Weasels in the Snow: Common Weasels and Stoats

A stoat in summer

In terms of the number of species, the weasel family is the most successful of the carnivoran families. That is, at least in part, due to their small size, allowing them to fill niches unavailable to larger animals such as bears, lions, or wolves. The members of the family that take this to the extreme are, of course, those for which it is named: the weasels themselves.

The term "weasel" isn't a truly scientific one. It's used to refer to all those musteline animals that are neither polecats, mink, nor stoats, and that isn't a natural group of animals. A true evolutionary unit should consist of a common ancestor and all of its descendants, but the term "weasel", while it would plausibly include the common ancestor, arbitrarily excludes some of that animal's descendants. In reality, therefore, the animals commonly referred to as "weasels" include some that are closer to, say, polecats, than they are to other "weasels", and, as a whole, they represent at least three, and probably four, different evolutionary lines.

The common weasel (Mustela nivalis) is the epitome of the idea that, for weasels, small is good. Known in America as the least weasel, at as little as 12 cm (5 inches) long ignoring the tail, it is the smallest member of it's family, and thus, the smallest of all carnivorans. Although it prefers forests or farmland, it is happy to live almost anywhere that there is cover, including mountains and semi-desert, and this adaptability has allowed it to inhabit a wider stretch of the world than any other carnivoran species except the wolf.