The ermine has a circumpolar distribution. It is found in the north temperate and frigid sectors of Eurasia and North America. In the New World, it ranges from east to west in a broad belt from the Arctic Ocean and adjacent islands of the Canadian Archipelago southward into the northern United States.
Males are generally twice as large as females.
Their short, moderately fine fur is white in the winter and the tip of the tail is black. They are specialist predators on small, warmblooded vertebrates, preferably mammals of rabbit size and smaller. When mammalian prey is scarce, ermine eat birds, eggs, frogs, fish, and insects. In severe climates, ermine frequently hunt under snow and survive entirely on small rodents and lemmings. Daily meals are essential to meet the ermine's exorbitant energy and heat production demands.