Mustela nivalisleast weasel

Geographic Range

Least weasels are found throughout the Palearctic region (excluding Ireland, the Arabian Pennisula, and the Arctic Isles), in Japan, and in the Nearctic, from Alaska and northern Canada south to Wyoming and North Carolina (Honacki, 1982). A population of least weasels was introduced to New Zealand as well (Sheffield, 1994).


Least weasels can survive in a wide variety of habitats, including open forests , farmlands, meadows, prairies, steppe, and semi-deserts. Least weasels avoid deep forests, sandy deserts, and open spaces. They are well adapted for the tundra (Sheffield, 1994).

Physical Description

The body of least weasels is long and slender, with a long neck; a flat, narrow head; and short limbs. This animal has large black eyes and large, round ears. The feet have five fingers with sharp claws. Mass is dependent upon location, North American populations are the smallest and those found in northern Africa have the largest mass. Fur color is chocolate brown on the back and white with brown spots on the underparts. The summer coat is about 1 cm in length. The winter coat, which is about 1.5 cm in length, turns to all white in northern populations and remains brown in the southern populations (Sheffield, 1994).

  • Sexual Dimorphism
  • male larger
  • Range mass
    30.0 to 55.0 g
    1.06 to 1.94 oz
  • Range length
    165.0 to 205.0 mm
    6.50 to 8.07 in


In North America, central Europe, and the former USSR, breeding can occur throughout the year, but the most breeding occurs in the spring and late summer. Gestation in least weasels lasts from 34 - 37 days. Litters may range from 1 - 7. A higher number of offspring per litter can be found in northern populations. Newborns weigh from 1.1 g to 1.7 g and are wrinkled, pink, naked, blind, and deaf. After 49 - 56 days, they have reached their adult length. By week 6 males are larger than females. In 9 - 12 weeks the family groups begin to break up, and in 12 - 15 weeks least weasels reach their adult mass. Females that are born in the spring are sexually mature in three months and may breed in their first summer. Summer and autumn born females are not as well developed and cannot breed until the next summer (Sheffield, 1994).

  • Breeding interval
    Least weasels can breed once or twice each year.
  • Breeding season
    Least weasels breed in spring and late summer.
  • Range number of offspring
    1.0 to 7.0
  • Average number of offspring
  • Range gestation period
    37.0 (high) days
  • Range weaning age
    18.0 (low) days
  • Range age at sexual or reproductive maturity (female)
    4.0 to 8.0 months
  • Range age at sexual or reproductive maturity (male)
    4.0 to 8.0 months

Newborns weigh from 1.1 g to 1.7 g and are naked, blind, and deaf. They are nursed and cared for in the burrow by their mother. After 49 to 56 days, they have reached their adult length. By week 6, males are larger than females. In 9 to 12 weeks the family groups begin to break up, and in 12 to 15 weeks the weasels reach their adult weight.

Females care for and nurse their young until they become independent.


Least weasels probably only live for several years after reaching adulthood and most die before reaching adulthood.

  • Range lifespan
    Status: captivity
    9.1 (high) years


Males and females live apart from each other except during the breeding season. Because the range of a female is smaller than a male's, one or more female may live within one male's range. Females have the ability to fend off other females and males from her home range. Males, once out of the breeding season, display dominance over females. Least weasels are very active, both day and night. The young spend their time play fighting and play mating.

Weasels watch the movement of their prey before they attack. When they kill, they go for the neck of the victim.

The dens of the weasel are not permanent but are those taken from the prey.

(Sheffield, 1994).

Communication and Perception

Least weasels possess keen senses of smell, hearing, touch, and sight. As with most mammals they rely heavily on their sense of smell, communicating among themselves and locating prey by detecting scents.

Food Habits

The diet of least weasels is composed of small mammals, mainly rodents. When rodents are scarce, weasels will eat birds' eggs and nestlings. Their diet also ranges from insects to lizards. In the extreme northern populations they will eat the carcasses of brown lemmings. Males are better hunters and are more likely to hunt larger prey, while females will continue looking for small rodents (Sheffield, 1994).

