Arctic fox

• 1. Introduction

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  In July 2019, the Norwegian Polar Institute told the story of a young female Arctic fox that had been fitted with a GPS tracker. Researchers released her on the east coast of Spitsbergen, in the Svalbard group of islands. She crossed the polar ice to Greenland in 21 days, a distance of 1,512 kilometers. From there she kept going to Ellesmere Island in northern Canada. By the time her tracker stopped working, she had covered a recorded 3,506 kilometers in 76 days. She averaged just over 46 kilometers a day and managed as much as 155 kilometers on her best day.

  Her species, Vulpes lagopus, is also called the white fox, the polar fox or the snow fox. It is a small fox native to the Arctic regions of the Northern Hemisphere and common across the tundra. How does an animal that weighs only a few kilograms survive an Arctic winter? Why do some of these foxes turn white while others stay dark? And why does a species with several hundred thousand members still have populations close to vanishing?

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• 2. Built for Cold

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  Seventy percent of the Arctic fox's winter coat is fine underfur, and that dense layer gives it the best insulation of any mammal. The fox sheds twice a year, swapping a short, darker summer coat for a long, lighter winter one. The winter coat is twice the length of the summer coat, and the thickness of the fur rises by 140 percent.

  Males average 55 centimeters from head to rear, and the whole animal is built compact. It has a rounded body, a short muzzle, short legs and short, thick ears. That shape gives the fox a low ratio of surface area to volume, so less of its body is exposed to the cold. Foxes that live further north are smaller than their southern relatives. The fluffy tail, about a third as long as the body, does extra work. When the weather turns bad, the fox curls up tightly and tucks its legs and head behind that tail, which leaves the least possible surface area exposed. It also stays out of the wind by resting in its den.

  On snow and frozen ground, the pads of an Arctic fox stay above the temperature at which tissue freezes. The fox manages this with a countercurrent heat exchange in the blood of its legs. It sends more blood to a network of tiny vessels in the surface of the pad, the part that actually touches the snow. At the same time, it narrows the blood vessels in the center of the pad, which saves energy and limits heat loss. The fox keeps its mobility and feels no pain, and its paws hold their temperature independently of the core of its body.

  In winter, the fox's total basal metabolic rate falls 27 percent below its summer level, and the rate per unit of body mass falls 37 percent. That rate is the energy the body burns at rest, so slowing it conserves fat. The fox does not hibernate. Instead it reduces how much it moves, and in autumn it builds up fat reserves, sometimes adding more than 50 percent to its body weight. Its core temperature stays constant all year. Researchers who sequenced its genes found two under positive selection. One, called GLTPD1, is involved in fatty acid metabolism. The other, called AKT2, is tied to glucose metabolism and insulin signaling. Both help the fox endure extreme cold and long stretches without food.

  Some scientists now doubt a figure long repeated for the lowest temperature the fox can face before it must burn extra energy, because it was never tested with the proper equipment. The coat that does all this work also comes in more than one color, and the choice is written in the fox's genes.

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• 3. Blue and White Morphs

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  Two genetically distinct coat colors exist in the Arctic fox, known as the white morph and the blue morph. The white morph is generally brown in summer, with lighter brown to cream sides and belly. In winter it turns entirely white. The blue morph is generally black, brown or grey in summer and becomes only slightly lighter in winter. Two similar mutations to a gene called MC1R cause the blue color and its relative lack of seasonal change.

  The blue allele is dominant over the white allele, which makes the numbers surprising. About 90 percent of the global population is white. Among the continental circumpolar population, 97 to 99 percent are white, and in Svalbard the figure is 84 to 97 percent.

  In Iceland, blue foxes make up 65 to 70 percent of the population. In Fennoscandia, they make up 25 to 30 percent.

  Coat color even predicts where a fox is likely to live. The white morph mostly lives inland, where it blends in with the snowy tundra. The blue morph occupies the coasts, where its dark color blends in with cliffs and rocks.

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• 4. Hunting by Nose and Ear

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  A carcass left behind by a polar bear can be smelled by an Arctic fox from 10 to 40 kilometers away. The fox may even use its nose to track down the bears themselves. Its sense of smell reaches through snow as well. It can find frozen lemmings buried under 46 to 77 centimeters of snow, and it can detect a seal lair beneath 150 centimeters of it.

  The fox's hearing covers a functional range from 125 hertz to 16 kilohertz. It is less sensitive than the hearing of a dog or of the kit fox. Even so, the fox can easily hear lemmings burrowing under 4 to 5 inches of snow. Once it has located its prey, it pounces and punches through the snow to catch it.

