Mustela erminea
(Ermine or Short-tailed Weasel)

Order: Carnivora
Order Description: Carnivores
Family: Mustelidae
Family Description: Weasels, Skunks and Others

The ermine wears two "coats", a light brown summer coat with white underneath, and an all white winter coat. The tail is black tipped. The white winter coat is the source of the common name "ermine", which is a French word for "white winter color". The ermine has a typical weasel shape; a very long body, short legs and a pointed face with an almost triangular head. Its tail is about 30 to 40% of its head and body length. The short-tailed weasel is most likely the smallest mammalian carnivore in Idaho. There is pronounced sexual dimorphismClick word for definition in ermines. Males tend to be 40 to 80% larger than females. Total length is 8 to 13.6 inches (200-340 mm), tail length is 2.2 to 4 inches (55-101 mm), and they weigh 2.1 to 7 ounces (60-200 g).

They range from Alaska and Canada, south through most of the northern U.S. down the Rocky Mountain chain to central California, northern Arizona, northern New Mexico, and east to Iowa, the Great Lakes region, Pennsylvania, and northern Virginia.

This species has adapted to a variety of habitats from low-elevation marshes to alpine meadows, or any location where there is an abundance of prey (small rodents). But they prefer wooded areas with thick understory near watercourses.

They feed primarily on small mammals, but will also eat other small vertebrates and insects. They are good climbers so they can raid bird and squirrel nests.

Ermine are relatively pure carnivores. One study showed the following proportions in their diet: 36% voles, 16% shrews, 11% deer mice, 9% rabbits, 4% rats 4% chipmunks and a small percentage of beetles, grasshoppers, and frogs. Their predatory skills are remarkable when you imagine that an ermine killing a snowshoe hare is comparable to a pet poodle killing a yearling cow! Tracks in the snow indicate that they can repeatedly leap three times their body length, even in deep snow. Could you leap 15 to 18 feet from a standing start time after time? The ermine, like all weasels, has a very high metabolic rate and a consequently large demand for food. This is due, in part, to their long, slim body shape, a shape that allows higher than usual body heat loss. Their daily activity seems to alternate between periods of rest and activity, although diurnalClick word for definition (activity during the day) activity seems more common during the summer. During the winter it often hunts under the snow and probably uses rodent burrows. Hunting in rodent burrows may be common. They have been observed hunting in ground squirrel burrows repeatedly entering a nest burrow and emerging each time with a young ground squirrel until the nest burrow is empty. When inactive, they occupy a den under logs, stumps, roots, brushpiles, or rocks. Ermines appear to maintain territories which have a wide range in size. Home ranges of from 8 to 500 acres (2.2 -200 ha) have been recorded with sub-territories existing within the larger home range. In a southern Ontario study male home range averaged 50 to 63 acres (20-25 ha) while female range was smaller; and most individuals remained on the study site less than 1 year. There are numerous anecdotes about ermines' "mousing" abilities. They can acclimate to humans and are often found near farms, or rural cabins where they can easily feed on deer mice attracted to the buildings. On occasion they can become very effective predators on chickens or other domestic small animals.

Breeding occurs in early to mid summer. ImplantationClick word for definition is delayed as the fertilized egg divides and begins to develop for about 2 weeks. It then remains inactive until the following spring when it implants in the uterusClick word for definition of the mother. Once implantation occurs, development is rapid and the young are born 4 weeks later. Female produces litterClick word for definition of 4-9 young (average 6-7), born mid-April or early May. Females reach sexual maturity in 3-4 mo; males are probably sexually mature in 12 months. Young are very small at birth, only about 0.07 ounce (2 g), but development is rapid. By the fifth week weaningClick word for definition is underway and the young are fed some meat. They continue to nurse for 7 to 12 weeks.

Status: Predatory wildlife
Global Rank: G5
State Rank: S5

Information written by Donald Streubel,© 2000
Photos from Yellowstone- National Park Service, Aug 2000 & Jan 2001.
Map image provided by
Stephen Burton,© 2000
Design by Ean Harker©1999, 2000.