Mustela nivalisleast weasel

Geographic Range

Least weasels are widespread and abundant throughout the Northern Hemisphere. They are a circumboreal species native to North America, from Alaska, south throughout Canada and the northern United States, and Europe (excluding Ireland, Greenland, and Iceland). They have also been introduced onto islands such as New Zealand, the Azores, Crete, and Malta. (Naughton, 2012; Sheffeld and King, 1994)


Least weasels are adaptable and able to thrive in a multitude of habitats. They are found in prairie grasslands, coniferous and deciduous forests, open tundra, bushy taiga, and rainforests that undergo a dry season in the summer months. Least weasels are comfortable above and below ground, maneuvering easily through both leaf litter, subterranean, and subnivean tunnels. Weasels have dens of different substrates in different habitats but do not burrow or dig dens; they use the abandoned dens from prey or other fossorial species. Weasels only temporarily reside in their dens, and many dens can be found in a single least weasel’s territory. Weasels commonly choose dens at the base of trees in habitats with tree stands, such as coniferous, deciduous or mixed forests. When trees are not available, weasels will reside in brush or log piles, and tall grass patches, such as those found in prairie habitats or on agricultural lands. The vegetation present in the habitat is not as important as the amount of the vegetation available to use as cover for ambush hunting, as having enough cover is vital for a successful hunt. (Innes and Hay, 1991; King, 1989; Murphy and Dowding, 1994; Naughton, 2012; Sheffeld and King, 1994; Ylönen, et al., 2003)

Physical Description

Least weasels are the smallest carnivorous predator in the world and have a typical mustelid body shape: a long, tubular body, with short limbs and a short tail that is less than a quarter of the head-body length. The cranium is long and flat, with short, round ears, long vibrissae and large, dark eyes. The white paws are pentadactyl (five-toed) with non-retractile claws at the end of each of the five digits. The dental formula is 3/3 incisors, 1/1 canines, 3/3 premolars and 1/2 molars; 34 teeth in total. The pelage color changes seasonally in northern populations, but not in more southerly populations. In the winter, the coat is pure white, but always lacking the black tail tip found in two similar species: ermines, or stoats (Mustela erminea), and long-tailed weasels (Mustela frenata). In summer, the coat turns rusty-chocolate brown on the dorsum and remains white on the ventrum. Body mass varies globally. The largest least weasels are found in warm climates, such as individuals from populations of North Africa, which can weigh over 70 grams and can reach total lengths (including the tail) over 217 mm. The smallest weasels are in North American populations and weigh, on average about 45 grams, and average about 190 mm in length (including the tail). Sexual dimorphism is present in least weasels, as the males are larger than the females by approximately 20 to 30 cm and 30 to 50 grams. There are approximately 10 subspecies of least weasel, all of which can be distinguished only by geographic location. Physically, the individuals are indistinguishable. There are no major differences between the geographic populations that have been documented that are useful in determining individuals from given regions, although individuals from warmer climates tend to have larger body sizes and masses than those from colder climates. However, using body size and mass is not an accurate method of determining an individual's subspecies. Geographically, populations are defined by inconclusive ranges, as there is still debate as to the definitive boundaries of each subspecies range and there is no current agreement as to where one population ends and the next begins. Because of the lack of concrete geographic boundaries, the exact number of subspecies has yet to be defined. (Casey and Casey, 1979; Feige, et al., 2012; Innes and Hay, 1991; Iversen, 1972; Naughton, 2012; Sheffeld and King, 1994; Zub, et al., 2012)

  • Sexual Dimorphism
  • male larger
  • Range mass
    30 to 100 g
    1.06 to 3.52 oz
  • Average mass
    55 g
    1.94 oz
  • Range length
    165 to 217 mm
    6.50 to 8.54 in
  • Average length
    190 mm
    7.48 in
  • Range basal metabolic rate
    137.5 to 205.7 cm3.O2/g/hr


Least weasels are polygynandrous, which means males and females mate numerous times with multiple partners. Although least weasels have a highly promiscuous mating system, males do not compete for females. The breeding season is mostly confined to the spring and summer months (as birthing earlier in the year highly increases the survival rate of the young), but breeding is known to occur intermittently throughout the year. Females are in estrous for an average of four days and will mate with several males over the estrous period. (Sheffeld and King, 1994; Sundell, 2003)

