Beavers: Castoridae

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BEAVERS: Castoridae



Beavers are among the largest of the rodents. They have a combined head and body length of 31 to 58 inches (80 to 140 centimeters). The flat, paddle-like tail is about 9.8 to 17.7 inches (25 to 45 centimeters) long. The tail is broad and scaly. A typical beaver can weigh 33 to 75 pounds (15 to 33 kilograms), with a few beavers weighing in at 100 pounds (45 kilograms). Males and females are similar in size.

Beavers' bodies are stocky with short limbs. Each limb has five clawed digits. The back feet, which are larger than the front, are webbed. The claws on the hind feet's first and second toes are split, appearing as a double claw. They have long, curved incisors, chisel-shaped teeth at the front of the mouth, that are an orange-brown color. The incisors grow continuously.

Their eyes are small and their ears are short. Their ears are set far back on their heads, which are broad and rounded. Beavers can close both their ears and nostrils when underwater. Beavers have a skin fold inside their mouths, which allows them to grasp onto items in their teeth without water entering their throat.

Beavers' fur is dense, made up of a fine coat of soft fur, called underfur, beneath long guard hairs, coarse hairs that form the outer fur and protect the underfur. The short underfur helps the beaver with water shedding and insulation. Fur color is a glossy yellowish brown to black. Their bellies are slightly paler in color, ranging from a brown to yellowish brown. The tail and feet are black.

The family name "Castoridae" refers to beavers' castor glands, or "castors." A gland is a group of special cells that make substances so that other parts of the body can work. This pair of glands, along with a pair of anal glands, releases a pungent, musky odor. Both sets of glands lie at the base of the tail.


Beavers are found in North America, northern Europe, and northern Asia. After a decrease in population, these animals have been reintroduced to Russia, Scandinavia, and Argentina. They are also found in Chile.


Beavers live primarily along streams, ponds, lakes, swamps and other waterways, in areas where they can build dams. They are found mainly in areas with a year-round water flow, but are found occasionally in roadside ditches, drainage ditches, and sewage ponds. They are have also become more common in urban areas.


Beavers feed primarily on the bark and outer layers of deciduous trees such as birch, willow, alder, sweet gum, magnolia, maple, and dogwood. They eat twigs, leaves, and roots of trees and shrubs. They also eat various parts of aquatic plants, especially the young shoots of water lilies. During the warmer months, they may add grasses, corn, and other plants to their diet.


Beavers are generally nocturnal, active at night. They are active year round. These animals are semiaquatic, living partly on land and partly in water, and are graceful moving about in water. They use their webbed feet and paddle-like tails to swim.

Beavers are hard workers and are considered the engineers of the animal kingdom because of the complex dams and lodges they build. Dams can be extensive, reaching over 10 feet (3 meters) high and stretching hundreds of feet long. A typical dam is 65 to 98 feet (20 to 30 meters) long. Mud and stones may set the foundation, base, for the dam. Brush and poles are added with the butt ends facing upstream, and mud, stones, and soggy vegetation are used as plaster on top of the poles. A dam is built higher than the water level. With maintenance and upkeep, dams are used by several generations of beavers.

Beavers may create multiple homes in their territory. Homes can take the form of a burrow, hole or tunnel, along a bank to make a den, or a wood lodge. Built of sticks and mud, the dome-shaped lodge is generally surrounded by the water backed up by the dam. The lodge may eventually reach more than 6.6 feet (2 meters) above the surface of the water. Each home may have several underwater entrances, which must reach below the winter ice. In some areas, especially near large rivers, beavers dig complex dens instead of building lodges. Burrows also may have underwater entrances that lead to the dry areas.

Beavers live in colonies, groups, of four to eight related individuals. Generally, the colony consists of a mated pair of adults and young that are less than two years old. There is usually only a single breeding female in a colony. A single beaver colony sometimes maintains several dams to control water flow. The oil that beavers' glands produce is used to mark their territory. This oil is also used to grease the beaver's fur coat to make it water repellent. Constant grooming and this oil keeps beavers' fur waterproof. It uses its second claw on its hind feet for grooming. Males and females display territorial behavior and will fight trespassing beavers. Communication is through postures, scent marking, tail slapping, and vocalizations, including a whistling or whining call.

In the winter, beavers anchor sticks and logs underwater to feed on during winter. If their pond freezes over, they swim beneath the ice and feed on previously stored food. The senses of hearing, smell, and touch are well developed.


Today's beaver had a mega cousin that lived millions of years ago and was one of the largest rodents ever known. The giant beaver was estimated at 7 to 8 feet (2.1 to 2.4 meters) long and weighed 450 to 700 pounds (204 to 318 kilograms). The giant beaver roamed North American marshlands until about 10,000 years ago, when they disappeared. The giant beaver ate plant materials and spent a lot of time in the water. Unlike today's beavers, giant beavers had ridged cutting teeth and did not build dams. Fossil evidence of the giant beaver ranges from Florida to northern Canada.

