Spirit of the Tundra

Natural History, Dec, 1998 by J. David Henry

I have often been charmed by the arctic fox's ability to appear as if out of nowhere and then, just as mysteriously, to disappear.

Years ago, I was far out on the open tundra of Canada's Banks Island, many miles from my base camp and hundreds of miles from Sachs Harbour, the nearest Inuvialuit community. I was scanning the rolling hills for birds, when I felt an intense stare at the back of my neck. I turned around, and there, just fifty yards away, was an arctic fox studying me. It was an adult but was barely as high as my knees. I don't know how long it had been observing me, but now it peered at me with some alarm. Then the little animal--only slightly larger than a house cat--threw back its head, gave a single, shrill bark, and disappeared in a trot over a ridge. I chased after it over the hummocky tundra, but when I got to the top of the ridge, the fox was nowhere to be seen. The polar desert stretched out for miles in front of me--no trees, no shrubs, no deep valleys, just the gently rolling land, tufts of arctic grasses, and scattered wildflowers. Yet the fox was gone.

In the years since this brief encounter, I have traveled to arctic fox country many times and have often been charmed by the animal's ability to disappear, seemingly before my eyes, as if it were more spirit than substance. Recently, however, in many parts of its range, the fox seems in danger of a disappearance more alarming than charming. The species--which inhabits tundra areas around the globe--suffers from human hunting and trapping and, increasingly, from competition with one of its closest relatives, the red fox.

The arctic fox (Alopex lagopus) is a young species, the oldest known fossils dating back only 250,000 years. But recent DNA analysis shows that despite its relative youth, the species has already evolved a number of unique genes.

The arctic fox may have evolved from the swift fox (Vulpes velox) on the tundra surrounding the continental ice sheets. Whatever its origins, the arctic fox has become so well adapted to its harsh northern environment that at first it is hard to imagine how the more southerly red fox (V. vulpes) could pose much of a threat.

Prospering on the top of the planet means being able to cope with temperatures as low as -50 [degrees] F and gale-force blizzards. Among the attributes enabling the arctic fox to tolerate such bone-chilling cold are small size and foreshortened body proportions, which reduce heat loss by minimizing surface area. An adult is only about two-thirds the size of a red fox. Females weigh between six and twelve pounds, males between eight and fourteen. The arctic fox also has a rounder, more compact head than the red fox, with roundish ears and shorter muzzle, neck, tail, and limbs.

Natural selection has further sculpted this ice fox. Approximately 70 percent of the arctic fox's coat consists of a fine underwool--long strands of hair that coil and coil, creating a layer of thick, dense fur next to the skin. In red foxes, only about 20 percent of the coat is invested in underwool. The winter coat of an arctic fox is twice as long as its summer coat; the bottom surface of its paws is completely covered by dense, short fur; and extensive layers of fat form between the muscles and the skin. All these features provide more insulation against the cold. If sheltered completely from the wind, an arctic fox in prime winter coat does not begin to shiver (that is, raise its metabolism to warm itself) until the air temperature drops below -50 [degrees] F. Red foxes under the same conditions start to raise their metabolism much sooner, at a relatively mild -9 [degrees] F.

The arctic fox is also well adapted to the boom-and-bust ecology of the arctic tundra. An average arctic fox litter contains ten offspring, or kits, as compared with the red fox's five. Yet when lemmings and other prey are abundant, arctic fox vixens have been known to give birth to as many as twenty-five, among the largest litters documented for any mammal. When prey is scarce, the foxes may fail to raise any young. Whether the embryos are reabsorbed in the vixen or the kits are born but die early of starvation, we do not yet know.

Hungry kits start eating solid foods at three or four weeks of age, and one researcher documented that a pair of arctic foxes and their litter consumed approximately 18,000 lemmings during the ninety-day period in which the young were being raised. Adult arctic foxes can be run ragged supplying food to their whelps: they frequently seem agitated and stressed during this time, and they lose a good deal of body fat. In a typical year, however, fat deposits will peak again in November and remain impressively high throughout the long, dark arctic winter.

The red fox is more at home at lower latitudes, where it inhabits prairies, shrublands, and forests. But this is an adaptable, opportunistic species, and it is not by chance that the red fox has the widest current distribution of any existing species in the mammalian order Carnivora. Humans have intruded on the north-ernmost reaches of the earth, and their arrival is often accompanied by alterations to the land that favor the red fox. Forest clearing, for example, opens up the landscape, creating prime hunting habitat for red foxes. Thus, as human development moves northward, so do red foxes--often moving out onto the tundra and supplanting arctic foxes as they go.