How Do Arctic Foxes Survive the Cold? Fur, Feet and Fat

The warmest fur of any fox

The Arctic fox lives year-round on the tundra and sea ice, where winter air temperatures often drop below −40 °C (−40 °F) and the wind makes it feel colder still. Its first line of defense is its winter coat, which is about twice as long as its summer coat.

About 70% of the winter coat is fine, woolly underfur. This dense layer traps still air next to the skin, and still air is an excellent insulator. Longer guard hairs on top shed snow and wind. Measurements of fur insulation have consistently put the Arctic fox’s winter pelt among the warmest of all mammals.

The fur also changes with the seasons. Arctic foxes molt twice a year, swapping the short summer coat for the thick winter one in autumn and back again in spring. Most foxes turn white in winter, which gives them camouflage on the snow as well as warmth; the reasons are explained in why Arctic foxes change color.

A body shaped to save heat

Every square centimeter of exposed skin loses heat, so the Arctic fox keeps its surface area small for its size. Compared with the red fox, it has:

• a rounder, more compact body
• a shorter muzzle
• small, rounded, thickly furred ears
• shorter legs

This pattern, with smaller ears and limbs in colder climates, is a well-known trend among warm-blooded animals. The contrast is clearest with the fennec fox of the Sahara, whose enormous ears do the opposite job and help it shed heat. In the Arctic fox the body parts that lose heat fastest, such as the nose, ears, legs and feet, make up a smaller share of the body surface than in the red fox.

Furry feet that never freeze

The Arctic fox’s scientific name, Vulpes lagopus, means “hare-footed fox”, because in winter the soles of its feet are covered in dense fur, like those of a snowshoe hare. The fur insulates the pads and gives extra grip on ice and snow.

Underneath, the circulation is just as clever. The arteries and veins in the legs run side by side and act as a countercurrent heat exchanger: warm blood flowing down toward the foot passes heat to the cold blood returning to the body, so most of the heat stays in the trunk. At the same time, the fox sends just enough blood to a network of small vessels in the footpads to keep them slightly above the freezing point of tissue, about −1 °C (30 °F). The feet stay cold but working, and the fox loses very little heat through them even when it stands on snow far colder than that.

Fat reserves and a slower winter metabolism

Arctic foxes prepare for winter by eating heavily in late summer and autumn. Some increase their body weight by more than 50%, storing fat under the skin and around the organs. This fat is both extra insulation and an energy store for the lean months, when lemmings are hidden under the snow and storms can stop hunting for days.

In winter the fox also lowers its energy needs. Studies have found that its resting metabolic rate is markedly lower in winter than in summer, and it moves around less when food is scarce. Unlike some Arctic mammals, though, the Arctic fox does not hibernate; it stays active all winter and keeps a normal body temperature.

Stored food helps too. In summer Arctic foxes bury surplus eggs, lemmings and birds in caches, and they dig these up during the winter and the following spring. Read more in what Arctic foxes eat.

Curling up, dens and snow shelters

Behavior is the last piece of the puzzle. To rest in the cold, an Arctic fox curls into a tight ball, tucks its legs and nose underneath and wraps its long, bushy tail over its face and feet. This posture shrinks its exposed surface to a minimum and protects the least-insulated parts of the body.

When the weather turns bad, the fox gets out of the wind. It may shelter in its den, squeeze among boulders or dig a hollow in a snowdrift, where the snow itself acts as insulation. Arctic fox dens are large burrow systems dug into raised, well-drained ground, often facing south, and they can be used by generations of foxes.

How well does it all work?

Older experiments from the mid-20th century suggested that a resting Arctic fox did not need to burn extra energy to stay warm until the air dropped to around −40 °C (−40 °F). Later researchers have questioned how that figure was measured, and more recent work suggests the point at which a resting fox must start working harder to stay warm is considerably higher. Either way, the Arctic fox copes with conditions that would quickly kill most mammals of its size.

Too warm in summer?

The same features can make summer tricky, since Arctic summers can reach 10–20 °C (50–68 °F) in some areas. The fox copes by molting into a much thinner coat, letting more heat escape through its nose, ears, legs and feet, and panting like a dog to cool down by evaporation. Its main challenge is no longer the cold but finding enough food to raise its pups.