Pronounced sexual dimorphism, as well as great geographical and individual variability throughout the extensive geographic range of the Least Weasel, has led to the naming of many taxa of species or subspecies rank. Sheffield and King (1994) recognised ten subspecies, whilst Abramov and Baryshnikov (2000) later recognised 19 subspecies.

The Tonkin Weasel M. tonkinensis and the Sichuan Weasel M. russelliana were included as part of M. nivalis by Abramov and Baryshnikov (2000) (and in previous versions of its Red List assessment), however, they are now treated as distinct species following Groves (2007).

The Egyptian Weasel M. subpalmata was considered to be distinct from the Least Weasel (Abramov and Baryshnikov 2000, Wozencraft 2005), and has been assessed for the Red List as a separate valid species but it is not supported by recent molecular data (Rodrigues et al. 2016), however, M. subpalmata is retained at present pending review by the Small Mammal Specialist Group Red List Authority.

A weasel population found in the central mountains of Taiwan at the end of 1990s and considered by its finders to be a new species allied to Erminea M. erminea, was later regarded to be part of M. nivalis (Abramov 2006, Lin et al. 2010). High level of genetic variation in M. nivalis throughout the species range is revealed by recent molecular studies (Kurose et al. 2005, Lebarbenchon et al. 2010, Sato et al. 2020).

European regional assessment: Least Concern (LC)
EU 27 regional assessment: Least Concern (LC)

The Least Weasel is a widespread species that is locally abundant in at least parts of its range. Although declines have been observed in the UK and are suspected in other parts of Europe, it is not thought that these approach the threshold for the population decline criterion of the IUCN Red List (30% in ten years or three generations), for this reason, it is listed Least Concern for both Europe and for the EU27 member states. Monitoring is required, and if new evidence emerges suggesting that declines are of a greater magnitude than is presently thought, the species should be reassessed.

The Least Weasel has a very large circumboreal distribution, covering most of Europe. It occurs throughout the Palearctic, from the British Isles (except Ireland) eastward to the Urals and including some Arctic islands. The Least Weasel has been introduced to many Mediterranean islands and to the Azores.

Outside Europe, this species is found in northern Asia, North Africa and North America, from Siberia to Afghanistan, Mongolia, eastern China, and Korea. It is also found on some Pacific islands: Sakhalin and Kuril islands (Shumshu, Paramushir, Kunashir), the Japanese islands (Hokkaido and northern Honshu), and Taiwan. It is distributed in North America from Alaska south to the Appalachian Mountains. It has been introduced to Sao Tome off West Africa and New Zealand; populations in North Africa (Morocco, Algeria and Tunisia) also were introduced by humans (Lebarbenchon et al. 2010, McDevitt et al. 2012, Rodrigues et al. 2017).

In the European part of its range, there are documented population declines in some areas (e.g., Britain: Battersby 2005), and suspected declines in others. In Eurasia, it is relatively common, but not often seen (Sheffield and King 1994). Local population densities of 0.2 to 1.0 individuals per hectare can occur in favoured habitats when prey are abundant (Sheffield and King 1994). However, over wider areas, the average population density may be as low as 1 to 7 per 100 hectares (Goszczynski 1977). Populations fluctuate both seasonally and annually, sometimes involving large increases of up to 10-fold, concurrently or within nine months of a population peak of small rodents, and lasting 6 to 18 months (Sheffield and King 1994).

Throughout the distribution range, the Least Weasel occurs in different habitats – from Arctic tundra to sand deserts and alpine mountains (up to 3,860 m asl). Habitat selection is usually determined by the local distribution of rodents. It is often found in open forests, farmlands and cultivated fields, grassy fields and meadows, riparian woodlands, hedgerows, alpine meadows and forests, scrub, steppes, semi-deserts and deserts, prairies, and coastal dunes (Sheffield and King 1994). It is well adapted to tundra and snowy environments and ranges above the tree line in Arctic and alpine areas. Snow presents a little obstacle to the distribution, hunting, or movements of M. nivalis. Least Weasel is a specialist predator of small mammals (especially rodents), although it also occasionally feeds on birds’ eggs, fishes, lizards, amphibians, invertebrates and carrion. Small rodents (voles, mice, lemmings, etc.) are the main diet for M. nivalis, but it can successfully attack and kill animals much larger than itself. Least Weasels are sufficiently small to follow and kill small rodents in confined spaces, such as narrow tunnels and burrows. In northern regions, the Least Weasel spends part of the year hunting under the snow, searching for vole and Lemming nests in the subnivean environments.

The Least Weasel is a solitary animal – no pair bond between adults. Males and females live separately for most of the year, except during the breeding season. Home range is around 1–30 ha, and varies with sex and region (King and Powell 2007). The size of the home range depends upon habitat, population density, season, and prey density. Females spend more time hunting in rodent tunnels and can therefore live on disproportionately small home ranges. Den sites may include burrows taken over from prey, holes in tree trunks or under tree roots through forest habitats or in piles of logs, ditches, and isolated patches of scrub in open habitats. Breeding may continue throughout the year but is concentrated in spring and late summer. In the Arctic zone, when small rodents are abundant, M. nivalis can breed during winter under the snow. When prey is abundant, adults can produce a second litter in late summer or autumn, and early–born young females can produce their first litter at this time. No delayed implantation (in contrast to M. erminea). Gestation is 34–36 days. The litter size averages 4–5, range 1–11. The family splits up after 3–4 months. Most wild individuals do not survive 1–2 years, but captives have lived up to 10 years.

The Least Weasel is preyed upon by most larger predators, including raptors, snakes, people, and larger carnivorans, including foxes, domestic cats and dogs. Threats include incidental poisoning with rodenticides (Sheffield and King 1994) and persecution. The Least Weasel prefers open agricultural habitats, which are declining owing to changes in agricultural practices (rural abandonment) in parts of Europe, as open fields undergo succession. The Least Weasel is rare and of little or no commercial value.

There is no use or trade of this species.

This species is found in many protected areas. It is listed on Appendix III of the Bern Convention (Pulliainen 1999) and is protected under national and sub-national legislation in a number of range states. Monitoring is required to quantify the population trend in Europe.