Why Do Arctic Foxes Change Color? Summer and Winter Coats

Two coats a year

The Arctic fox is one of only a handful of mammals that turns white in winter, along with animals such as the mountain hare, the snowshoe hare and the stoat. It does not bleach its old fur. Instead it molts, shedding one coat and growing a completely new one.

The winter coat is long, dense and, in most foxes, pure white. It is about twice as long as the summer coat, and roughly 70% of it is fine underfur, which makes it one of the warmest coats of any mammal. The summer coat is short and thin, usually brown or gray-brown on the back and head with a paler cream or yellowish belly.

The spring molt is the most noticeable. From around April into early summer foxes look scruffy and patchy as tufts of white fur fall out to reveal the darker coat beneath. In autumn, roughly from September to November, the new white coat grows in gradually over several weeks.

Why white in winter and brown in summer?

The main reason is camouflage. A white fox is hard to see against snow, which helps it creep up on prey such as lemmings and birds and makes it harder for eagles, wolves and bears to spot. In summer, a white fox would stand out sharply against brown tundra and dark rocks, so a brown coat is the better disguise.

Warmth is the second reason. The white coat is not warm because it is white, but because it is long and dense. The color and the extra thickness come together with the autumn molt, so the fox gets camouflage and insulation at the same time. The thin summer coat helps it shed heat during the brief Arctic summer. More on this in how Arctic foxes survive the cold.

What tells the fox when to change?

The timing of the molt is controlled mainly by day length, as in other mammals with seasonal coats. As the days shorten in late summer and autumn, the change in daylight affects hormones, including melatonin from the pineal gland, which switch on the growth of the winter coat. Lengthening days in spring trigger the reverse.

Because the molt follows the calendar of daylight rather than the weather, a fox cannot speed up or delay it much when snow arrives early or late. In most years this works well, since snow cover in the Arctic usually follows the seasons closely.

Blue foxes: the color morph that stays dark

Not every Arctic fox turns white. The species has two genetically distinct color forms, or morphs:

The difference comes from variants of a single gene, MC1R, which controls dark pigment in the fur. The blue variant is dominant, so a fox needs only one copy to be blue, yet blue foxes remain the minority across most of the range. Blue and white foxes are the same species and can appear in the same litter.

The blue morph is most common where snow is less constant. On rocky coasts and islands such as Iceland, where it makes up the majority of foxes, a dark coat blends in well with cliffs, boulders and dark shorelines for much of the year. On the open, snowy tundra inland, white foxes have the advantage. In the fur trade, the dark pelts were sold as “blue fox”, which is why fur farmers released blue foxes on many islands in Alaska.

Do pups change color too?

Arctic fox pups are born in late spring or early summer with short, dark, woolly fur, and they stay dark for the first weeks of life. They grow a more adult-looking coat during their first summer, and white-morph youngsters turn white along with the adults in their first autumn.

Climate change and the white coat

A seasonal white coat only works if there is snow. As the Arctic warms, snow arrives later in autumn and melts earlier in spring in many areas. That can leave white foxes standing out against bare ground for longer each year, a so-called camouflage mismatch that has been well studied in snowshoe hares and is thought to affect other color-changing animals too.

Less snow and a warmer climate also help the larger red fox spread north into Arctic fox country, where it competes for dens and food and kills Arctic foxes. Scientists see the shrinking value of the white coat as one part of a wider set of pressures on Arctic foxes, explained in are Arctic foxes endangered?.