The stoat (Mustela erminea), also known as the short-tailed weasel, is a species of Mustelidae native to Eurasia and North America, distinguished from the least weasel by its larger size and longer tail with a prominent black tip. The name ermine is often, but not always, used for the animal in its pure white winter coat, or the fur thereof. In the late 19th century, stoats were introduced into New Zealand to control rabbits. The stoats have had a devastating effect on native bird populations (see stoats in New Zealand).
It is classed by the IUCN as Least Concern, due to its wide circumpolar distribution, and because it does not face any significant threat to its survival. It was named one of the world's top 100 "worst invasive species" by the IUCN Species Survival Commission's Invasive Species Specialist Group.
The ermine luxury fur is often used by Catholic monarchs, Pontiffs and Cardinals who sometimes use it as the mozetta cape, and devotional images such as the Infant Jesus of Prague. It also forms part of the parliamentary and coronation robes of British peers of the Realm.
The winter fur is very dense and silky, but quite closely lying and short, while the summer fur is rougher, shorter and sparse. 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 the reverse direction. The moult, initiated by photoperiod, starts earlier in autumn and later in spring at higher latitudes. In the stoat's northern range, it adopts a completely white coat (save for the black tail-tip) during the winter period. Differences in the winter and summer coats are less apparent in southern forms of the species. In the species' southern range, the coat remains brown, but is denser and sometimes paler than in summer.