What does the Arctic fox say?
The Arctic fox is a small carnivore, around half the size of a red fox, with a small and compact body, covered with thick fur. The Arctic fox is light-weight as they rarely weigh more than four kilograms. There are two colour variants with the white fox entirely white in the winter, but in the summer it turns to a brown colour with yellow areas on the body’s underside, while the blue fox is completely brown in the summer and turns steel blue during the winter. There are more white foxes in the mountain areas while coastal areas are dominated by blue foxes.
Arctic foxes live in the tundra of the Arctic and have a circumpolar range. They number around several hundred thousand with the foxes commonly found in Siberia, North America, Greenland and Svalbard. In Sweden and Norway, the Arctic fox is sparsely distributed right from the tundra up north all the way down the Scandinavian mountain chain. The Arctic fox is even found in Iceland.
Arctic foxes crossing sea ice bridges
The Arctic fox did not arrive in Iceland due to human introduction, instead scientists believe that the Little Ice Age formed a bridge of sea ice between 200 and 500 years ago allowing Arctic foxes to migrate to the island from different Arctic regions including Russia, North America and Greenland. This is not the first time the Arctic fox has used sea ice to colonise Iceland. Ancient Arctic foxes crossed the sea ice during previous ice ages to reach Iceland well before human settlement. When global temperatures rose, the sea ice melted isolating the ancient foxes on the island before the Little Ice Age reconnected Iceland to the mainland. During this Little Ice Age temperatures plummeted between the 1600s and 1800s with rives including the Thames freezing over.
Analysing DNA of ancient Icelandic foxes
The scientists analysed DNA samples from the ancient remains of Icelandic Arctic foxes dating from two late 9th to 12th century archaeological sites and compared the findings to DNA from modern Arctic foxes. They found that the ancient Arctic foxes shared a single genetic signature while the modern population possesses five unique signatures. The scientists reasoned that the explosion in genetic diversity was due to Arctic foxes migrating across the sea ice during the Little Ice Age. Even today, Arctic foxes routinely travel hundreds of kilometres across sea ice.
Adapting to the cold
The Arctic fox has adapted well to living in the cold, extreme climate of the Arctic. Because they have short legs and a round body they have a small body area, enabling them to stay warm. Their winter coat has one the highest insulating capacities of any other mammal. During the winter months even their paws are covered in fur. These adaptations mean the Arctic fox can survive even in temperatures of minus 40 degrees Celsius without increasing their metabolism rate. In fact, they have been known to withstand temperatures of even minus 70 degrees Celsius!
Another way the Arctic fox survives in brutally cold temperatures is having veins in its legs very close together. This means the warm blood that flows into the leg heats up the colder blood returning. The result is the legs having a lower temperature than the rest of the body, reducing heat loss.
Storing fat and food for the winter
To survive through the winter with minimal supplies of food, the Arctic fox stores body fat. During the summer and autumn months the Arctic fox eats as much as it can with the food building up an insulating layer of fat and fat reserves that it uses up during the winter months. Additionally, the Arctic fox will store food, burying it ready for use in the winter. However, these hidden supplies cannot be relied on as the winter storms can be severe and be covered in thick snow. When the winter storms are at their worst, Arctic foxes will hide in a sheltered place and let themselves be covered by the snow or they dig into snow drifts. These adaptations mean a healthy Arctic fox can in fact function without food for several weeks.
A feast of sea birds and rodents
The Arctic fox’s typical diet depends on where it’s located in the Arctic. The Arctic foxes that live in the coastal areas of Iceland, Svalbard and Western Greenland have a diet of sea birds, seal carcasses and fish. Because of the abundance of food in coastal areas Arctic foxes usually have between 6 and 8 pups, all of which have a good chance of survival as the coastal area’s food supply is stable and predictable. In contrast, the inland Arctic foxes that live in the tundra and alpine regions of Scandinavia, North America and in the north-east of Greenland feast on rodents, including lemmings and so they have variable years of food supply. Every 3-4 years there is a peak in rodent populations resulting in many pups being born. During the lean years, Arctic foxes may have only a few pups or even none. Because of the variability in food supply between coastal and inland regions, it means there is a difference in territorial size. Because coastal areas have a consistent supply of food it means the Arctic foxes have small territorial areas while in the inland areas of the Arctic, the foxes’ territory is large and may not even overlap with its home range.
To survive and reproduce, Arctic foxes live in pairs with the male and female helping each other defend their home range and raise their pups. Researchers believe they live in pairs as raising pups is very energy-intensive and so improves their chances of survival. Sometimes females from previous litters may even stay at home with the pair to help feed and care for the next lot of pups. When Arctic foxes give birth it’s not out in the open, instead they give birth in dens. Dens are usually dug in a sand or gravel bank in the lower areas of mountains and have multiple entrances: it can even be up to 10, while the largest dens have hundreds of entrances and have been used for centuries.
Pup sizes dependent on food supplies
Arctic females usually breed in their first year but that depends on whether there is abundancy of food. The Arctic fox mates around March and April with pups born typically in May-June. The size of litters is also dependent on availability of food with litters of 10-16 common for inland Arctic foxes who have abundant supplies of rodents. The average nonetheless is around 6 pups per litter. When an Arctic fox is born its blind and has thin fur. For the first three to four weeks the pups stay inside the den and learn how to walk and play. After 10-12 weeks they become adventurous and will start exploring the world alone. By the time autumn arrives the pups have left to find their own home range as well as a partner and den.
Tagging Arctic foxes
To learn more about Arctic foxes, the Swedish Arctic fox project is studying the demography and genetics of the Arctic fox as part of a wider conservation and monitoring programme that includes the Norwegian Environment Agency and WWF. As part of the study, the researchers visit Arctic fox dens in the Swedish mountain ranges and tundra and see if they are inhabited. At the same time the researchers do extensive surveys of small rodents and birds. Arctic foxes are also tagged on their ear, enabling tagged foxes to be recognised within several hundred metres with a spotting scope.