Foxes of the blue phase remain dark or charcoal colored year round but become somewhat lighter in winter. Newborn arctic fox pups of both color phases are covered with short velvety dark brown fur. This fur lengthens and becomes lighter, especially on the flanks, after the pups reach 2 weeks of age. The contrast between the back and belly increases as the back darkens during their first three months. Blue phase pups acquire their characteristic dark color by the time they are 2 months old.
Most dens have southerly exposure.
They extend from 6 to 12 feet underground. Enlarged ground squirrel burrows with several entrances are often used as dens. Mating occurs in early March and early April. Gestation lasts 52 days. Litters average seven pups but may contain as many as 15 pups.