  Lemmings are the most common prey wherever they live, and a family of foxes can eat dozens of them each day. The menu is wide. It includes voles, other rodents, hares, birds, eggs, fish and carrion. The fox scavenges carcasses left by wolves and polar bears, and in times of scarcity it even eats their feces. In April and May, it preys on ringed seal pups while the young animals are confined to a snow den and relatively helpless. Berries and seaweed also appear in its diet, so the fox may be considered an omnivore. In some places in northern Canada, migrating birds that breed in the area provide an important food source. On the coast of Iceland and on other islands, the diet consists mostly of birds.

  Snow goose eggs in Canada show how the fox plans ahead. A fox gathers these eggs at a rate of 2.7 to 7.3 eggs an hour and stores 80 to 97 percent of them. Isotope analysis shows that a stored egg can still be eaten after a year. After 60 days, its usable energy has dropped by only 11 percent, from about 816 kilojoules in a fresh egg. The fox is a significant egg predator and takes the eggs of every tundra bird species except the largest. At the beginning of winter, one fox carries about 14,740 kilojoules of energy in fat alone. An average fox of 3.5 kilograms needs 471 kilojoules a day to survive the winter. Eggs stored in summer are dug up the following spring, just before the foxes breed.

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• 5. Dens and Pups

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  Some Arctic fox dens are tunnel systems covering as much as 1,000 square meters. In spring, the fox looks for frost-free, slightly raised ground, and these dens are often dug into eskers, which are long ridges of sediment deposited in formerly glaciated regions. A den can stay in use for many decades and shelter many generations of foxes. Foxes prefer dens with many entrances that are easy to reach and clear of snow and ice. They favor dens that face south toward the sun, which keeps them warmer. Large, maze-like dens help them escape predators, especially when red foxes are nearby. When red foxes are absent, Arctic foxes move into dens that red foxes used before. Shelter quality matters more to them than the closeness of spring prey.

  Breeding usually takes place in April and May, and gestation lasts about 52 days. Litters range from 3 pups to as many as 25, the largest litter size in the order Carnivora. Litter size follows food. White foxes live mainly inland and depend on lemmings, whose numbers rise and fall in a cycle of 3 to 5 years. When lemmings are plentiful, a white fox may bear 6 to 18 pups. When food is scarce, it often does not breed at all. Blue foxes live on the coast, where food is steadier, and they have around 5 to 6 pups each year.

  Newborn pups weigh only 60 to 90 grams. They are blind and deaf until they are 14 to 16 days old. They leave the den at 3 to 4 weeks, they are weaned by 7 weeks, and their fur stays dark until they are 8 weeks old. Natal dens often sit in rugged terrain that may protect the pups, and parents sometimes move a litter to a nearby den to avoid predators.

  Arctic foxes are primarily monogamous, and both parents care for the young. Other family members occasionally help. When predators and prey are both abundant, the foxes become more likely to take multiple mates, and more complex social structures appear. Larger groups can guard a single territory more effectively, which raises pup survival. When resources are scarce, competition grows and fewer foxes share a territory. On the coasts of Svalbard, complex social groups are more common than among inland foxes, which stay monogamous because food is more limited. Scandinavia has more complex structures than other populations because red foxes are present there. Iceland is a special case, where monogamy is the most common arrangement. One-year-olds often stay inside their parents' territory even though predators are absent and resources are thin, which may point to kin selection.

  Most wild Arctic foxes do not live past their first year, and those that survive puphood average 3 to 4 more years. Some reach 11. The oldest age ever recorded was 16, in captivity.

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• 6. Range and Roaming

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  The Arctic fox occurs all the way around the pole, in tundra habitats of northern Europe, northern Asia and North America. Its range includes Iceland, Fennoscandia, Svalbard, other islands in the Barents Sea, northern Russia, islands in the Bering Sea, Greenland, Alaska and Canada as far south as Hudson Bay. It mostly inhabits tundra and pack ice. It also lives in the boreal forests of Canada, from northeastern Alberta to Newfoundland and Labrador, and on the Kenai Peninsula in Alaska. Foxes have been found at elevations up to 3,000 meters above sea level and have been seen on sea ice close to the North Pole. During the last ice age, the range was much larger, and fossils turn up across much of northern Europe and Siberia.

  Iceland has only one land mammal that is native to it, and it is the Arctic fox. The animal reached the isolated North Atlantic island at the end of the last ice age by walking over the frozen sea. In Súðavík, the Arctic Fox Center holds an exhibition on the species and studies how tourism affects the population. Other places have a more broken history. The fox is locally extinct on Jan Mayen. It was once locally extinct on Bear Island too, until foxes returned on their own by crossing sea ice.