Male least weasels defend territories, typically against other males, but during the breeding season, they abandon their territories in search of females in estrous. Because of sexual dimorphism and rigid dominance hierarchies between males and females, males are able to invade a female’s territory at any point in the year. Females defend a territory largely against other females, but will ferociously defend a home range, regardless of the intruders’ sex, during late-stage pregnancy and lactation. Ovulation is induced, occurring via baculum stimulation, which is straight, not curved, with a hook-shaped tip, which is a distinguishing feature of least weasels. Courtship is a rough process, which includes fighting, biting, and tumbling about until the male is able to grasp the female at the nape of her neck. Once the position is accomplished, the male mounts the female and copulation, which can last over an hour, ensues. Prolonged copulation is required in order to stimulate female ovulation. Prey density has a dramatic effect on the number of litters per year and the number of young per litter. During years of high prey abundance and in areas such as the Arctic, where lemming populations can reach astronomical numbers, up to 15 offspring can be born, and up to three litters per year can occur, partially because of post-partum estrus and partially because of high prey density. On average, there are only one or two reproductive events per year. Gestation lasts approximately one month, and an average of four or five altricial young, called “kits”, are born. Kits are born hairless and helpless, and weigh between 1.0 to 1.7 grams at birth. (East and Lockie, 1965; Erlinge, et al., 1982; Innes and Hay, 1991; King, 1989; Sheffeld and King, 1994; Sundell, 2003)

  • Breeding interval
    Least weasels breed one to three times per year, depending on prey density.
  • Breeding season
    Their breeding season is concentrated from March to June (although breeding is known to occur year-round).
  • Range number of offspring
    1 to 13
  • Average number of offspring
  • Range gestation period
    34 to 37 days
  • Average gestation period
    35 days
  • Range weaning age
    18 to 56 days
  • Range time to independence
    8 to 10 weeks
  • Range age at sexual or reproductive maturity (female)
    3 to 8 months
  • Average age at sexual or reproductive maturity (female)
    5 months
  • Range age at sexual or reproductive maturity (male)
    5 to 9 months

Soon after copulation, males depart and females assume the role of caring for offspring, with no help from the male. Altricial young are born hairless, blind, deaf, and helpless and rely completely on maternal care for their survival. Kits rely on their mother’s milk for about 32 days, after which, weaning begins, although the mother may bring meat to the kits as early as two weeks post-parturition. At about 47 days, the kits are able to kill prey for themselves and at about nine to ten weeks, the kits disperse and are independent. It has been demonstrated that killing is an innate behavior and kits in captivity kill prey with no previous experience or exposure to prey; however, kits that remain with their mother and gain practical hunting experience have higher success rates than those isolated from their mother. (Naughton, 2012; Sheffeld and King, 1994; Sundell, 2003)


Because least weasels are so small, have limited abilities in storing fat, and have such high metabolic demands, it is likely that they do not live long in the wild. The majority of young do not make it to the age of weaning, especially in the second and third litters of the year, likely due to the increased predation risk on kits in nests. The average lifespan for wild individuals is short; only 1 or 2 years, whereas the longest recorded captive lifespan is 10 years. (Naughton, 2012; Sheffeld and King, 1994; Sundell, 2003)

  • Range lifespan
    Status: wild
    4 (high) years
  • Range lifespan
    Status: captivity
    10 (high) years
  • Typical lifespan
    Status: wild
    1 to 2 years
  • Average lifespan
    Status: wild
    2 years
  • Typical lifespan
    Status: captivity
    5 to 6 years
  • Average lifespan
    Status: captivity
    6 years


