beavers / subject background
Beaver Facts
The Beaver is an amphibious rodent (order Rodentia) of the family Castoridae and the genus Castor. There are two species of beaver: the North American Castor canadensis, and the similar European Castor fiber. The beavers' prehistoric ancestors include Castor californiensis and Castoroides ohioensis, a creature which weighed between 700 and 800 pounds. The film Beavers features the Castor canadensis which, while native to North America, can now be found in Europe and Asia as well.
The contemporary beaver has soft brown fur, a leathery paddle-shaped tail (crisscrossed by a scale-like pattern) and characteristic rodent teeth which are used principally for gnawing wood. The beaver is the largest of rodents, with the exception of the amphibious capybara of South America. A mature beaver measures about 48 inches in length and generally weighs between 40 and 60 pounds, although some may weigh much more than this — the record was set in 1921 by a beaver from Wisconsin, which was reported to weigh 110 pounds. Beavers have poor eyesight which is compensated for by acute hearing and an excellent sense of smell.
The leaves, twigs and bark of deciduous trees make up the beavers' principal diet, with the aspen tree being a particular favorite when available. In order to effectively digest this diet, bark and wood are pre-digested in a special gland, excreted, re-eaten and then re-digested (in a manner similar to the digestive process of a rabbit). Beavers do eat a variety of other vegetation, including floating duckweed, pond lily leaves and roots, bulrushes, bracken fern, tender green grasses, and even algae.
Excellent swimmers, beavers can swim submerged for half a mile or more. They can constrict muscles in the ears and nose to prevent water from entering and they can also close their lips behind their incisors to keep water out of the mouth while cutting submerged branches. If not overly active, these creatures can stay under water for at least 15 minutes. When swimming in winter, beavers will make use of air bubbles and air pockets trapped under the ice to extend the time they can stay submerged. In an emergency situation, oxygen-rich blood otherwise being used by muscular areas of the beaver's body can be redirected to the brain.
Beavers live in colonies. A colony may contain as little as two or as many as a dozen beavers and is located in a stream, lake or pond. Often a colony of beavers will create their own pond by building one or more dams across a stream or other water source.
Within a colony the beaver is a highly social creature. Beavers exhibit intolerance to alien beavers however, and have been known to attack and even kill an intruding one. To mark the boundaries of the colony's territory, beavers secrete a yellow-orange liquid called castoreum from the castor glands, a pair of glands located under the skin at the base of the tail.
Beavers are nocturnal animals; while they are seen during the day, much of the beavers' work is accomplished during the evening and night hours. One beaver or two beavers working alternately, can gnaw through the trunk of a tree fairly quickly. A tree three to four inches in diameter can be felled in under an hour. During the course of a year, two beavers may fell as many as 400 trees, some of them over a foot and a half in diameter.
Usually trees are cut within about 150 feet of the shoreline. When a tree has been felled, several beavers may participate in removing branches, cutting up limbs and dragging or floating the material to a chosen site. At the site, branches and limbs are progressively interwoven to produce a solid structure which is then sealed with rocks, mud and grass.
A dam may grow to well over 100 feet in length and the pond behind it may grow to cover several acres, providing the beaver colony with a good refuge from land-based predators. Some dams have been reported to reach well over a thousand feet in length, creating a lake with numerous lodges. In the giant screen film Beavers, the large dam is some 300 feet in length. In preparation for winter, the beavers will work to ensure that the dam is sufficiently high and the pond deep enough to prevent the water from freezing to the bottom.
For living quarters, the beavers will construct a dome-shaped house or lodge, also made of wood, mud and grass. The lodge is located in the middle of the pond, or attached to the bank of a lake or stream. Typically a lodge is about six to ten feet in diameter. Located inside the lodge and above the water level is a living chamber accessible only through underwater tunnels, of which there may be several. With the exception of a vent hole at the top of the lodge, the walls are coated with mud to provide good insulation. Over a period of years a lodge can become very large (over 20 feet in diameter and 12 feet in height) and develop several living chambers. The walls may be built up until they are over two feet thick. Beavers may also burrow into soft earth on the banks of streams or ponds, creating one or more chambers for rest and feeding.
A beaver colony contains only one active adult pair who will mate for life. Mating takes place in water in early February and beaver kits are born in early May (a gestation period of 100 to 110 days). A female will typically produce her first litter around her third birthday. A litter will usually consist of three to seven kits, although litters of up to 12 have been recorded. As with many creatures, the size of the litter varies with the age of the female, the availability of food, and the density of the population.
Kits will nurse for two to two and a half months. By the fall, the beaver kits are assisting in the winter preparations of the colony: gathering and storing food, and building dams and houses. By the time winter sets in and the pond freezes over, the kits will weigh anywhere from 11 to 28 pounds, depending upon the food available in their habitat.
Most of the young beavers will leave the colony in early spring in search of a good place to start a new family. These migrating beaver are particularly susceptible to attacks from predators such as wolves, coyotes, lynxes, bobcats, mountain lions, wolverines, bears and otters, or even from other beavers whose territory they have transgressed. Generally only a few beaver from a litter will live to reach five years of age (the survival rate may be as low as 12% ). Beavers may live to be much older however, and some have been found to have lived as long as 20 years.
The beaver has long been of economic and religious importance for the native peoples of North America. For thousands of years Indians have trapped the beaver for its meat and fur, and saved a space in their religious ceremony for the creature. With the coming of Europeans to the continent in the 16th and 17th Century, beaver fur became an important commodity in the trade between Indians and Whites, and between North America and Europe. Indeed, the fur trade, of which beaver pelts were a major component, provided much of the impetus for early exploration and settlement of the continent. The trapping and selling of beaver pelts continues in the present age.
To many, the beaver is a destructive animal and a nuisance. This is particularly the case with farmers and others who have found their land being flooded as a result of these creatures hard work. Yet, within the realm of nature the beaver plays an important role, transforming the environment as no other creature on Earth save humans. Through dam-building, the beaver creates a water habitat for an infinite variety of new creatures. When the dam eventually breaks and the water drains from the pond, the land beneath will become rich and fertile meadow.