  During winter, 95 percent of Arctic foxes make only commuting trips that stay within their home range. These trips last less than 3 days and happen between 0 and 2.9 times a month. Nomadism accounts for 3.4 percent of the foxes. Loop migrations, where a fox travels to a new range and then returns home, are the rarest at 1 percent. Foxes in Canada that roam or migrate travel from the Canadian archipelago to Greenland and northwestern Canada. Males and females do not differ significantly in how long or how far they go. Foxes near coastal goose colonies are less likely to migrate, while those facing low lemming numbers are more likely to head out over sea ice. Nomadic behavior becomes more common with age.

  Residency is the norm because it lets foxes keep their territories, and the cost of leaving is high. Migratory foxes die at more than three times the rate of resident foxes.

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• 7. Name and Ancestry

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  In 1758, Carl Linnaeus described the species in the 10th edition of Systema Naturae and named it Canis lagopus. The type specimen came from Lapland, Sweden. The word vulpes is Latin for fox. The specific name lagopus comes from the Ancient Greek words lagōs, meaning hare, and pous, meaning foot. It refers to the hair on the fox's feet, which resembles that of cold-climate hares.

  The Arctic fox is a true fox of the genus Vulpes, in the fox tribe Vulpini, which has 12 living species. It belongs to the subfamily Caninae of the dog family, Canidae. At one time it had a genus of its own, called Alopex, but more recent genetic evidence places it in Vulpes with most other foxes. Its lineage split from the red fox about 3.17 million years ago and from its sister group, the kit fox, about 0.9 million years ago. Besides the common Arctic fox, Vulpes lagopus lagopus, four other subspecies have been described. They are the Bering Islands, Greenland, Iceland and Pribilof Islands Arctic foxes.

  The Tibetan Plateau holds fossils of Vulpes qiuzhudingi, an extinct fox from the early Pliocene, between 5.08 and 3.6 million years ago. It is believed to be the ancestor of the modern Arctic fox, in what is called the out of Tibet hypothesis. The Pliocene was about 2 to 3 degrees Celsius warmer than today, and the mid-Pliocene Arctic summer was 8 degrees warmer. Isotope analysis of fossils leads researchers to claim that the plateau had tundra-like conditions then. They say it harbored cold-adapted mammals that later spread to North America and Eurasia during the Pleistocene, which ran from 2.6 million to 11,700 years ago.

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• 8. Fragile Populations

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  Since 2004, the International Union for Conservation of Nature has rated the Arctic fox as least concern. The total population must be in the order of several hundred thousand animals. Counting them is hard, because the best long-term records are hunting bag records and questionnaires, which carry several sources of error. Numbers also swing widely from year to year.

  The Scandinavian mainland population is acutely endangered, despite decades of legal protection from hunting. Fewer than 200 individuals remain across Norway, Sweden and Finland. Finland declared the fox a protected species in 1940, yet its population has not recovered, and it is classed as critically endangered there. Sweden's Vindelfjällen Nature Reserve has the Arctic fox as its symbol. Extreme fur prices around the start of the 20th century drove heavy hunting, even when the foxes were already scarce. The population has stayed at low density for more than 90 years. A 1997 estimate counted about 60 adults in Sweden, 11 in Finland, 50 in Norway and around 20 on the Kola Peninsula. That adds up to around 140 breeding adults. Even after a local lemming peak, the population tends to collapse back to levels dangerously close to nonviability.

  On Medny Island, in Russia's Commander Islands, mange caused by an ear tick introduced by dogs in the 1970s cut the fox population by some 85 to 90 percent. About 90 animals remain. They are being treated with antiparasitic drugs, but the outcome is still uncertain. Uncontrolled trapping elsewhere has almost wiped out two subpopulations. The fox's numbers also follow its prey, so they are especially vulnerable in years when the lemming and vole populations crash.

  The Aleutian Islands show how fur trading can backfire. Pelts with a slate-blue color were especially valuable, so foxes were carried to Aleutian Islands that had never had any during the 1920s. The program increased the number of blue foxes, but the foxes preyed on Aleutian Canada geese, which conflicted with efforts to protect that species. The fox had been introduced to the Aleutians in the late 19th century, and today that population is being eradicated to preserve the local birds.

  The larger red fox is gaining ground at the Arctic fox's expense. One explanation points to climate change, since a light coat offers less camouflage when there is less snow. Red foxes dominate where the two ranges overlap, killing Arctic foxes and their kits. Another explanation involves the gray wolf, which historically kept red fox numbers down. As the wolf was hunted to near extinction across much of its range, red foxes multiplied and took over the role of top predator. In parts of northern Europe, programs now allow the hunting of red foxes in the Arctic fox's former range.

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