  • Primary Diet
  • carnivore
    • eats terrestrial vertebrates


Least weasels are aggressive and fierce and will attack animals much larger than themselves. Young in nests are preyed on by snakes, while adults may be preyed on by large birds of prey, such as owls and hawks.

Ecosystem Roles

Least weasels are important predators of small mammals in the ecosystems in which they live.

Economic Importance for Humans: Positive

Least weasels have been hunted and trapped by humans throughout the world (Sheffield, 1994). The help keep in check the populations of many species of rodents that are potentially harmful to agriculture.

  • Positive Impacts
  • controls pest population

Conservation Status

Least weasels are generally widespread and abundant. Localized populations may be threatened by habitat destruction, but these animals are generally not threatened.


Tanya Dewey (editor), Animal Diversity Web.

Toni Lynn Newell (author), University of Michigan-Ann Arbor.



Living in Australia, New Zealand, Tasmania, New Guinea and associated islands.

World Map


living in the Nearctic biogeographic province, the northern part of the New World. This includes Greenland, the Canadian Arctic islands, and all of the North American as far south as the highlands of central Mexico.

World Map


living in the northern part of the Old World. In otherwords, Europe and Asia and northern Africa.

World Map


young are born in a relatively underdeveloped state; they are unable to feed or care for themselves or locomote independently for a period of time after birth/hatching. In birds, naked and helpless after hatching.

bilateral symmetry

having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.


an animal that mainly eats meat


uses smells or other chemicals to communicate

  1. active during the day, 2. lasting for one day.

animals that use metabolically generated heat to regulate body temperature independently of ambient temperature. Endothermy is a synapomorphy of the Mammalia, although it may have arisen in a (now extinct) synapsid ancestor; the fossil record does not distinguish these possibilities. Convergent in birds.

female parental care

parental care is carried out by females


union of egg and spermatozoan


forest biomes are dominated by trees, otherwise forest biomes can vary widely in amount of precipitation and seasonality.

internal fertilization

fertilization takes place within the female's body


referring to animal species that have been transported to and established populations in regions outside of their natural range, usually through human action.


offspring are produced in more than one group (litters, clutches, etc.) and across multiple seasons (or other periods hospitable to reproduction). Iteroparous animals must, by definition, survive over multiple seasons (or periodic condition changes).


having the capacity to move from one place to another.

native range

the area in which the animal is naturally found, the region in which it is endemic.


active during the night


having more than one female as a mate at one time


rainforests, both temperate and tropical, are dominated by trees often forming a closed canopy with little light reaching the ground. Epiphytes and climbing plants are also abundant. Precipitation is typically not limiting, but may be somewhat seasonal.

seasonal breeding

breeding is confined to a particular season


remains in the same area


reproduction that includes combining the genetic contribution of two individuals, a male and a female


lives alone


uses touch to communicate


Coniferous or boreal forest, located in a band across northern North America, Europe, and Asia. This terrestrial biome also occurs at high elevations. Long, cold winters and short, wet summers. Few species of trees are present; these are primarily conifers that grow in dense stands with little undergrowth. Some deciduous trees also may be present.


that region of the Earth between 23.5 degrees North and 60 degrees North (between the Tropic of Cancer and the Arctic Circle) and between 23.5 degrees South and 60 degrees South (between the Tropic of Capricorn and the Antarctic Circle).


defends an area within the home range, occupied by a single animals or group of animals of the same species and held through overt defense, display, or advertisement


A terrestrial biome with low, shrubby or mat-like vegetation found at extremely high latitudes or elevations, near the limit of plant growth. Soils usually subject to permafrost. Plant diversity is typically low and the growing season is short.


reproduction in which fertilization and development take place within the female body and the developing embryo derives nourishment from the female.


Honacki, J.H., ed.; Kinman, K.E., ed.; Koeppl, J.W., ed. 1982. mammal Species of the World; A Taxonomic and Geographic Reference. Association of Systematic Collections, U.S.A.

Sheffield, J.R.; King, C.M. (2 June 1994). "Mustela nivalis." Mammalian Species. The American Society of Mammalogists, 454.