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Mustela erminea

What do they look like?

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 42mm to 120mm. 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 fur on the back is chocolate brown while the fur on the belly extending to the upper lip is yellowish white.

  • Sexual Dimorphism
  • male larger
  • Range mass
    25 to 116 g
    0.88 to 4.09 oz
  • Range length
    170 to 330 mm
    6.69 to 12.99 in
  • Average basal metabolic rate
    1.276 W

Where do they live?

Ermine are distributed all the way around northern part of the globe. They are found in the north temperate regions of Eurasia and North America. In the New World, they range from east to west in a broad belt from the Arctic Ocean and adjacent islands of the Canadian Archipelago southward into the northern United States. Ermine are absent from the Great Plains.

What kind of habitat do they need?

Ermine prefer woodlands near rivers, marshes, shrubby fencerows, and open areas adjacent to forests or shrub borders. Although ermine live primarily on the ground, 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 plant material, and fur and feathers from prey. Side cavities of burrows are used for storing food and as bathrooms.

How do they reproduce?

Both male and female ermine have many different mates.

Ermine mate in late spring to early summer. Females produce only 1 litter per year. Young are born in April or May after an average pregnancy of 280 days, which includes an 8 to 9 month period in which the young do not develop. Longer days beginning in March trigger the young to begin developing. Litter size ranges from 3 to 18 offspring and averages 4 to 9. Young are blind and helpless. They are covered with fine white hair, and a prominent dark mane of dense fur develops around the neck by the third week (function unknown). The young grow quickly and are able to hunt with their mother by their eighth week. Although females do not reach adult size until a least 6 weeks after birth, they are able to mate when they are 60 to 70 days old, often before they are weaned. Males do not breed or grow to adult size until their second summer.

Females in nature may survive for at least 2 breeding seasons, while males generally do not survive this long. The number of offspring that each ermine has in its lifetime depends on the amount of food that is available.

  • How often does reproduction occur?
    Ermine generally breed once yearly.
  • Breeding season
    Ermine mate in late spring to early summer.
  • Range number of offspring
    3 to 18
  • Average number of offspring
  • Average number of offspring
  • Average gestation period
    280 days
  • Average gestation period
    43 days
  • Average weaning age
    8-10 weeks
  • Average age at sexual or reproductive maturity (female)
    60-70 days
  • Average age at sexual or reproductive maturity (female)
    Sex: female
    95 days
  • Average age at sexual or reproductive maturity (male)
    2 years
  • Average age at sexual or reproductive maturity (male)
    Sex: male
    365 days

Only females care for their offspring, nursing and protecting them until they become independent. The young are born blind and helpless.

  • Parental Investment
  • no parental involvement
  • altricial
  • pre-fertilization
    • provisioning
    • protecting
      • female
  • pre-hatching/birth
    • provisioning
      • female
    • protecting
      • female
  • pre-weaning/fledging
    • provisioning
      • female
    • protecting
      • female
  • pre-independence
    • provisioning
      • female
    • protecting
      • female

How long do they live?

The average life span of an ermine is 1 to 2 years; the maximum is 7 years. (Ruff and Wilson, 1999)

  • Range lifespan
    Status: wild
    7 (high) years
  • Typical lifespan
    Status: wild
    1 to 2 years
  • Average lifespan
    Status: wild
    1-2 years
  • Range lifespan
    Status: captivity
    12.5 (high) years

How do they behave?

The ermine's slender, agile body allows it to move swiftly both above ground and through underground burrows. Females hunt in tunnels more than males, which may explain the higher number of males that are trapped. Ermine can also run easily across snow. This ideal predator hunts in a zigzag pattern, moving by a series of leaps of up to 50 cm each. Ermine investigate every hole and crevice, often stopping to survey their surroundings by raising their heads and standing upright on their hindlegs. They may travel up to 15 km in one night.

Adult males dominate females and young. Females tend to remain in their birth place throughout their lives. Males spread out and claim large territories that usually contain or overlap females' territories.

Male and female ermine only associate with one another during the breeding season.