Least weasels, like other members of the weasel family, are solitary except during the breeding season, where males, normally sedentary, will travel some distance to find females. Least weasels form sex-based dominance hierarchies, where older males are dominant over females and juvenile males. Males are thought to be dominant over females largely due to sexual dimorphism: the males are larger than the females, in both mass and length. Dominant males make physical and vocal threats, and will viciously attack submissive males, which will retreat, sometimes squealing as they leave. These vicious attacks rarely happen to females, as females fight back unlike submissive males, although females will submit to the dominant male. Least weasels must eat very regularly to avoid starvation and death and are often found foraging at all hours, day or night. Weasels can consume over 50% of their body weight every day and even more so in the winter months. Food caching is a common occurrence, as weasels frequently kill prey larger than themselves, yet they only eat a few grams of meat per meal. Caching is especially important for lactating females with kits, as lactation is such an energetically expensive time. Caches are concealed around the den entrance, as are latrine sites. An individual will scent-mark around a den site using secretions from their anal glands. When cornered or startled, anal gland secretions can be discharged, which release a foul-smelling fluid that can deter an antagonist. Weasels also occasionally perform the “weasel war dance”, a colloquial term used to describe a intermittent series of leaps and twists, often accompanied by bark-like vocalizations, stiff limbs, an arched back, and erection of their dorsal and caudal hairs. There are a few hypotheses as to why this “dance” is performed, but at this time, there is no solid scientific evidence explaining this behavior. One hypothesis is that the war dance is thought to perplex prey and to give the weasel a hunting advantage, although there are cases where the weasels perform this “dance” lacking an audience. Weasels of all ages perform the dance, yet it is more common in younger individuals, especially kits playing with their siblings. (Innes and Hay, 1991; King and Powell, 2007; King, et al., 2001; Naughton, 2012; Sheffeld and King, 1994)

  • Range territory size
    0.2 to 1.0 km^2
  • Average territory size
    6.0 km^2

Home Range

Least weasels use abandoned dens left by prey species or other fossorial animals and will maintain a territory around the temporary den site only if the rodent population within the territory is large. The boundaries are marked by releasing scent secretions from anal glands, but there is marked overlap between territories. Males, being dominant, will enter a female’s territory at any given time. In general, each individual has its own defined territory, but there is sometimes overlap as males have a larger territory size than females. Overlap also occurs when weasel population density is high or due to dominance, where a single male’s territory will cover other individual’s territory. This is thought to be because the dominant male is able to travel without conflict, unlike a subordinate weasel. Non-dominant weasels rarely overlap territories because of strong territoriality and fighting between lower ranked individuals. It is commonly the dominant male that overlaps others, both males and females. Territory size also varies with prey density, where higher prey density leads to smaller home ranges because of less required travel time to find prey. (King, 1975; Naughton, 2012; Sheffeld and King, 1994)

Communication and Perception

Least weasels rely heavily upon olfaction for intra- and interspecific communication and have been known to hunt “with nose” while searching for prey in subterranean environments. Although the sense of smell is thought to be the most important sense, weasels also use vision and hearing while hunting. Least weasels have long vibrissae (“whiskers”), which may help detect vibrations and for spatial orientation in an environment. Captive weasels have been recorded making many vocalizations: barks, hisses, chirps, squeaking, squealing, and trilling. These sounds vary in response to the stimulus. Hissing is interpreted as a response to a lesser threat, while chirping is in response to a more urgent threat. Squeals are emitted when the weasel is cornered, and quieter trills are thought to be greetings between mothers and kits, and also greetings between kin. Kits are vocal, using squeaks and chirps to communicate with their mother and siblings. (Burn, 2008; King and Powell, 2007; Naughton, 2012; Ylönen, et al., 2003)

Food Habits

Least weasels, like many other mustelid species, have a reputation for killing prey much larger than themselves, then caching the remains. Least weasels are highly specialized rodent predators and as such, they rely heavily upon rodent species for food. Least weasels are, however, opportunistic feeders and will not overlook an easy meal, such as carrion. Field voles, wood mice, and bank voles constitute much of a least weasel’s diet in more southern populations; almost 100 percent of a weasel's diet is made up of rodents if they are abundant. When rodents are scarce, least weasels will also feed upon birds’ eggs, lizards, amphibians, small fish, and invertebrates. Rodents, especially collared lemmings in the northern arctic regions, are vitally important for weasel reproductive success. Least weasel reproduction is tightly interconnected with lemming abundance, as there are not many other prey species for northern populations of least weasels. Northern populations of weasels cycle more apparently than those found in southern populations due to the strong food requirements placed upon lemmings, which also undergo population cycling; however, least weasel populations naturally peak and subside, even with alternative food sources available, like populations in more southern regions. (Andersson and Erlinge, 1977; Day, 1968; King, 1975; King, et al., 2001; Naughton, 2012; Sheffeld and King, 1994; Sundell, 2003)

  • Animal Foods
  • birds
  • mammals
  • amphibians
  • reptiles
  • fish
  • eggs
  • carrion
  • insects


