, but the population trend has not been quantified in Spain or Portugal. It is abundant in north-western and central Mongolia, but is rare in the eastern plains. Despite population fluctuations, it is a widespread and abundant species, common in suitable habitats. However, in some parts of its range it is rare.
Overall, the population trend for this species is thought to be stable.
Native in the following countries:
Afghanistan; Albania; Andorra; Austria; Azerbaijan; Belarus; Belgium; Bosnia and Herzegovina; Bulgaria; Canada; China; Croatia; Czech Republic; Denmark; Estonia; Finland; France; Georgia; Germany; Greece; Hungary; India; Ireland; Italy; Japan; Kazakhstan; Kyrgyzstan; Latvia; Liechtenstein; Lithuania; Luxembourg; the former Yugoslav Republic of Macedonia; Moldova; Mongolia; Montenegro; Netherlands; Norway; Pakistan; Poland; Portugal; Romania; Russian Federation; Serbia; Slovakia; Slovenia; Spain; Sweden; Switzerland; Tajikistan; Turkey; Ukraine; United Kingdom; United States; Uzbekistan.
This species has a circumboreal range throughout, Europe, Asia, and North America, from Greenland and the Canadian and Siberian Arctic islands south to about 35°N. In Europe, it is present as far south as 41ºN in Portugal, and is found on most islands with the exception of Iceland, Svalbard, and some small North Atlantic islands. It also does not occur on Mediterranean islands. Its vertical range is from sea level to 3,000 m a.s.l.
On a range-wide scale, no major threats are known. Locally the species may be threatened by unrestricted trapping and habitat loss due to timber harvest or natural disturbance. In the Iberian Peninsula the species is dependent on two Arvicola species, and these are declining, so loss of prey base may be a threat. Habitat loss (e.g. as a result of urbanization) is also a problem in parts of the range. The species is commonly hunted in Russia, where there is also a limited fur trade. In western and central Europe, the stoat was frequently hunted for its white winter fur up until at least the 1930s, with c.30,000 pelts sold in Finland alone during that decade. Availability of prey is the principal factor controlling population density, but disease, parasites and other pressures can also contribute.