Illinois Natural History Survey - University of Illinois

Least Weasel

Susan Post


Least weasels, Mustela nivalis, are not only the smallest members of the weasel family, but also the smallest member of the Order Carnivora  in the world. They do not appear to be common in any part of their range and usually pass unnoticed until one is trapped or revealed by tracks in fresh snow. They are found in Europe, northern Asia, North Africa, and North America. In North America they are found from Alaska and northern Canada, south to Wyoming and North Carolina. In Illinois they are found north of an east-west line running through Edgar–Adams counties. They can be found anywhere that mice and other rodents are plentiful. They will adapt to a variety of habitats, but prefer rolling or flat countryside, short-grass fields, and the edges of cultivated fields.


With their long, slender skulls and sinuous bodies, least weasels are built to squeeze into nooks and crannies. They are small enough to follow their favorite prey—mice—into their underground burrows. If their heads can fit into burrows, their bodies soon follow. They are diminutive in size, only seven inches long, and less than an inch in diameter with very short tails (about one inch in length). Weasels are brown with white under parts and toes. The tail tips contain a few black hairs. Their heads are long and oval shaped; the eyes are black and the ears short and pointy. Northern individuals are completely white in the winter, but in the eastern part of their range, from southern Michigan south, they may remain brown. They have a well-developed anal scent gland, which they use for marking territories and defense. In fact, the name weasel comes from the Old English word “weosule” which means flowing and refers to the musky secretions of the animal.

Least weasels are active any time of the year, day or night. They can run up to six miles an hour. When hunting they move about their home range (about two acres), investigating every hole and crack, even climbing trees or bushes to examine the nests of birds. The least weasel is a specialist on small mammals, eating mice, rats, and voles. Bird eggs, nestlings, and insects are also fair game. They consume about 40% of their body weight per day, eating 9 or 10 times each day. Pouncing on their prey, the weasels wrap their legs and bodies around victims, and then kill them with swift bites, usually at the base of the skull. Even if not hungry, least weasels will kill and store their excess food in side passages of their burrows.

Nests of least weasels are constructed of grasses lined with fur or feathers and have been found beneath corn shocks in shallow burrows bordering streams. They will even take over the nests or burrows of their prey species.  Weasels will den in the abandoned burrows of other small mammals, such as a mice, gophers, or ground squirrels, adding a lining of mouse hair to any already present nesting material.

Lease weasels have at least two litters of young per year, each with one to six young. The young are born after a gestation of 34–37 days and are wrinkled, pink, and naked. Eighteen days after birth the young have fur and are eating meat. Their eyes will not open for at least another week. Within 40 days they can kill prey, and by 12 weeks they are fully independent and leave their families.

While hawks, owls, and foxes are important predators, the worst enemy of the least weasel may be a cold winter with no food. 


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