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Beavers

 

Beavers are the largest living rodents in North America, with adults averaging 40 pounds in weight and measuring more than 3 feet in length, including the tail. These semi-aquatic mammals have webbed hind feet, large incisor teeth, and a broad, flat tail. Once among the most widely distributed mammals in North America, beavers were eliminated from much of their range in the late 1800's because of unregulated trapping. With a decline in the demand for beaver pelts, and with proper management, they became reestablished in much of their former range and are now common in many areas. 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 that have enough building material available will create ponds by building dams across creeks or other watercourses and impounding water. "Busy as a beaver" as the old saying goes especially in autumn.

Beaver dams create habitat for many other animals and plants. In winter, deer and elk frequent beaver ponds to forage on shrubby plants that grow where beavers cut down trees for food or use to make their dams and lodges. Weasels, raccoons, and herons hunt frogs and other prey along the marshy edges of beaver ponds. Migratory water birds use beaver ponds as nesting areas and resting stops during migration. Ducks and geese often nest on top of beaver lodges since they offer warmth and protection, especially when lodges are formed in the middle of a pond. The trees that die as a result of rising water levels attract insects, which in turn feed woodpeckers, whose holes later provide homes for other wildlife. The beaver's incisors (front teeth) are harder on the front surface than on the back, and so the back wears faster. This creates a sharp edge that enables a beaver to easily cut through wood. Like many rodents, beavers construct nesting dens for shelter and for protection against predators. These may be burrows in a riverbank or the more familiar lodges built in the water or on the shore. Beavers eat the leaves, inner bark, and twigs of aspen (a favorite food), alder, birch, cottonwood, willow, and other deciduous trees. Beavers also eat shrubs, ferns, aquatic plants, grasses, and crops, including corn and beans. Coniferous trees, such as fir and pine, are eaten occasionally; more often, beavers will girdle and kill these trees to encourage the growth of preferred food plants, or use them as dam building material.

Beavers have large, sharp, upper and lower incisors, which are used to cut trees and peel bark while eating. The incisors grow their entire lives, but are worn down by grinding them together, tree cutting, and feeding. Fermentation by special intestinal microorganisms allows beavers to digest 30 percent of the cellulose they ingest. When the surface of the water is frozen, beavers eat bark and stems from a food "cache" (a safe storage place) they have anchored to the bottom of the waterway for winter use. They also swim out under the ice and retrieve the thick roots and stems of aquatic plants, such as pond lilies and cattails. Like many rodents, beavers construct nesting dens for shelter and for protection against predators. However, the basic interior design varies little and consists of one or more underwater entrances, a feeding area, a dry nest den, and a source of fresh air. Beavers flood areas for protection from predators, for access to their food supply, and to provide underwater entrances to their den. Flooded areas also wet the soil and promote the growth of favored foods. Beavers living on water bodies that maintain a constant level (e.g., lakes, large rivers) do not build dams. Dams are constructed and maintained with whatever materials are available wood, stones, mud, and plant parts. They vary in size from a small accumulation of woody material to structures 10 feet high and over 165 feet wide. 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 waters. Beavers keep their dams in good repair and will constantly enlarge the dams as the water level increases in their pond. A family of beavers may build and maintain one or several dams in their territory.

In cold areas, dam maintenance is critical. Dams must be able to hold enough water so the pond won't freeze to the bottom, which would eliminate access to the winter food supply. Depending on the type of water body they occupy, beavers construct freestanding lodges or bank dens. Lodges and bank dens are used for safety, and a place to rest, stay warm, give birth, and raise young. 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 lead to tunnels that meet at the center of the mound, where a single chamber is created. Bank dens are dug into the banks of streams and large ponds and beavers may or may not build a lodge over them. The 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 den during winter. A mated pair of beaver 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 (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.) The kits then go off on their own in search of mates and suitable spots to begin colonies, which may be several miles away. Beavers live in colonies that may contain 2 to 12 individuals. The colony is usually made up of the adult breeding pair, the kits of the year, and kits of the previous year or years. Populations are limited by habitat availability, and the density will not exceed one colony per ½ mile under the best of conditions. Because of their size, behavior, and habitat, beaver have few enemies. When foraging on shore or migrating overland, beavers are killed by bears, coyotes, bobcats, cougars, and dogs. Other identified causes of death are severe winter weather, winter starvation, disease, water fluctuations, floods, and falling trees.

