How the Arctic Fox Survives −50 °C

The Arctic fox (Vulpes lagopus) lives where the annual mean temperature is below freezing and winter darkness lasts for months. It does not hibernate, does not migrate in any organised way, and does not shelter for the season. It simply keeps working through the polar winter, and its body is a catalogue of how to make that survivable.

The best coat in the animal kingdom#

Measured by insulation per unit thickness, Arctic fox winter fur outperforms that of any other mammal, including polar bear and Arctic wolf. It is a two-layer system: a dense woolly underfur that traps still air, and longer guard hairs that shed wind and moisture. The species name lagopus — 'hare-foot' — refers to the fur that grows over the pads, insulating the feet and improving grip on ice.

The result is that an Arctic fox's lower critical temperature — the point at which it must burn extra energy to stay warm — sits near −40 °C. Below that it increases metabolic output only modestly, and shivering does not begin in earnest until roughly −70 °C, colder than any temperature it will encounter in most of its range.

Warm body, cold feet — on purpose#

Keeping the feet at core temperature on sea ice would be ruinously expensive. Instead, arteries carrying warm blood into the leg run alongside veins carrying cold blood back, and heat transfers between them before it reaches the extremity. The paw is held just above freezing while the core stays at 38 °C, and the amount of heat leaking into the ice is minimised. The same counter-current heat exchange appears in reindeer legs, gull feet and whale flippers.

Two colour morphs, two strategies#

The morphs are the same species and interbreed freely; the ratio between them tracks how much of the winter landscape is actually white. On Iceland and coastal Greenland, where wind strips the rock bare and food comes from the shoreline, blue foxes predominate. The moult between summer and winter coats is triggered by day length, which is why a warming Arctic — where snow arrives late — leaves white foxes conspicuously mismatched against brown ground for weeks.

Eating in a landscape with no plants worth eating#

Inland, the Arctic fox is a lemming specialist, and its breeding success rises and falls with the lemming cycle. In a peak year a vixen may produce a litter of a dozen or more cubs — the largest litters of any wild canid — and in a crash year many pairs do not breed at all.

Coastal foxes live differently: seabird colonies, eggs, washed-up fish and marine carrion sustain them, and the population is far more stable. Both groups cache heavily, burying eggs and carcasses in permafrost that functions as a natural freezer, and both will follow polar bears across the sea ice to scavenge seal kills — a risky commute that can take a fox hundreds of kilometres from land.

The pressure they are actually under#

Globally the species is listed as Least Concern, but that number conceals a serious regional problem. In Fennoscandia the population was reduced to a few hundred animals by historic fur trapping and has never fully recovered, and it now faces a second threat: as the climate warms, red foxes are expanding north. Red foxes are larger, dominant in direct competition, and will kill Arctic fox cubs and take over their dens. Where the tree line and the shrub tundra move north, the Arctic fox loses ground it cannot get back.

For the opposite end of the same design problem — a fox built to dump heat rather than conserve it — see the fennec fox.

If you want to see how differently the same problem can be solved, put this next to the fennec fox, which is built to dump heat rather than hold it. And for the animal doing the squeezing in Fennoscandia, the northward spread is part of the wider story in where do foxes live and are foxes endangered.