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.


Common weasels are found throughout Europe, except for Ireland and some of the smaller islands, into North Africa, and across the whole of Asia north of the Middle East and the Himalayas, reaching as far as Japan. Nor does it stop there, for exactly the same species lives in Alaska, Canada, and across much of the Great Plains of the USA. There is, admittedly, considerable variation across this vast range with numerous subspecies and American weasels, for example, being smaller than their European cousins, but, nonetheless, the evidence seems to suggest that they are really are just one species. Even when there is debate that these may represent two or more different species, the proposed dividing line isn't across the oceans, as one might expect, but runs through the middle of Sweden - it's been argued that the weasels of most of Europe may be distinct from those of Lapland, but that the latter are still the same as those in Canada and America.

Although they are capable of taking a wide range of prey, common weasels feed almost entirely on small rodents, especially voles. Their small size and slender, sinuous bodies give them a significant advantage here, for they can pursue such animals into their burrows, scampering down narrow passages and giving their prey no means of escape. In fact, they often take over the burrows of animals they have killed, using them as their own shelter.

However, there is a price to be paid for this ability. Small animals lose body heat easily, and therefore need a faster metabolism than larger animals do, and so, proportional to their size, require a higher calorie intake. The slender bodies of weasels make this worse, because their body surface is larger, relative to their mass, than it might otherwise be, causing them to lose energy even faster.

As a result, weasels need to eat a lot. A common weasel has to eat between five and ten meals every day, and consumes up to a third of its body mass daily - and maybe twice that if it's pregnant. When you have to eat that frequently, even a long sleep isn't an option, and weasels take a number of short naps through the day. While they prefer to be nocturnal, using the cover of darkness to hide from larger carnivores that might try to eat them, their constant need for food means that they also have to be up and about for at least some of the daylight hours.

It helps that they are efficient killers, quickly dispatching prey with a powerful bite to the back of the neck. They can even take animals larger than themselves, holding on with their jaws, forcing the animal to the ground, and then raking with their claws to finish it off. While, of course, they aren't large enough to consume even a small rabbit at a single sitting, taking one down does at least fill their bellies for quite a while.

Weasels also have to be active through the winter. even at times of heavy snow. For most of the year, they have brown fur with distinct white underparts, but in much of their range, they moult in the late autumn, producing a pure white coat (or, in some places, just a paler one) that hides them against the snow, and remains until the spring. This would probably be valuable to fur trappers were it not for the fact that the common weasel is just too small for it to be worthwhile catching and skinning them. Indeed, common weasels are useful animals to have on the farm, because all of that eating helps to keep down the population of mice.

The stoat (Mustela erminea) belongs to a separate evolutionary line to the common weasel, with the two last having had a common ancestor over three million years ago, before the start of the Ice Ages. Nonetheless, the two animals are remarkably similar in both appearance and habits. Stoats are somewhat larger, with heavier bodies and longer limbs, weighing between 210 and 350 grams (7 to 12 ounces), compared with a weight of less than 100 grams (3½ ounces) for most common weasels. However, they are much the same colour, and, in some parts of the world, the largest common weasels are actually larger than the smallest stoats, so the easiest way to tell them apart is that the latter have a much longer tail, which is black for almost half its length, rather than just at the tip.

Like weasels, stoats are adaptable animals, and represent a single species found in Europe, north and central Asia, and North America, albeit with even more distinct subspecies than the common weasel. However, they seem to avoid warmer, drier, climes, and they are not found, for example, around the Mediterranean. They are, however, found in Ireland, apparently having got there under their own steam, which the common weasel failed to do.

The lifestyle of stoats is very similar to that of weasels, although they eat a slightly wider range of prey, including, for example, a much higher proportion of rabbits - the largest stoats can even take down a full-grown hare, an animal much larger than themselves. They store some of their food in chilly larders through the winter, but they will often switch to hunting different prey depending on what is available at certain times of the year, and have been reported to go as far as regularly dining on fruit when meat is in short supply.

Like common weasels, stoats from northern climes moult in the winter, producing a thick white coat. Unlike weasels, they keep the black patch on the end of their tails, and it has been suggested that this can serve to distract potential predators, who may end up directing their attention away from the stoat's main body. That common weasels don't do the same may be due to the fact that their tails are much shorter, making the tactic less useful. Unfortunately for the stoat, it is just large enough for this luxuriant white fur to be worth money, and thousands are killed for it every year. Indeed, the American name for the stoat, and, for that matter, that in almost every other European language besides English, is the same as the name for its winter pelt: ermine.

A stoat in winter

Not all stoats become white in winter, however. For example, those in England, where heavy snow is unusual and often short-lived, they rarely do so, while their neighbours in northern Scotland do so all the time, and those in between just develop some white patches. There's an obvious advantage to that, since being white makes you rather visible if it isn't snowing, but how do they manage it? It isn't just a genetic difference between the populations, because stoats taken from southern England and kept in the cold, do turn white - something they never do in the wild. On the other hand, there is apparently more to it than just temperature, since they regain their summer coats as the days lengthen, even if the room they are kept in remains cold. The details are unclear, although the moult appears to be under hormonal control.

Stoats and common weasels both vigorously defend their patch of land from same-sex members of their own species, although, even outside the breeding season, they aren't so bothered by the other sex, perhaps because males are much larger than females, and maybe aren't eating exactly the same food. They both prefer to mate in spring and summer, but the course of their pregnancies are quite different.

As one might expect for a small animal, common weasels have a short pregnancy, lasting only around five weeks, after which they give birth to a litter of around five young, initially weighing only one or two grams (about one twentieth of an ounce) each. The young are weaned within two months, and leave home within three, by which time they are almost fully grown. That's such a short time that the mother is capable of breeding again almost immediately afterward, and even the young can become parents themselves before the year is out.

Stoats, however, are quite different, and apparently better adapted for the prospect of a long winter. Although they mate at the same time of year, they don't give birth for another ten months, holding the embryo in suspended animation for all but the last month of that period - and thereby ensuring they will be born in the spring. Litters are about the same size, although the young are slightly larger, and covered with a short white fuzz, while newborn common weasels are hairless. The young take slightly longer to wean and leave home than those of weasels, but even so, some females can breed in their first year (although males, apparently, have to wait until the next one).

Even though stoats are hunted for their fur, neither they, nor the common weasels, are at all endangered, and their adaptability keeps both species numerous. They have even been introduced to New Zealand, where they now roam wild, as well as to some Mediterranean islands, and, in the case of the common weasel, to the Azores and the island of São Tomé off the west coast of Africa. Well adapted to farmland, and too small to be much of a menace to human livestock, common weasels and stoats have managed to do pretty well alongside humans.

[Pictures from Wikimedia Commons. Cladogram adapted from Koepfli et al. 2008]