Characteristics
The long-tailed weasel, which is the largest of the three weasels found in Montana and varies from 13 to 18 inches in length, including a 6 1/2 inch tail.  It is the long tail which gives it its name. They are about 2 to 3 1/2 inches high and weigh up to 12 ounces.

It has a long, slender, muscular body with short legs. The head is small, with beady eyes, small ears and a pointed nose. They move with quick movements and a graceful, bounding gait. All three weasels change color with the seasons, and there is no color difference between the sexes. All the senses are well developed in the weasel.

The long tailed weasel is much the same in color as the short tailed weasel except that, in some areas, it does not become white in winter. The feet are dark brown in summer whereas the short tailed weasel has pale yellowish feet in summer. The tail has a black tip throughout the year. The molting pattern is different than in short tailed weasels, starting on the back in spring and causing a brindled appearance to the facial markings.

Life Cycle
The long-tailed weasel females mature at 3 to 4 months and males mature at about one year. The breeding season is in July. There is a period of delayed implantation with a gestation period of 9 to 10 months. The period of active pregnancy is 23 to 17 days.

Litter sizes varies from four to thirteen, with an average of six to eight. The young are born in April or May in nests constructed in underground dens or hay piles. Mouse nests and burrows are often used and heavily lined with fine grass and mouse fur. The male begins to bring food to the den about 1 month after the young are born. The young are weaned at the end of 5 weeks and are able to hunt for themselves by 7 or 8 weeks of age. The family stays together until late summer and then disperses. The life expectancy of weasels is short, probably less than a year, although they are capable of living as long as 6 years.

Food
Weasels prey on small rodents such as mice, rats, voles, hares, rabbits, and chipmunks. They also take shrews, birds, birds eggs, frogs, bats, insects, earthworms and may occasionally kill domestic chickens.

The weasel hunts by tirelessly and persistently investigating every small hole, crevice, bush or rock pile it encounters. They will track prey by following their scent trails and generally attack prey by ambushing and pouncing on it. They are very quick and kill by piercing the base of the skull with their teeth. The weasel frequently kills more than it can eat and often caches leftover food. The weasel can consume up to one third of its own weight in a 24 hour period.

Habits
Weasels are curious, alert and bold. They are persistent hunters who seldom remain long in their dens and may be abroad hunting at any hour, although they are usually most active at night. Weasels are active year round. Weasels occasionally hunt in pairs but, for the most part, are solitary except during breeding and rearing season. They are good swimmers and can also climb trees. All species emit a strong musk odor when alarmed, and the weasel may stamp its feet when annoyed. Weasels may mark their trails with droppings. Home ranges very from 30 to 400 acres.

Population
Weasel populations often cycle with mouse populations. Several parasites can infect weasels, such as guinea worm and kidney worms. These probably have little impact on the population.

Weasels are subject to predation from hawks, owls, foxes, coyotes, dogs, cats and man.

In agricultural areas, weasels are more common due to the practice of storing grain which provides ideal conditions for mice.

Habitat
Weasels prefer woodlands or open country with hedgerows, thickets or fence rows. They are usually found near water but are not semi-aquatic as is the mink. They frequent stone piles, brush heaps, wood piles, hay stacks, log piles and old abandoned buildings.

The long-tailed weasel is found in almost all land habitats near water.  It has the broadest ecological and geographical range of the North American weasel. It avoids dense forests.

The dens of weasels are shallow chambers about 6 inches underground with two to three entrances and are lined with mouse fur and grass.

Economic Value
The fur of the long-tailed weasel commands a higher price because it is so much larger.

Weasels play an important role in helping to control rodent populations.