Least weasels are long and slender, with a long neck, a narrow head, and short limbs. They have large, black eyes and large, round ears. The feet have five fingers with sharp claws. The mass of least weasels varies depending upon their location. In North America least weasels range in weight from 30 to 55 grams, with males being slightly larger than females. Total length ranges from 165 to 205 mm, tail length ranges from 22 to 40 mm. Fur color is chocolate brown on their 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 southern populations.
Least weasels are native to the Nearctic and Palearctic regions and have been introduced to the Australian region. They are found throughout Europe and northern Asia (excluding Ireland, the Arabian Pennisula, and Artic islands), in Japan, and throughout North America. In North America they range from Alaska and northern Canada south to Wyoming and North Carolina. A population of least weasels was introduced to New Zealand as well.
Least weasels do well 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 regions.
Pregancy in least weasels lasts from 34 to 37 days. Litters may range from 1 to 7 young. A higher number of offspring per litter can be found in northern populations. In the wild it is possible to have two litters per year, but there is a high death rate in the second litter. Females that are born in the spring are mature in four 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. Males reach sexual maturity at 8 months old.
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.
Least weasels probably only live for several years after reaching adulthood and most die before reaching adulthood.
Males and females live apart from each other except during the breeding season. Because home ranges of females are smaller than male home ranges, one or more females may live within a male's range. Females have the ability to fend off other females and males from their 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 wrap their body and limbs around the victim and kill with a bite to the base of the skull. Least weasels use abandoned burrows rather than making their own.
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.
The diet of least weasels is composed of small mammals, mainly rodents like white-footed mice and meadow voles. When rodents are scarce, weasels will eat birds' eggs and nestlings, insects, and lizards. The size of prey that least weasels are able to hunt depends on burrow size of the prey. If the weasel is too large to fit into the burrow it is unlikely that they will be able to hunt those animals. Because females are smaller, they are able to hunt smaller prey than males. Least weasels will kill more prey than they can eat at the time and will store this surplus in their burrows for later consumption.
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.
Least weasels play an important role in controlling rodent populations.
Least weasels have been hunted and trapped by humans throughout the world. They help keep in check the populations of many species of rodents that are potentially harmful to agriculture.
Least weasel populations are not considered threatened.
Tanya Dewey (editor), Animal Diversity Web.
Toni Lynn Newell (author), University of Michigan-Ann Arbor.
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.