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Beavers are the largest living rodents in North America, with adults averaging 40 pounds in weight and measuring more than three feet in length, including the tail. Beavers are the only species that can actually create its own habitat, and it does so by impounding water. In doing so, beavers are considered a keystone species, meaning their presence in nature greatly impacts other wildlife, providing quality wetland habitat for many dozens of other species of wildlife.
Dams increase water depth, allowing beavers to store food where it will not be frozen into ice in the winter. Flooding also lets beavers construct underwater entrances to their lodge, which in turn protects them from predators. The higher water level promotes the growth of favored aquatic food plants. The limiting factor in dam height is that the dam and water elevation cannot be higher than the living quarters, or the den will be flooded. Dam maintenance is critical, and beavers keep them in good repair. Beavers living in lakes and rivers with a constant water level do not build dams and may not build a typical lodge. Beavers construct and maintain dams with whatever materials are available – wood, stones, mud, and plant parts. Dams are typically three to four feet high but may be higher. Most dams are less than 50 feet long, but they can be much longer depending on the physical aspects of the basin in which the impoundment lies. The feel and sound of flowing water stimulate beavers to build dams. However, they routinely let a leak in a dam flow freely, especially during times of high water. Beavers may enlarge their dam to increase the impoundment and allow access to more food. They are limited by the physical characteristics of that basin and the elevation of the den floor within their lodge. A family may build and maintain one or several dams in its territory.
Beaver dams create habitat for many other animals and plants. Moose use the highly nutritious emergent and submergent aquatic plants found in the deeper beaver flowages. In winter, deer and moose may frequent beaver ponds to forage on shrubby plants that grow where beavers cut down trees for food, dams, or lodges. Deer benefit from lush meadows that develop along flowages when beaver dams no longer hold water. Otters, mink, raccoons, and herons hunt frogs and other prey along the marshy edges of beaver ponds. Waterfowl such as black ducks, wood ducks, hooded mergansers, and green-winged teal are closely tied to these flowages to forage, raise young, and rest during migration. Ducks and geese may even nest on top of beaver lodges, which offer warmth (from the beavers that live below) and protection (especially when lodges are located in the middle of a pond). Trees killed by rising water levels provide perch sites for avian predators, habitat for insects, and food for insect-eating birds such as woodpeckers. These trees also develop cavities that many species of animals require for nesting.
Lodges and Bank Dens
Lodges and bank dens are used for safety and as a place to rest, stay warm, give birth, and raise young. Lodges consist of a mound of branches and logs plastered with mud. One or more underwater openings lead to tunnels that meet at the center of the mound, where there is a single chamber. Beavers build freestanding lodges in areas where the bank or water levels aren't sufficient for a safe bank den. Beavers can also dig into the banks of streams and large ponds, leading to the term "bank beavers." They may or may not build a lodge on top. Bank dens may also be located under stumps, logs, or docks. One family can have several lodges or bank dens but will typically use only one area during winter.
Leaves, inner bark, and twigs of deciduous trees and shrubs make up the primary food. "Popple" (aspen) is the favorite followed by birch, cottonwood, willow, oak, and maple. Beavers will also eat herbaceous plants, grasses, and some aquatic plants. Beavers rarely eat coniferous trees such as fir, spruce, and pine. More often, beavers will use these trees as dam-building material or girdle and kill them to encourage the growth of preferred food plants. Beavers have large, sharp incisors that grow continually. The animal wears them down by cutting trees, peeling bark, and feeding. Fermentation by intestinal microorganisms allows beavers to digest thirty percent of the cellulose they ingest. Beavers store food for the winter months by stashing stems underwater, anchoring them to the bottom of the lake or stream. When ice makes it impossible to forage on land, they feed on the bark and stems in their cache and on the thick roots and stems of aquatic plants, such as pond lilies and cattails. Surviving such harsh conditions also requires dense fur, a well-insulated den, and fat reserves. Beavers do not hibernate, but are less active during winter, spending most of their time in the lodge or den.
This semi-aquatic mammal has a few distinctive characteristics, however the best known is the beaver's tail. The tail is covered with leathery scales and sparse, coarse hair. A large beaver tail may be 15 inches long and six inches wide. The tail is important both in the water and on land. It serves as an area for storing fat, and because it is nearly hairless, it releases body heat, helping the beaver to regulate its body temperature. In the water, the animal uses its flexible tail as a four-way rudder. When diving after being frightened, a beaver loudly slaps the water with its tail; the sound warns all beavers in the vicinity that danger is near, and perhaps serves to frighten potential predators. On land, the tail acts as a prop when a beaver is sitting or standing upright. It also serves as a counterbalance and support when a beaver is walking on its hind legs while carrying building materials with its teeth, front legs, and paws. Contrary to common belief, beavers do not use their tails to plaster mud on their dams.
Additionally, beaver are well known for their webbed hind feet and large orange teeth which they use to cut down trees, shrubs, and other available vegetation for food and building materials. Tooth marks look like twin grooves, each groove measuring one-eighth inch or more. They generally eat all of branches and twigs under three-quarters of an inch in diameter. Their teeth have indeterminate growth, meaning their teeth will continue to grow throughout their lifetime.
Although beavers are often active during the day, they are primarily nocturnal – one of the best times to observe them is in the evenings.
A beaver’s harvesting will become most intense in late fall, all family members concentrate on repairing and building up dams and the family lodge in preparation for winter.
Reproduction & Family Structure
A mated pair of beaver can live together for many years, sometimes for life. Beavers breed between January and March; females produce a litter of one to eight kits (average four) between May and June. The number of kits is related to the amount of food available – more food, more kits – 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; some leave at 11 months and a few females may stay until they are three years of age. Beavers are sexually mature at age two. At or just before this age they leave on their own or are driven off to find a mate and establish their own colony. Beavers causing new problems in sub-optimum habitat are often these newly dispersing individuals. Beavers live in colonies that may contain two to twelve individuals. The colony is usually made up of the adult breeding pair, the kits of the year, and kits of the previous year. Populations are limited by habitat availability. Under the best of conditions, colonies will be at least half a mile apart.
Survival & Threats
Once among the most widely distributed mammals in North America, beavers were nearly eliminated from much of their range in the late 1800s because of unregulated trapping. With a decline in the demand for beaver pelts and with proper management, they re-established much of their former range and are now common to abundant in many areas.
Though an aquatic lifestyle and protected dens are effective survival strategies, black bears, coyote, lynx, bobcat, fisher, and dogs prey upon beavers that are foraging on shore or migrating overland. Other identified causes of death are severe winter weather, winter starvation, disease, water fluctuations, water pollution, floods, and falling trees.
Management & Conservation
Beavers are still considered a valuable furbearer and managed with a regulated trapping season, which runs October-April. In addition to the pelt, people use beaver meat, tail, teeth, and caster glands for making a variety of products. Trappers must register the beavers they catch and report their effort so the Department can ensure the harvest is sustainable.
Living with Wildlife
How to Prevent or Resolve Conflicts with Beavers
Despite an appreciation for these animals and our best intentions to live with them, beavers can become a problem if their eating habits or dam/den building activity flood or damage property. Before beginning any control action, assess the problem fairly and objectively. Are beaver really causing damage or creating hardship requiring control action? The very presence of beavers is often seen as a problem when, in fact, the beavers are causing no harm.