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Mustela nivalis - Least Weasel

Physical Description:

Mustela nivalis is the smallest of any Wisconsin carnivore, ranging from 6.5 – 8 inches long including the tail, which is less than 20% of the total length.  Sexual dimorphism is common in mustelids, and it is obvious in the least weasel.  Adult males can be 189 to 206 mm in length, while females range from 172 to 188 mm.  Adult males weigh between 41 – 50 grams, while adult females weigh only 37 – 41 grams (Jackson, 1961).  Sexual dimorphism in weasels may be due to the different roles in reproduction.  Males are polygynous, so the bigger the body size, the better chance of obtaining mates.  In females, a smaller body size gives them an advantage in hunting efficiency, which is vital when rearing young.  Sexual dimorphism is also more apparent in larger subspecies.  For example, males in the UK and southern Sweden may weigh 100% more than females in the same population.  The skulls of males are also larger, stronger, and have more strongly developed sagittal and occipital crests than females (Sheffield and King, 1994).  However, Bergmann’s Rule doesn’t seem to apply for Mustela nivalis.  The northernmost subspecies M. n. pygmaea, found in northern Russia, is smaller than the American least weasel (Simms, 1979).

Summer fur color on its upper body is chocolate brown, with undersides being white.  Tail is brown with no black tip, unlike its relatives M. frenata and M. erminaea which have the black tip.  Winter pelage is entirely white, and is usually acquired by mid-November (Jackson, 1961).  The dental formula is i 3/3, c 1/1, p 3/3, m 1/2 = 34.  This carnivorous dentition includes carnassials used for shearing and slicing meat (Sheffield and King, 1994).


Mustela nivalis is an extremely rare species throughout its range, especially in Wisconsin.  It is found in North America, Europe and Asia, and has also been introduced to New Zealand, Malta, Crete, and the Azore Islands (Sheffield and King, 1994).  As for Wisconsin, it has only been recently noticed as being distributed statewide.

In Mammals of Wisconsin, Jackson showed that Mustela nivalis was only found in southern Wisconsin, south of latitude 45 degrees N (Jackson, 1961).  Recently, the subspecies M. n vulgaris has been found statewide (Sheffield and King, 1994; Wildlife Primer, 1998).  In North America, Mustela nivalis is expanding its distribution, especially south and west.

M. nivalis can be traced back to an early ancestor, Mustela praenivalis, from Europe between the late Pliocene and the early Pleistocene.  M. nivalis fossils were commonly found in European cave deposits associated with large numbers of small rodent fossils in the caves.  M. nivalis is believed to have emigrated to North America across the land bridge during the late Pleistocene, which was also during the Wisconsin glaciation (Sheffield and King, 1994).

Ontogeny and Reproduction:

Copulation initiates ovulation in the least weasel.  Although other mustelids in Wisconsin have delayed implantation, M. nivalis does not.  This provides the females with time to have more than one litter per year (Sheffield and King, 1994; Simms, 1979).  The total gestation period is 34 – 37 days (Sheffield and King, 1994), with litter size varying between 3 – 6 young.  Young are blind, deaf and hairless when born, and are pinkish in color (Jackson, 1961).  They weigh between 1.1 to 1.7 grams when born, and they grow very quickly.  By day 49, all the young are adept to killing mice, and by days 49 – 56, they have reached full adult size.  Least weasels may grow fast, but they also have a very short life expectancy.  Annual survival rates are 20% for males and 25% for females.  Life expectancy in the wild is less than 1 year, with a mean age of 0.79 – 1.16 years old; however, M. nivalis can live 7 – 10 years in captivity (Sheffield and King, 1994).

