At full adult size total body length from head to rump is 170 mm to 330 mm. Males are generally twice as large as females, with males weighing from 67 to 116 grams and females from 25 to 80 grams. The tail length is about 35% of the total body length, ranging from 42 mm to 120 mm. Ermine have the typical weasel form: long body, short legs, long neck supporting a triangular head, slightly protruding round ears, bright black eyes, and long whiskers. Their short, moderately fine fur is white in the winter and the tip of the tail is black. In the summer, the dorsal fur is chocolate brown while the ventral fur extending to the upper lip is yellowish white.
Ermine are carnivores that hunt primarily at night. They are specialist predators on small, warm-blooded 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. Ermine cache leftover meals as a way of dealing with these demands.
Ermine prefer riparian woodlands, marshes, shrubby fencerows, and open areas adjacent to forests or shrub borders. Although ermine are primarily terrestrial, they climb trees and swim well. Tree roots, hollow logs, stone walls, and rodent burrows are used as dens. Dens are usually around 300 mm below ground. Ermine line their nests with dry vegetation, and fur and feathers from prey. Side cavities of burrows are used as food caches and latrines.