Weasel Biology

Identification | Biology | Damage ID | Management | Resources

Reproduction 

Long-tailed weasels mate in late summer, mostly from July through August. Females are induced ovulators and will remain in heat for several weeks if they are not bred. There is a long delay in the implantation of the blastocyst in the uterus, and the young are born the following spring, after a gestation period averaging 280 days.  

Average litters consist of 6 young, but litters may include up to 9 young. The young are blind at birth and their eyes open in about 5 weeks. They mature rapidly and at 3 months of age the females are fully grown. Young females may become sexually mature in the summer of their birth year. 

Denning Cover 

Tree roots, hollow logs, stone walls, and rodent burrows are used as dens. Dens are usually around a foot in depth below ground. Weasels line their nests with dry vegetation, and fur and feathers from prey. Side cavities of burrows are used as food caches and latrines. 

Behavior 

Weasels are active in both winter and summer; they do not hibernate. Weasels are commonly thought to be nocturnal but evidence indicates they are more diurnal in summer than in winter. The weasels appear to prefer hunting certain coverts with noticeable regularity but rarely cruise the same area on two consecutive nights. 

Habitat and Home Range 

Weasels prefer riparian woodlands, marshes, shrubby fencerows, and open areas adjacent to forests or shrub borders. Although weasels are primarily terrestrial, they climb trees and swim well.  

Home range sizes vary with habitat, population density, season, sex, food availability, and species. The least weasel has the smallest home range. Males use 17 to 37 acres, females 3 to 10 acres. The short-tailed weasel is larger than the least weasel, and has a larger home range. Male short-tailed weasels use an average of 84 acres, and females 18 acres, according to snow tracking. The long-tailed weasel has a home range of 30 to 40 acres, and males have larger home ranges in summer than do females.  

Weasel population densities vary with season, food availability, and species. In favorable habitat, maximum densities of the least weasel may reach 65 per square mile; the short-tailed weasel, 21 per square mile; and the long-tailed weasel, 16 to 18 per square mile. Population densities fluctuate considerably with year-to-year changes in small mammal abundance, and densities differ greatly among habitats.  

Food Habits 

The weasel family belongs to the order Carnivora. With the exception of the river otter, all members of the weasel family feed primarily on insects and small rodents. Their diet consists of whatever meat they can obtain and may include birds and bird eggs.  

As predators, they play an important role in the ecosystem. Predators tend to hunt the most abundant prey, turning to another species if the numbers of the first prey become scarce. In this way, they seldom endanger the long-term welfare of the animal populations they prey upon.  

Long-tailed weasels typically prey on one species that is continually available. The size of the prey population varies from year to year and from season to season. At times, weasels will kill many more individuals of a prey species than they can immediately eat. Ordinarily, they store the surplus for future consumption, much the same as squirrels gather and store nuts.  

Where pocket gophers (Geomys bursarius) occur, they are the primary prey of long-tailed weasels. In some regions these gophers are regarded as nuisances because they eat alfalfa plants in irrigated meadows, and native plants in mountain meadows where livestock graze. Because of its predation on pocket gophers and other rodents, the long-tailed weasel is sometimes referred to as the farmer’s best friend. This statement, however, is an oversimplification of a biological relationship.  

Weasels prefer a constant supply of drinking water. The long-tailed weasel drinks up to 0.85 fluid ounces daily.