It is an opportunistic carnivore, and grows up to 30 cm long. It eats rabbits; rodents such as the mouse, vole and rat; other small mammals; birds and their eggs and young; and sometimes fish, reptiles, amphibians, and invertebrates. It is a very skillful tree climber and can descend a trunk headfirst, like a squirrel. The stoat is capable of killing animals much larger than itself. When it is able to obtain more meat than it can eat it will engage in "surplus killing" and often stores the extra food for later. Like other mustelids it typically dispatches its prey by biting into the base of the skull to get at the centers of the brain responsible for such important biological functions as breathing. Sometimes it will also make preliminary bites to other areas of the body. In most areas it coexists with the Least weasel (Mustela nivalis, also known as the European common weasel), and to avoid competition, the Least weasel, the smallest member of order Carnivora, generally takes smaller prey and the stoat slightly larger prey. Where the Least weasel is absent the stoat is smaller (~70 g). Males are much larger than females and generally take larger prey.
The stoat can be found throughout Canada, the western United States, Europe, and Asia in the northern temperate, subarctic and arctic regions. It was introduced into New Zealand to control the rabbit population and is considered a pest because it eats the eggs and young of the native birds. Although it inhabits northern latitudes it is built long and thin, leading to an increased surface area-to-volume ratio and increased dissipation of heat from its body. The advantage of this shape is that it is one of the few species able to follow burrowing animals into their own homes. It partly compensates for this shape by having short legs, small ears, a fast metabolism and, in winter, thick fur.
It is a member of the weasel family, which also includes other weasels, mink, otters, badgers, polecats, the wolverine, martens, the tayra, the fisher and in some taxonomical classifications skunks. This is one of the most species-rich families in order Carnivora. The stoat's coat is a rich medium brown with an off-white belly. It is often referred to as an ermine and in winter, the coat is thicker and the colour changes to clean white. In all seasons it has a pronounced black tip on its tail. The black tip probably serves as a decoy to predators, which would include almost any carnivore large enough to eat a stoat (e.g. wolves, foxes, wolverines, and some birds of prey). This kind of coat is very similar to the coat of the long-tailed weasel (Mustela frenata), a related animal of about the same size which also molts into white in the northern part of its range, and it is easy to confuse these kinds of weasels. Another name for the stoat is the "short-tailed weasel" because its tail is shorter than that of the long-tailed weasel. In general it is found farther north. These animals can be distinguished from the Least weasel because the Least weasel always lacks a black tip on its tail.
The stoat is territorial and relatively intolerant of others in its range, especially others of the same sex. Within its range, it typically uses several dens, often taken from prey species. It usually travels alone, except when it is mating or is a mother with older offspring. It breeds once a year, producing several young per litter, and its mating system is promiscuous. Copulation occurs during the mating season with multiple partners and is often forced by the male, who does not help raise the offspring. Sometimes it occurs when the female is so young she has not even left the den(!) In spite of being such a small animal, the stoat's gestation is among the longest reported for mammals (11 months) because of the adaptation of delayed implantation, in which an egg is not implanted in the uterus until months later. The animal's "real" gestation is much shorter. This is presumably an adaptation to the highly seasonal environment in which the stoat lives. Communication (and also location of prey) occurs largely by scent, since the stoat as typical of mammals has a sensitive olfactory system. As a result much of this communication is missed by human scientists. However, stoats are believed to identify females in estrus by scent, and also the sex, health and age of prey. Some kinds of rodents such as voles have counter-adapted by being able to shut down reproduction (which makes females slower and easier to catch) if they smell the odor of mustelids. The stoat's visual resolution is lower than that of humans and color vision is poor, although night vision is superior. Like most other non-primate mammals they have dichromatic color vision (they can see blue and yellow, but not red). Tactile information is conferred by the vibrissae, or whiskers. Stoats are largely nocturnal/crepuscular but will sometimes come out during the day.
The skins were prized by the fur trade, especially in winter coat, and used to trim coats and stoles. In Europe these furs were a symbol of royalty. The ermine was also considered a symbol of purity in Europe. In some areas of Japan, because of its adorable appearance and somewhat elusive nature it is still considered a symbol of good luck.
In heraldry, the term "ermine" is used to mean a white field strewn with small bell-shaped designs called ermine-spots. This represented a white ermine pelt decorated with black hairs from the tail tip. Variants included "ermines" or "counter ermine" (white spots on black), "erminois" (black spots on gold), and "pean" (gold spots on black); commentators have said that there are many more, but some of these seem to exist only in theory.