  • Range territory size
    0.1 to 0.2 km^2

Home Range

Ermine territories change size with prey abundance. The maximum home range size is about 20 hectares (0.2 square kilometers). When there is a lot of food, individuals don't need to travel as far to hunt and can live in smaller areas of about 10 hectares (0.1 square kilometers). Home ranges of males are usually twice the size of female home ranges. These solitary mammals keep others out by patrolling their territory boundaries and marking them with scent.

How do they communicate with each other?

Ermine have keen senses of smell, vision, hearing, and touch that help them to locate prey. Most members of the weasel family are fairly quiet animals, but some vocalizations may be used for communication. Ermine probably use odor to let other ermine know they are ready to mate.

What do they eat?

Ermine are carnivores that hunt mostly at night. They are 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 often hunt under snow and survive entirely on small rodents and lemmings. Ermine must eat daily to meet their high energy demands. They store leftover meals as a way of dealing with these demands.

When an ermine senses a prey animal, it approaches as closely as possible. With incredible speed it grasps the back of the victim's head and neck with sharp teeth, and wraps its body and feet around the victim. The victim dies from repeated bites to the base of the skull. Ermine have keen senses that help them locate prey. Hares and rodents are mainly followed by scent, insects by sound, and fish by sight.

  • Primary Diet
  • carnivore
    • eats terrestrial vertebrates
  • Animal Foods
  • birds
  • mammals
  • amphibians
  • fish
  • eggs
  • insects

What eats them and how do they avoid being eaten?

Ermine are fierce and aggressive, although small, animals. Potential predators are larger carnivores including red fox, gray fox, martens, fishers, badgers, raptors, and occasionally domestic cats.

What roles do they have in the ecosystem?

Ermine are important predators on small mammal communities in the ecosystems in which they live.

Many ermine die from a parasitic worm that infects the nose. Eventually the worm causes holes to form in the skull and puts pressure on the brain, causing death. Shrews are believed to carry this parasite.

Commensal or parasitic species (or larger taxonomic groups) that use this species as a host
  • nematode worms

Do they cause problems?

Ermine, and other Mustela species can take domestic fowl when they can gain access to them.

How do they interact with us?

Humans trap thousands of ermine each season, but the demand for pelts has recently decreased. The white winter fur has long been used in trimming coats and making stoles. Ermine are excellent at catching mice, which makes them valuable to humans.

  • Ways that people benefit from these animals:
  • body parts are source of valuable material
  • controls pest population

Are they endangered?

Ermine are not considered threatened or endangered, although hunting pressure in some areas may impact populations severely. (Ruff and Wilson, 1999)

Some more information...


Allison Poor (editor), University of Michigan-Ann Arbor.

Heather Loso (author), University of Michigan-Ann Arbor.


Baker, Rollin H. 1983. Michigan Mammals, pg.472-478. Michigan State Univeristy

Press, Michigan.

Edger, Judith L. 1990. Patterns of geographic variation in the skull of Nearctic Ermine (Mustela erminea). Canadian Journal of Zoology, 68:1241-1248. National Research

Council of Canada, Ontario.

Jones, J. Knox and Elmer C. Birney. 1988. Handbook of Mammals of the North-Central States, pg. 254. University of Minnesota Press, Minnesota.

King, Carolyn M. 1983. Mammalian Species,195:1-8. The American Society of Mammalogists, New York.

Kurta, Allen. 1995. Mammals of the Great Lakes, pg. 228-231. University of Michigan Press, Michigan.

Nowak, Ronald M. and J.L. Paradiso. 1983. Walker's Mammals of the World, 2:988-989.

The John Hopkins University Press, Baltimore and London.

"Animal Life Histories Database" (On-line).

Ruff, S., D. Wilson. 1999. The Smithsonian Book of North American Mammals. Washington [D.C.]: Smithsonian Institution Press in association with the American Society of Mammalogists.

University of Michigan Museum of ZoologyNational Science Foundation

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Loso, H. 1999. "Mustela erminea" (On-line), Animal Diversity Web. Accessed March 03, 2024 at

BioKIDS is sponsored in part by the Interagency Education Research Initiative. It is a partnership of the University of Michigan School of Education, University of Michigan Museum of Zoology, and the Detroit Public Schools. This material is based upon work supported by the National Science Foundation under Grant DRL-0628151.
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