Ermine or Weasel

Geographic Range

Ermine have a circumpolar distribution. 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. 


Ermine prefer riparian woodlands, marshes, shrubby fencerows, and open areas adjacent to forests or shrub borders. Although ermine are primarily terrestrial, they climb trees and swim well. Tree roots, hollow logs, stone walls, and rodent burrows are used as dens. Dens are usually around 300mm below ground. Ermine line their nests with dry vegetation, and fur and feathers from prey. Side cavities of burrows are used as food caches and latrines. (Ruff and Wilson, 1999)

Physical Description

Mass: 25 to 116 g (0.88 to 4.08 oz)

Length: 170 to 330 mm (6.69 to 12.99 in)

At full adult size total body length from head to rump is 170mm to 330mm. 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 dorsal fur is chocolate brown while the ventral fur extending to the upper lip is yellowish white.


Breeding interval: Ermine generally breed once yearly.

Breeding season: Ermine mate in late spring to early summer.

Number of offspring: 3 to 18; avg. 4-9

Gestation period: 280 days (average)

Time to weaning: 8-10 weeks (average)

Age at sexual or reproductive maturity (female): 60-70 days (average)

Age at sexual or reproductive maturity (male): 2 years (average)

Ermine are a polygynous-promiscuous species, with males and females mating opportunistically.

Ermine mate in late spring to early summer. Females are polyestrous, but produce only 1 litter per year. Young are born in April or May after an average gestation period of 280 days, which includes an 8-9 month period of developmental delay. Longer days beginning in March trigger the resumption of fetal development. Litter size ranges from 3-18 offspring and averages 4-9. The sex ratio is unequal. 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 60-70 days old, often before they are weaned. Males do not breed or gain adult dimensions until their second summer.

Females in nature may survive for at least 2 breeding seasons, while males generally do not survive this long. Reproductive success is highly dependent on food availability.

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


Longest known lifespan in wild
7 years (high)

Expected lifespan in wild
1-2 years (average)


The ermine's lithe, 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, progressing 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 disperse and attain large territories that usually encompass or overlap females' territories.

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

Home Range

Ermine population densities fluctuate with prey abundance. When conditions are good, an individual may occupy a 10ha area. The maximum home range size is about 20ha. Home ranges of males are usually twice the size of female home ranges. These solitary mammals maintain exclusive boundaries that are patrolled and marked by scent. (Ruff and Wilson, 1999)

Communication and Perception

Ermine have keen senses of smell, vision, hearing, and touch that help them to locate prey. Most mustelids are fairly quiet animals, but some vocalizations may be used in intra-specific communication. Chemical cues are probably the main means of communication reproductive readiness to potential mates.

Food Habits

Ermine are carnivores that hunt primarily at night. They are specialist 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 frequently hunt under snow and survive entirely on small rodents and lemmings. Daily meals are essential to meet the ermine's exhorbitant energy and heat production demands. Ermine cache leftover meals as a way of dealing with these demands. (Ruff and Wilson, 1999)

Once a potential prey is identified, the ermine 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. (Ruff and Wilson, 1999)


Ermine are fierce and agressive, although diminutive, animals. Potential predators are larger carnivores including red fox, gray fox, martens, fishers, badgers, raptors, and occassionally domestic cats.