Beavers usually mate for life and are monogamous, have one mate. If one of the pair dies, the beaver may then find another mate. Females are dominant. Mating takes place once a year from January to March. Gestation, pregnancy, is 100 to 110 days. Females generally have three to four offspring, called kits, but can have anywhere from one to nine. Offspring generally will nurse for two to three months.


Beavers were once common throughout Europe, Asia, and North America before people began hunting them for their thick, glossy pelts, fur. People used these pelts for coats and hats. People also dined on beaver and used the scents produced by their castor glands for perfume. In Europe, beavers were almost hunted to extinction, no longer existing, by 1860 c.e.

Beavers also can affect the water flow of an ecosystem. By constructing dams and burrowing into banks, they increase the wetland area and overall growth in an area. This helps organisms around the area flourish. For people, beaver's altering of a landscape can be a nuisance. Damming can flood roads, crops, and homes.


Populations of beavers, once extremely low, are gaining in numbers. The Eurasian beaver is listed as Near Threatened, not currently threatened, but could become so, by the World Conservation Union (IUCN).


Physical characteristics: Also commonly called simply the American beaver, the North American beaver weighs from 33 to 75 pounds (15 to 35 kilograms). They have yellowish brown to black fur.

Geographic range: North American beavers are found in Alaska, Canada, throughout the continental United States, and the extreme northern areas of Mexico. These animals are not found in desert regions or southern Florida. They have also been introduced in Finland, Russia, and Argentina.

Habitat: Like all beavers, the North American beaver is aquatic and lives near water in the form of a pond, stream, lake, or river.

Diet: North American beavers eat a variety of plant material. They prefer the cambium, the soft layer between the wood and bark, and leaves of trees such as aspen, birch, aspen, willow, cottonwood, and alder. Their diet also can include aquatic plants, such as pond weeds, water-lilies, and cattails. North American beavers also eat grasses, shrubs, and herbs.

Behavior and reproduction: North American beavers build more extensive dams that alter the landscape than their European counterparts. They are primarily nocturnal but are also frequently active during the day. As the weather gets cooler, beavers stockpile food for the winter by storing it underwater in their lodge or den. When they are able to break through the winter ice, these animals continue to cut down trees. In the northern areas, this underwater food storage may be the beaver's main food supply for months. In the southern areas, beavers are more active year around.

North American beavers and people: North American beavers are part of Native American myths. An Apache myth says that beavers have the magic of the medicine men. Beavers have played an integral role in the development of the United States and Canada. These animals were highly valued for the pelts. The beaver pelt became a unit of currency in colonial times, often leading to fights over trapping territories. The potential for profit, money, encouraged trappers to continue to move west, and settlers soon followed the trappers. Beavers were hunted so intensively throughout North America that the population was reduced by 90 percent by the late twentieth century.

Altering its environment with dams and the creation of ponds benefits the beaver's ecosystem. The ponds help control runoff and help the fish and other organisms flourish. There are over fifty species of animals that live in beaver ponds. The damming of streams raises the level of the water. This causes the tree species that cannot survive in permanently wet soil to die, allowing for the spread of other species. Some people consider these animals a pest. The cutting of trees can damage crops and timber. Their creation of dams can cause flooding that can also harm woodlands and farms.

Conservation status: The IUCN does not list the North American beaver as a threatened species. ∎



Clutton-Brock, Juliet, and Don E. Wilson, eds. Smithsonian Handbooks: Mammals. New York: Dorling Kindersley Publishing, 2002.

Macdonald, David, ed. The Encyclopedia of Mammals. New York: Facts on File Publications, 1984.

Nowak, Ronald M. Walker's Mammals of the World, 5th ed. Baltimore and London: The Johns Hopkins University Press, 1991.


Hair, Marty. "Busy Beavers Work to Build Homes." Knight Ridder/Tribune News Service (February 26, 2004): K5424.

Stewart, Doug. "I'll Be Dammed! Once Nearly Extinct, Beavers are Making a Comeback—Sometimes a Little Too Close to Home." Time (March 29, 2004): 42–43

Wilkinson, Todd. "The Benefits of Beavers." National Parks (January– February 2003): 30–32

Web sites:

Lindsey, Donald W., and Christy Brecht. "American Beaver." Discover Life. (accessed on June 1, 2004).

Myers, P. "Castoridae." Animal Diversity Web. (accessed on June 1, 2004).

"North American Beaver, Canadian Beaver." BBC Science & Nature: Animals. (accessed on June 1, 2004).

"Rodents: Castorida." Animals Online. (accessed on June 1, 2004).