Ecology and Behavior:

The least weasel’s diet is usually comprised of small mammals, especially voles.  In Wisconsin, mice, voles, small birds, insects and chipmunks comprise the majority of their diet (Wildlife Primer, 1998).  M. nivalis has a very high metabolic rate, so these weasels must eat 8 – 10 times a day.  This is very unlike most carnivores who gorge themselves one day, and then can go several days without eating.  The size of a meal that they consume is not very large, so what isn’t eaten will be cached for later (Gillinham, 1984).  The method of caching their prey is very important, especially when prey is scarce.  A study was done to compare hunting efficiency to density of voles.  When there was a higher density of voles (16 voles/ha), more voles were killed, and hunting success was about 50%.  When there were less voles (8 voles/ha), the predation rate decreased dramatically, thus giving it a Type II functional response curve (Sundell et al., 2000).

Although they are a predator, they are also preyed upon.  They are preyed upon by foxes, snakes, domestic cats and dogs, and even M. frenata and M. erminaea (Sheffield and King, 1994).  There are also records of Mustela nivalis  being eaten by great horned owls and rough-legged hawks, with least weasel remains also found in barn owl pellets. Mustela nivalis are hosts to lice, ticks and fleas, and there have also been two species of roundworms found in them, Molineus patens and Physoloptera maxillaries (Jackson, 1961).

The habitat that is selected by M. nivalis is usually associated with a high density of rodents.  These areas include farmlands, grassy fields and meadows, hedgerows and open forests.  Mustela nivalis prefer to stay out of dense forests, which lack food and cover (Sheffield and King, 1994).  In Wisconsin they seem to prefer meadows and fields, and that may be why they have been found more commonly in the southern part of the state (Wildlife Primer, 1998).  In central Wisconsin they are also found in marsh habitats, where the water is at or near the surface (Jackson, 1961; Long, 1970).

Den site selection is similar to that of their normal habitat site selection.  Den sites are usually rock piles, underneath abandoned buildings, woodpiles, abandoned mole and other small mammal runs, and brush heaps (Jackson, 1961; Wildlife Primer, 1998).  Dens are lined with fur that is taken from their victims, and may be an inch thick when matted down.  A second nest is often built as a retreat (Jackson, 1961).

The least weasel uses bounds to get from place to place, and never seems to tire.  They travel hunting routes, checking for prey in every hole and crevice they come across.  They make their kill by biting the neck of their victim.  When eating, they start by eating the brain and head first, with any excess cached away.

They are active day and night throughout the year (Sheffield and King, 1994).  In a study of least weasel activity, activity level seemed to be evenly distributed throughout the day between sunrise and sunset, with most of their inactive period being in the evening (Sundell et al., 2000)

Home ranges are determined by scent markings, dominance and food supply.  Outside of the breeding season, males defend their territory against males, and females defend their territory against females.  During this time, males have dominance over females, and will often cross into other female’s territories.  During the late stages of pregnancy, females will defend their territories against males and females.  During the breeding season, males will travel a long ways to find receptive females, while females will stay within their territory.  Once the male finds a receptive female, he will grab her by the top of her neck after a short scuffle, and will copulate with her.  The pair will copulate many times over several days to ensure fertilization.  The male takes no part in the rearing of the young (Sheffield and King, 1994).

Literature Cited:

Gillingham, B.J.  1984.  Meal size and feeding rate in the least weasel (Mustela nivalis) 65: 517 – 519.

Jackson, H.T.  1961.  Mammals of Wisconsin: 343-345. The University of Wisconsin Press.

Long, C.A.  1970.  Mammals of central Wisconsin. Reports on the fauna and flora of Wisconsin.

Sheffield, S.R., and C.M. King.  1994.  Mustela nivalis. Mammalian Species. 454: 1 – 10.

Simms, D.A.  1979.  North American weasels: resource utilization and distribution. Canadian Journal of Zoology 57(3): 504 – 520.

Sundell, J., K. Norrdahl, E. Korpimaki, and I. Hanski.  2000.  Functional response of the least weasel, Mustela nivalis nivalis. Oikos 90: 501 – 508.

Wisconsin wildlife primer, wildlife habits and habitat.  1998.  Wildlife and Your Land: 13.

Reference written by Brandon Stefanski, Biol 378: Edited by Chris Yahnke.
Page last updated 4-28-04.

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