Arctic fox

The Arctic fox (Vulpes lagopus)—also called the white fox, polar fox, or snow fox—is a small fox that lives in the Arctic regions of the Northern Hemisphere, especially on the tundra. It is built for cold weather, with thick, warm fur that turns white in winter to blend in with the snow. Its fur is the best insulator of any mammal, and its body is round and compact, which helps it retain heat. Most wild Arctic foxes die before their first birthday, though a few can live up to 11 years. Their body length ranges from 46 to 68 cm (18 to 27 in). They eat small animals like lemmings, voles, ringed seal pups, fish, waterfowl, and seabirds, as well as carrion, berries, seaweed, insects, and other small invertebrates. During breeding season, they form monogamous pairs and raise their young together in complex underground dens, sometimes with help from other family members. Their natural predators include golden eagles, Arctic wolves, polar bears, wolverines, red foxes, and brown bears.

The Arctic fox is smaller than the red fox, standing 25–30 cm (9.8–11.8 in) tall at the shoulder. Males average 3.5 kg (7.7 lb), with a range of 3.2–9.4 kg (7.1–20.7 lb); females average 2.9 kg (6.4 lb), ranging from 1.4–3.2 kg (3.1–7.1 lb). Males measure about 55 cm (22 in) from head to rear (range 46–68 cm), while females average 52 cm (20 in) (range 41–55 cm). Foxes living farther north tend to be smaller than those farther south. The tail is roughly one-third the length of the body.

Arctic foxes shed their fur twice a year. Their summer coat is short and darker, while the winter coat is long, thick, and twice as long as the summer coat. There are two genetically distinct color morphs: white and blue. The white morph is brown with lighter sides and belly in summer, turning entirely white in winter. The blue morph is black, brown, or gray in summer, becoming slightly lighter in winter. Two similar mutations to the MC1R gene cause the blue color and reduce seasonal color change. The blue allele is dominant over the white, but 90% of the global population—97–99% of the circumpolar population and 84–97% of Svalbard’s—is white. The blue morph is most common in Iceland (65–70%) and Fennoscandia (25–30%).

To survive the cold, the Arctic fox has a dense, multilayered winter coat, with 70% fine underfur for insulation. Its compact body, short muzzle, short legs, and short, thick ears give it a low surface area to volume ratio, so less heat escapes. The nose, ears, legs, and feet lose the most heat, which helps with cooling in summer. Like dogs, the fox has a nose that evaporatively cools the brain during warm weather or exercise. About 22% of its total body surface area dissipates heat quickly, compared to 33% in red foxes. The fur’s thermal conductivity is the same in summer and winter, but winter fur is 140% thicker, lowering thermal conductance. In summer, thermal conductance is 114% higher than in winter, yet the body’s core temperature stays constant year-round.

The Arctic fox regulates its leg temperature with a countercurrent heat exchange system. It keeps its feet above the tissue freezing point (−1 °C or 30 °F) even on cold ground, without losing mobility or feeling pain. It does this by increasing blood flow to a capillary network in the pad surface that contacts the snow, while constricting blood vessels in the center of the pad to save energy and minimize heat loss. The paw temperature stays independent from the core temperature; if the core drops, the pad remains above freezing.

In winter, the Arctic fox’s average mass-specific and total basal metabolic rates (BMR) are 37% and 27% lower than in summer. It slows its metabolism to conserve fat and reduce energy needs. Its lower critical temperature is −7 °C (19 °F) in winter and 5 °C (41 °F) in summer. The old belief that it could handle temperatures below −40 °C (−40 °F) is now considered inaccurate because it was never tested with proper equipment. The fox also has genes that help it survive extreme cold and starvation. Transcriptome sequencing found two positively selected genes: GLTPD1, involved in fatty acid metabolism, and AKT2, involved in glucose metabolism and insulin signaling.

The Arctic fox can hear frequencies from 125 Hz to 16 kHz, with a sensitivity of ≤ 60 dB in air and an average peak sensitivity of 24 dB at 4 kHz.