Mustela erminea
Ermine
Description
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.
Remarks
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. Pages 63-65. Vail-Ballou
Press Inc., Binghamton, NY.
Forsyth, Adrian. 1999. Mammals of North America: Temperate and Arctic Regions.
Pages 226-227. Firefly Books Inc., Buffalo, NY
Jackson, H.T.H. 1961. Mammals of Wisconsin. Pages 338-343. University of
Wisconsin press, Madison, WI.
Rue III, L.L. 1981. Furbearing animals of North America. Pages 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.
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