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Mustela erminea - Ermine


Ermine (Mustela erminea) have the typical weasel form of a long slender body, short legs, long neck, and a small head.  The body is 23-36 cm in length, and 5-5.5 cm tall.  The tail length averages about 1/3 of the total body length.  Males generally weigh 80-150 g, while females weigh 60-85 g. Ermine have a dark brown coat on its upper half, legs, and feet during summer, and a white underside along the inside of its legs (Burt 1957).  Starting around the middle of October, ermine begin to molt from brown to white. This molting is triggered by photoperiod, and takes about 2.5 months (Rue III 1981).  By December, the entire coats of ermine are white, except for the tip of the tail, which stays black year round. By middle of March, ermine will have molted back to their winter coats, though in warmer winters with little snow, this will occur sooner (Rust 1962).  The spring molt takes only 2 months to be completed.  Dental Formula: I 3/3 C1/1 P3/3 M ½ = 34.

Distribution in Wisconsin:

Ermine are found in the boreal, Holarctic region.  Primary habitats are woodlands and brushlands, though the ermine is also commonly found in open countrysides around woodpiles and barns.  In Wisconsin, ermine are found throughout the state.  However, ermine are more common in the northern parts of the state than in the southern parts.  This is because the much larger long-tailed weasels (Mustela frenata) are more prevalent in the southern part of the state, and can essentially out-compete ermine for food and territory (Jackson 1961).

Ontogeny and Reproduction:

Ermine are solitary animals, and only get together for the breeding season, which occurs between July and August.  They have 1 litter per year, consisting of 4-9 young, which are born between April and May after delayed implantation (Jackson 1961).  Nests are usually located under logs, stumps, or brush piles, and often have many tunnels leading in and out.    The young are born almost naked, except for a few white hairs on their heads and the back of their necks.  Baby ermine weigh only 2-3g at birth.  Between 4 and 5 weeks old, the young ermine’s eyes open and the baby ermine are now fully furred. By the time ermine are 2.5-3 months old they is fully-grown.  Females are sexually mature when they are 3-4 months old, though males aren’t sexually mature until a year old (Jackson 1961).  The young usually disperse from their nest by the end of summer to live on their own.  The average life span is 5-6 years.  Ermine vocalize among each other by using trills (Forsyth 1999).

Ecology and Behavior:

Ermine are primarily nocturnal carnivores.  Up to 80% of its diet consists of small mammals such as mice and shrews. Ermine do not have prey preferences; instead, the mammals they consume are in proportion to the abundance of the mammal (Aldous 1942).  Small birds, eggs, amphibians, and insects may also be eaten in times when mammals are scarce. While the slender body shape allow ermine to chase prey through burrows and hide from larger predators, it also causes ermine to lose much body heat compared to shorter, rounder animals.  For this reason, ermine have a metabolic rate 10% higher than mammals of the same weight, and therefore must consume more food. Another problem with the ermine’s anatomy is its small stomach size.  Ermine are restricted to eating less than 28g of food at a time, and can only do so once every few hours.  This also means that ermine have to eat every few hours (Forsyth 1999).  Ermine will often kill more than it can consume, taking any easy meal that presents itself, and caching its extra food. During winter months, ermine hunt and travel beneath the snow.  They hunt by carefully stalking their prey, then pouncing on the prey once its close enough.  Ermine often screech to startle their prey, and are quickly able to kill their prey by biting the base of the neck with its powerful jaws (Forsyth 1999).

Ermine also have many natural predators.  Barred and great horn owls, sharp shinned hawks, and foxes all prey upon the ermine.  Mink are known to limit the population sizes of ermine, either due to directly killing the ermine, or through indirect competition for the same food sources.

Ermine have very keen senses of hearing and smell.  They are able to hear slight rusting in grasses or mice squeaking from 50 feet away.  They have poor vision when looking at stationary objects, though they can see slight motions very well.  


Ermine are not chicken killers as once thought, and are more likely a beneficial mouse hunter for farmers.  Ermines are trapped commonly, though this does not put the population at risk (Burt 1957).

Literature Cited:

Aldous, S.E., J. Manweiler. 1942. The winter food habits of short-tailed weasel in northern Minnesota. Journal of Mammalogy 23: 250-255.

Burt, W. H.  1957.  Mammals of the Great Lakes Region: 63-65.  Vail-Ballou Press Inc., Binghamton, NY.

Forsyth, Adrian.  1999.  Mammals of North America: Temperate and Arctic Regions: 226-227.  Firefly Books Inc., Buffalo, NY.

Jackson, H.T.H.  1961.  Mammals of Wisconsin: 338-343.  University of Wisconsin press, Madison, WI.

Rue III, L.L.  1981.  Furbearing animals of North America: 187-203.  Crown Publishers Inc., New York, NY.

Rust, C.C. 1962. Temperature as a modifying factor in the spring pelage of Short-Tailed Weasels. Journal of Mammalogy 43: 323-328.

Reference written by Chris Reinke, Biol 378: Edited by Chris Yahnke.
Page last updated 4-28-04.

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