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The Adirondack’s smallest weasel is 23-31 cm (9-12 in) in length, including the short, 5.4-8.5 cm (2.1-3.3 in) tail. Ermine weigh 60-110 g (2.1-3.9 oz). Females are one-half to one-thirds the size of males. The ratio of tail length to total (combined head-body length) is the only reliable means of separating large ermine from small long-tailed weasels (M. frenata) because of their similar appearance and size. The tail of the ermine is less than 44 percent of head-body length. The ermine had a long, slender body and neck, flat triangular head and short legs. The small, beady eyes are black and the ears are short, rounded, and furred. The upper parts of the body are brown and the lower parts white to yellowish white, except in winter (December – March or April) when the coat is white. The tip of the tail is black in all seasons. Photoperiod, modified by ambient temperature, governs the biannual molt.

Range and Habitat

The range includes the boreal and montane forests of the Northern Hemisphere. The North American distribution includes most of Alaska, Canada, and south to northern California, New Mexico, Minnesota and Maryland in the U.S. The ermine occurs throughout the Adirondack Park at all elevations, and in all terrestrial habitats. It frequently uses the nest of prey for shelter, adding dried plant materials, fur and feathers to the nest lining. An ermine often has several nests that may be underground, within or under hollow logs, rock piles, and walls, and buildings such as old barns or sheds. 

Food and Feeding Behavior

The ermine is carnivorous, hunting small animals for food, although it may eat some fruit and carrion when prey is scarce. Voles and mice comprise much of the diet. Red squirrels, eastern chipmunks, moles, shrew, eastern cottontails, young snowshoe hares, earthworms, insects, frogs, snakes, birds and birds’ eggs constitute a lesser part of their diet. The ermine hunts by moving quickly over large areas, searching all cover carefully, and stalking and pouncing on animals that it locates by scent, sound, or sight. Ermine kill prey by biting the base of the skull, and when attacking larger prey, cling to the struggling victim with both front and hind limbs. The notion that the weasels suck the blood of the prey is a myth. However, before they begin to feed on flesh and internal organs, weasels often lick the blood from the wounds. An ermine requires a daily quantity of food equivalent 19-32 percent of its body weight, and because of the uncertainty of locating prey, often kills more than it can eat at once, caching the excess. In spite of this adaptive behavior, starvation is a common source of mortality, especially when small mammal populations decline.

Activity and Movement

Ermine alternate periods of rest or sleep with bouts of activity throughout a 24-hour cycle. They are active at all seasons, and in winter move on the surface of crusted snow and tunnel through soft snow, stopping occasionally to thrust the head above the surface, apparently to determine the route of travel. Ermine swim and climb well, but spend most of their time on the ground where they move by bounding, covering 50 cm (20 in) with each bound, or by running. They are capable of traveling at speeds up to 13 km/hr (8 mph) for short distances. When startled or pursuing prey, an ermine can leap 1.8m (6 ft).


The female bears her annual litter of 4-18 (average 6) in April or May. The nutrition available to the female is an important determinant of both liter size and survival; a female tends to have larger litters and more young survive to maturity when prey, especially small rodents, is abundant. At birth, young ermine weigh about 2 g (0.007 oz), are blind, and naked except for a thin layer of white hairs on the neck. Their eyes open at 28-48 days, the females first. They begin to eat some solid food at this age but continue to nurse until 7-12 weeks old. At 3 months, ermine are capable of capturing their own food. Age at first breeding is 6 months or less for females; one year for males.

The gestation period for this species is 280 days.Most embryonic development occurs during the last 28 days of pregnancy. Adults mate while the female is still nursing her current litter (mid-summer); juvenile females may be bred before they are weaned. Potential longevity is 6 or 7 years, but few ermine survive more than 12-15 months.


Few predators consistently prey on the ermine because of its aggressiveness and agility. Great horned owls, goshawks, foxes, coyotes, and even the long tailed weasel occasionally kill ermine.

Social Behavior

  • Social system - Ermine are asocial during most of their lives. A male seeks out family groups and brings freshly killed prey to the female, probably to appease her and thus permit him to mate with both the female and her daughters. Apart from this supplemental feeding, the male does not exhibit parental care. Adults occupy fixed home ranges which very in size with seasons and prey abundance. Home range size is from 4-200 ha (10-500 acre) with 10-40 ha (25-100 acre) an average. The home range of individuals of the opposite sex may overlap, but not those of the same sex. Males have larger ranges which may include those of several females. Local ermine populations vary in density, probably in response to a fluctuating prey base. Peak ermine densities reported from throughout the range are one ermine per 4-17 ha (10-42.5 acre).
  • Communication - Chemical signals play a key role in the life of the ermine, which has large anal glands. Other glands may be present, e.g., on the abdomen. Droppings (scat) deposited in conspicuous places within and along the boundaries of the home range mark occupied areas which transient avoid. Adults press or drag their abdomens over objects, another likely method of marking an area. When startled or attacked, an adult may first threaten by stamping the feet, and then discharge the powerful musk from the anal glands. Ermine are relatively silent, but can hiss, screech, grunt, and produce a soft “took-took-took” call. 

Additional References

Hall, E. R. 1951. American weasels. University of Kansas Publication, Museum of Natural History, 4:1-466.

Hamilton, W. J., Jr. 1933. The weasels of New York. American Midland Naturalist, 14:289-344.

King, C. M. 1983. Mustels erminea. Mammalian Species, 195:1-8.