The stoat (Mustela erminea), also known as the short-tailed weasel or the wild otter, is a small mammal of the family Mustelidae.

The stoat is an opportunistic carnivore and grows up to 11.81 in (30 cm) long. It eats rabbits and rodents such as the mouse, vole and rats and other small mammals. It also eats birds, their eggs, and young. They sometimes eat 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.

The stoat can be found almost throughout the northern temperate, sub arctic and arctic regions. It is found in Europe, Asia, Canada, and the U.S. (though it is absent from the south eastern U.S.). It was introduced into New Zealand in an unsuccessful attempt to control the rabbit population and is considered a pest because it eats the eggs and young of native birds. Although it inhabits northern latitudes it is built long and thin. This leads 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, and fast metabolism and, in winter, thick fur.

It is a member of the family Mustelidae. This includes other weasels, mink, otters, ferret, 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. In winter, the coat is thicker and the color changes to clean white when in areas that have an inch or more of snow for at least forty days of the year. 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. This kind of coat is very similar to the coat of the long-tailed weasel. The North American name for the stoat, the “Short-tailed weasel” arose because its tail length distinguishes it from the long-tailed weasel. In general it is found farther north. Both species 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. 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.