Why Do Stoats Turn White in Winter? Ermine Coat Explained

How the stoat’s winter molt works

A stoat replaces its whole coat twice a year. It does not bleach its brown fur or change the color of existing hairs. Instead, old hairs are shed and new ones grow in, and in the autumn the new hairs grow without the brown pigment that colors the summer coat.

The main trigger is day length, or photoperiod. As the nights grow longer in autumn, changes in the stoat’s hormones start the molt. The autumn molt is fairly quick and spreads from the belly up toward the back, so a stoat can turn from brown to white within a few weeks. The spring molt back to brown is slower and runs the other way, starting at the head and moving down the back toward the belly.

Temperature also plays a part. In cold years, or at higher latitudes and altitudes, the white coat appears earlier in autumn and stays later into spring. The winter coat is also denser and silkier than the summer coat, so the ermine is better insulated as well as better hidden.

Why does white fur help a stoat?

Stoats are both hunters and hunted. A brown stoat on fresh snow stands out to red foxes, owls, hawks and eagles, all of which kill stoats. A white stoat blends into the snow, especially when it is moving fast between drifts and tunnels.

Camouflage also helps the stoat as a predator. In winter it hunts voles and lemmings in runways under the snow and pounces on rabbits and birds above it. A white hunter can get closer to prey before it is noticed. This is the same strategy used by the Arctic fox, described in why do Arctic foxes change color, and by mountain hares and ptarmigans.

Why the tail tip stays black

The black tip of the tail is present in every coat, summer and winter. The leading explanation is that it acts as a decoy. A hawk or owl swooping on a white stoat on snow sees the moving black spot most clearly and strikes toward it, often missing the body.

This idea was tested in experiments published in the early 1980s, in which trained hawks were flown at moving model weasels. Models with a black spot partway along a long tail were missed more often. A black spot on a very short tail, like that of the least weasel, did not help and made the model easier to catch, which may explain why least weasels have no black tail tip.

Why some stoats stay brown

How white a stoat turns depends on where it lives. Across Scandinavia, Russia, the Alps and the Arctic, almost all stoats become fully white. In southern Europe and other mild lowlands, the winter coat stays brown, only thicker and sometimes paler.

Britain is a good example of the in-between zone. In the Scottish Highlands most stoats turn white, while in southern England white stoats are rare and most stay brown. Many stoats in northern England and lowland Scotland end up partly white, with brown patches on the head and back. In Ireland, the local stoats rarely turn white at all.

These differences seem to come partly from inheritance and partly from local conditions. Populations in snowy places have a strong built-in tendency to turn white, while in mild places colder winters can still make individual stoats whiter than usual, so the same area can see more white stoats in a hard winter.

Ermine fur in history

White ermine pelts, with their black tail tips, were among the most prestigious furs in Europe. They were sewn together to line coronation cloaks and to trim the robes of kings, peers and senior judges, and the pattern of white fur with black spots became the heraldic fur called “ermine”. The ermine pattern still appears on the traditional flag of Brittany in France.

Ermine was also a symbol of purity, linked to a medieval legend that a white ermine would rather be caught than soil its coat. Leonardo da Vinci painted Lady with an Ermine in about 1490. Today ceremonial robes often use rabbit fur or synthetic fur instead.

Climate change and coat-color mismatch

Because the molt is set mainly by day length rather than by snow, a stoat can turn white on schedule even in a year when the snow comes late or melts early. A white animal on brown ground is very easy to see, a problem called camouflage mismatch.

This has been studied most closely in snowshoe hares in North America, where hares that were mismatched with their background died more often from predators. Researchers expect that shorter snow seasons may put similar pressure on stoats and other color-changing animals. Over time, populations may shift toward staying brown in places where snow becomes less reliable.

Do other weasels turn white?

The least weasel also turns white in the north of its range and in mountains, though it never has a black tail tip. In North America, the long-tailed weasel can turn white in snowy areas and has a black tail tip like the stoat. For the full list of differences, see stoat vs weasel.