Humans remain the major predator of beavers. Historically, beavers have been one of the most commonly trapped furbearers. In the State of Washington, from 1991 to 2000, a annual average of 5,289 beavers were trapped. However, the average for the past three years has dropped to just over 1,000. Beavers live 5 to 10 years in the wild. Beavers are nocturnal, but are occasionally active during the day. They do not hibernate, but are less active during winter, spending most of their time in the lodge or den. Probably no animal leaves more obvious signs of its presence than the beaver. Freshly cut trees and shrubs, and prominent dams and lodges are sure indicators of their activity. Look for signs of beavers during the day; look for the animals themselves before sunset or sunrise. Approach a beaver site slowly and downwind. (Beavers have poor eyesight but excellent hearing and sense of smell). Look for a V-shaped series of ripples on the surface of calm water. A closer view with binoculars may reveal the nostrils, eyes, and ears of a beaver swimming.

If you startle a beaver and it goes underwater, wait quietly in a secluded spot and chances are that it will reemerge within one or two minutes. However, beavers are able to remain underwater for at least 15 minutes by slowing their heart rate. When seen in the water, beavers are often mistaken for muskrats. Try to get a look at the tail: Beavers have a broad, flat tail that doesn't show behind them when swimming, whereas muskrats have a thin tail that is either held out of the water or sways back and forth on the water's surface as the animal swims. Beavers stand their ground and should not be closely approached when cornered on land. They face the aggressor, rear up on their hind legs, and hiss or growl loudly before lunging forward to deliver extremely damaging bites. Beavers cut down trees, shrubs, and other available vegetation for food and building materials. Large stumps are pointed, 1 to 2½ feet high, and sometimes the tree trunk is still attached. Tooth marks look like twin grooves, each groove measuring 1/8 inch or more. There will be a pile of wood chips on the ground around the base of recently felled trees. Limbs that are too large to be hauled off are typically stripped of bark over the course of several days. The cut on small wood usually involves a 45-degree cut typical of rodents, but at a larger scale. Branches and twigs under ¾ inches in diameter are generally eaten entirely. Most harvesting is done within 165 feet of the water's edge. In areas with few predators, but a lean food supply, toppled trees and other signs of feeding may be found twice that distance from the den site. Beavers transport woody material even farther through upstream and downstream sites.

By late fall, all family members concentrate on repairing and building up dams and the family lodge in preparation for winter. Harvesting is at its most intense level at this time of year. Slides are the paths beavers make where they enter and leave the water. They are 15 to 20 inches wide, at right angles to the shoreline, and have a slicked down or muddy appearance. Beavers construct channels or canal systems leading to their ponds, using them to float food-such as small, trimmed trees-from cutting sites. Canals are also safe travel ways for swimming instead of walking. With receding water levels during summer, beaver activity shifts toward building and maintaining channels to access new food supplies. Channels often look man-made, have soft, muddy bottoms, and are filled with 15 to 25 inches of water. Beavers that live 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. Beaver droppings are seldom found on land; those that are will commonly be found in the early morning at the water's edge. Individual beaver droppings are usually cylindrical, up to 2½ inches long (sometimes shorter), and look as if they were formed of compressed sawdust. The diameter is an indication of the animal's size, with 1 inch being average for adults. The color of fresh deposits is dark brown, with lighter-colored bits of undigested wood, all turning pale with age. In order to warn each other of danger; beavers slap their tails against the water, creating a loud splash. Sounds also include whining (noises made by kits), a breathy greeting noise, and loud blowing when upset. The Beavers Tail; The tail of a large beaver may be 15 inches long and 6 inches wide. It is covered with leathery scales and sparse, coarse hairs.

The beaver's tail has important uses both in the water and on land. 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. The tail stores fat, and because it is nearly hairless, releases body heat, helping the beaver to regulate its body temperature.

 

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