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Living with Beavers

Biology

Beavers are the largest living rodents in North America, weighing in at an average of 40 pounds. Beaver are semi-aquatic mammals with webbed hind feet, large incisor teeth, and a broad, flat tail. They have poor eyesight, but excellent hearing and sense of smell. The beaver’s sharp incisors, which are used to cut trees and peel bark while eating, are harder on the front surface than on the back, and the uneven wearing creates a sharp edge that enables ease of cutting through wood. The incisors continually grow, but are worn down by grinding, tree cutting and feeding.

Beaver fur consists of long, coarse outer hairs and short, fine inner hairs. The fur can range in color, but is most of dark brown. Scent glands secrete an oily substance called castoreum, which the beaver uses to waterproof its fur. Beavers are territorial and mark their territory by creating small mounds of mud, leaves, and sticks, and which they coat with castoreum. More susceptible to predators on land, beavers tend to remain in the water as much as possible. They are excellent swimmers, and with valvular nose and ears and lips that close behind the four large incisors teeth, they are able to remain submerged for up to 15 minutes. The flat, scaly tail stores fat, and is used to signal danger by slapping the surface of the water.

Food and Feeding

Beavers are herbivorous and eat a variety of leaves, shoots, roots, aquatic herbs and the inner (cambium) layer of woody plants. Herbaceous plants are a major component of their diet in summer, but in winter woody plants and submerged roots are their primary food. In Montana, aspen, willow and cottonwood are preferred food, but beaver also eat alder, birch, maple, apple and other fruit trees, dogwood and many ornamentals. Fermentation by special intestinal microorganisms allows beavers to digest 30 percent of the cellulose they ingest. Coniferous trees, such as fir and pine, are eaten occasionally; however, beavers more often girdle and kill these trees for use in dam building material. When the surface of the water is frozen, beavers eat bark and stems from a food cache stored near the entrances to their bank dens and lodges. Food caches are seldom found where winters are mild.

Family Structure

Beavers live in colonies of 2 to 12 individuals. The colony typically consists of an adult breeding pair, the kits of the year, and kits of the previous year or years. A mated pair will live together for many years, sometimes for life. Beavers breed between January and March, and litters of one to eight kits (average four) are produced between April and June. The number of kits is related to the amount of food available and the female’s age. The female nurses the kits until they are weaned at 10 to 12 weeks of age. Most kits remain with the adults until they are almost two years old, and then go off on their own in search of mates and suitable spots to begin colonies, which may be several miles away.

Predators and Mortality

A dead beaver

Because of their size, behavior and habitat, beaver have few enemies. Beavers live 5 to 10 years in the wild. When foraging on shore or migrating overland, beavers are killed by predators including bears, wolves, coyotes, bobcats, cougars and dogs. Other identified causes of death are severe winter weather, winter starvation, disease, water fluctuations and floods, and falling trees. Humans remain the major predator of beavers. Historically, beavers have been one of the most commonly trapped furbearers. Mortality from current human activity most commonly takes the form of trapping or shooting for lethal control of nuisance beaver, recreational trapping, and road kill.

Lodges and Dens

Beavers are found where their preferred foods are in good supply---along rivers and in small streams, lakes, marshes and even roadside ditches containing adequate year-round water flow. In areas where deep, calm water is not available, beavers will select sites with abundant building materials and create ponds by constructing dams across creeks or other waterways to create pond and pool habitats.

Depending on the type of water body they occupy, beavers will construct freestanding lodges or will excavate burrows known as bank dens. Lodges and dens are used for safety, and a place to rest, stay warm, give birth, and raise young. Lodges and dens are not subsurface, and have enough air exchange to allow beaver, muskrats, and other air breathing fish and wildlife to breathe while conditions are otherwise limiting. Freestanding lodges are built in areas where the bank or water levels aren’t sufficient for a safe bank den. Lodges consist of a mound of branches and logs, plastered with mud. One or more underwater openings and tunnels lead to center of the mound, where a single chamber is created. Bank dens are dug into the banks of streams and large ponds, and in some sites a lodge may be built over the den. Bank dens may also be located under stumps, logs or docks. Beavers living in cold climates store branches of food trees and shrubs for winter use by shoving them into the mud at the bottom of ponds or streams near the entrance to their bank den or lodge, or may create a floating cache anchored by its own weight or large branches at the top of the cache frozen in ice or otherwise anchored. This latter cache type appears to be most common in larger rivers and other areas with rocky substrate.

Illustration of beaver dam and lodges