Arctic fox

The Arctic fox (Vulpes lagopus) goes by several names—white fox, polar fox, or snow fox—and is a small fox species that lives across the Arctic tundra of the Northern Hemisphere. It’s built for frigid conditions, with thick, insulating fur that turns white in winter to blend into the snow, plus a big, bushy tail. Most wild Arctic foxes die before their first birthday, though a rare few can reach 11 years old. Their bodies measure 46 to 68 cm (18 to 27 in) long and are rounded to reduce heat loss.

This fox hunts small prey like lemmings, voles, ringed seal pups, fish, waterfowl, and seabirds. It also scavenges carrion and eats berries, seaweed, insects, and other tiny invertebrates. During breeding season, Arctic foxes pair up monogamously and raise their pups together in complex underground dens, sometimes with help from other family members. Their natural enemies include golden eagles, Arctic wolves, polar bears, wolverines, red foxes, and brown bears.

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

Arctic foxes shed their fur twice a year. The summer coat is short and darker, while the winter coat is long, thick, and twice as long as the summer one to retain heat. Two genetically distinct color morphs exist: white and blue. The white morph is brown with lighter sides and belly in summer, turning pure white in winter. The blue morph is black, brown, or gray in summer and becomes only slightly lighter in winter. Two similar mutations in the MC1R gene cause the blue color and reduce seasonal color change. The blue allele is dominant over the white, yet 90% of the global population—and 97–99% of the circumpolar mainland population—is white. In Svalbard, 84–97% are white. The blue morph is most common in Iceland (65–70%) and Fennoscandia (25–30%). Arctic fox fur offers the best insulation of any mammal.

To survive the cold, the Arctic fox relies on a dense, multilayered winter coat, 70% of which is fine underfur. Its compact body, short muzzle, short legs, and short, thick ears give it a low surface-area-to-volume ratio, minimizing heat loss. The nose, ears, legs, and feet lose the most heat—useful for cooling in summer. Like dogs, the fox has evaporative cooling in its nose to keep the brain cool during warm weather or exercise. About 22% of the fox’s total body surface area dissipates heat quickly, compared to 33% in red foxes. The fur’s thermal conductivity stays the same in summer and winter, but winter insulation improves because fur thickness increases by 140%. In summer, the body’s thermal conductance is 114% higher than in winter, though core body temperature remains steady year-round.

The Arctic fox regulates leg temperature using countercurrent heat exchange. It keeps its paws above the tissue freezing point (−1 °C or 30 °F) even on cold ground by increasing blood flow to a capillary network in the pad surface, while constricting vessels in the pad’s center to save energy. Paw temperature stays independent of core temperature; if the core drops, the pads remain above freezing. The fox’s average basal metabolic rate (BMR) is 37% lower in winter than summer, and total BMR drops 27%, helping conserve fat stores by slowing metabolism. The lower critical temperature is −7 °C (19 °F) in winter and 5 °C (41 °F) in summer. The old idea that it could handle −40 °C (−40 °F) is now considered inaccurate, as it was never properly tested. Genetic studies have identified two positively selected genes: GLTPD1, involved in fatty acid metabolism, and AKT2, linked to glucose metabolism and insulin signaling—both helpful for surviving extreme cold and starvation.

The Arctic fox hears 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.