The mortality rate for least weasels is highest during the early days of life while they are still helpless in their nests. Juvenile mortality is common from predation by snakes and foxes, while adults are typically preyed upon by owls, such as tawny owls, and other birds of prey, such as falcons, eagles, and hawks. Least weasels may also be preyed upon by other larger weasel species, such as ermines and long-tailed weasels. Least weasels can counteract predation events by using their camouflaged pelage to blend in with substrate and aggressive behavior, such as vocalizations and biting, and by hiding in shelters. Least weasels will also release their anal glands, or musk glands, when startled or fearful. The secretions from the musk glands contain the strong-smelling sulfuric compounds thietane and dithiacyclopentane, these compounds are thought to deter attacks from predators, especially those that rely heavily upon olfaction. (Innes and Hay, 1991; Naughton, 2012; Powell, 1973; Sheffeld and King, 1994)

  • Anti-predator Adaptations
  • cryptic

Ecosystem Roles

Least weasels, as highly-skilled rodent predators, play an important role in maintaining or initiating cycles in rodent populations. Rodent cycling is a vital component of the tundra ecosystem and specialized predators, such as least weasels, are helpful for keeping lemming populations in check. Bird species in New Zealand, where least weasels were introduced, are negatively affected by weasel predation, especially ground-dwelling brown kiwis. (Feige, et al., 2012; Henttonen, 1987; King, et al., 2001)

Economic Importance for Humans: Positive

Least weasels are effective rodent predators. By preying on rodents, which can transmit disease, eat economically valuable crops, and cause extensive property damage, humans directly benefit both economically and health-wise from least weasels. Trappers are also able to benefit, albeit not strongly, from least weasels caught as bycatch in traps set for larger fur-bearers. Least weasel pelts do not have substantial economic value in Canada, but some weasel pelts are used as lining and trim on garments such as luxury coats and mittens. (Naughton, 2012; Sheffeld and King, 1994)

  • Positive Impacts
  • body parts are source of valuable material
  • controls pest population

Economic Importance for Humans: Negative

Although least weasels have been blamed for attacks on domesticated chickens and other avian livestock, there is little proof to suggest that least weasels prey upon any domestic livestock. (Naughton, 2012)

Conservation Status

While they are not considered rare in North America, least weasels are more common in Europe and Asia, and are not globally threatened. As a whole, populations of least weasels are considered stable. (Tikhonov, et al., 2013)


Gina Campbell (author), University of Manitoba, Jane Waterman (editor), University of Manitoba, Leila Siciliano Martina (editor), Animal Diversity Web Staff.



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

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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


uses sound to communicate


living in landscapes dominated by human agriculture.


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


flesh of dead animals.


uses smells or other chemicals to communicate


having markings, coloration, shapes, or other features that cause an animal to be camouflaged in its natural environment; being difficult to see or otherwise detect.

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

ranking system or pecking order among members of a long-term social group, where dominance status affects access to resources or mates


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


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


a distribution that more or less circles the Arctic, so occurring in both the Nearctic and Palearctic biogeographic regions.

World Map

Found in northern North America and northern Europe or Asia.

induced ovulation

ovulation is stimulated by the act of copulation (does not occur spontaneously)


An animal that eats mainly insects or spiders.


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


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


an animal that mainly eats fish


the kind of polygamy in which a female pairs with several males, each of which also pairs with several different females.


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.


an animal that mainly eats dead animals

scent marks

communicates by producing scents from special gland(s) and placing them on a surface whether others can smell or taste them


remains in the same area


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


lives alone

stores or caches food

places a food item in a special place to be eaten later. Also called "hoarding"


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).

tropical savanna and grassland

A terrestrial biome. Savannas are grasslands with scattered individual trees that do not form a closed canopy. Extensive savannas are found in parts of subtropical and tropical Africa and South America, and in Australia.


A grassland with scattered trees or scattered clumps of trees, a type of community intermediate between grassland and forest. See also Tropical savanna and grassland biome.

temperate grassland

A terrestrial biome found in temperate latitudes (>23.5° N or S latitude). Vegetation is made up mostly of grasses, the height and species diversity of which depend largely on the amount of moisture available. Fire and grazing are important in the long-term maintenance of grasslands.


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.


movements of a hard surface that are produced by animals as signals to others


uses sight to communicate


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

year-round breeding

breeding takes place throughout the year


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Zub, K., S. Piertney, P. Szafrańska, M. Konarzewski. 2012. Environmental and genetic influences on body mass and resting metabolic rates (RMR) in a natural population of weasel Mustela nivalis. Molecular Ecology, 21(5): 1283-1293.