American beaver

Class: Mammalia
Order: Rodentia
Family: Castoridae
Genus and Species: Castor canadensis
  • beaver standing on hind legs. Its paws are oddly hand-like and its large yellow teeth are visible in its jaw
  • beaver coming out of pond. Its fur is brown
  • A beaver walking on the ground
  • beaver gnawing. The light is striking it so that its head is a pale tawny color and not the usual dark brown
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American beaver

The largest rodent in North America, beavers are the rare species that significantly modifies their environment—by building dams—providing aquatic habitats for dozens of other species.

Physical Description

Beavers have a yellowish brown to almost black coat; a broad, flat scaly tail; large orange incisors; webbed rear feet and digitated front paws.

Beavers, having dual habitats, move between aquatic and terrestrial environments. Their stocky bodies have large volume to surface area ratios, giving them great ability to conserve warmth. On land, beavers are extremely awkward, making them vulnerable to attack from predators. However, in water, beavers swim up to 6 miles per hour (10 kilometer per hour) and, because of their over-sized lungs, can stay submerged for up to 15 minutes while traveling over one half mile.

The incisors, the long, dark orange visible teeth, are self-sharpening and can cleave a willow the size of a person's finger in a single bite. Incisors grow throughout a beaver's life and, through daily use, are continuously worn down. If the incisors become too long, they prevent the mouth from closing enough for the grinding molars to meet, possibly causing starvation. Beavers seem to have a smile upon their face because of lips that appear too small. The lips do not close over the incisors, but are pulled tightly behind them. This is an important adaptation to the aquatic side of their life, permitting them to work underwater without their mouth filling with water.

Their nose is equipped with a pair of slap valves that close when beavers dive, swim, or work underwater. The skin around the nose is bare, enhancing a beaver's tactile sense and gathering more information than if the nose were covered with fur.

Whiskers help detect objects near the sides of a beaver's face and head, which is especially helpful in narrow passageways and dark water.

The eyes have a thin, transparent membrane, called a nictitating membrane, which is pulled over the eyeball for underwater work. A beaver's sight is good only for short distances and at close range.

The ears are external, small and rounded and have valves that close for underwater work. The beaver's auditory sense is well developed which is important for hearing another beaver's danger signal—a tail slap on the surface of the water.

The front feet are small, dexterous, and well adapted to work on land. Beavers walk on five digits, grasp sticks with their front paws, and have well developed digging claws. They hold their food with their front paws and eat corn-on-the-cob style. The hind feet are quite different and are webbed for swimming. The hind feet are larger than the front and totally void of fur except on the dorsal surface. The hind feet also have a preening toe, the second from the inside, with a unique double toenail. The preening toe serves as a comb and prevents the fur from matting while maintaining waterproofing and insulating properties. These flexible toes are also used to remove burrs and parasites from the fur. Beavers are meticulous in keeping their fine, soft fur waterproofed.

The tail varies in shape from short and broad to long and narrow and is an individual and family trait. In general, it is about two inches (5 cm) thick at the base and about 0.24 inches (0.6 cm) thick at the tip. It is practically hairless and covered with black scales. There is a sharp demarcation between the fur and the scales, the fur remaining at full length and density right up to this line. The tail is used as a rudder in swimming, as a balance prop while working on land and to signal danger when slapped on the water. Beavers will also store fat in their tails, eating more in the fall so they can survive off the fat stored in their tails through winter if food is not available. The beaver's vertebrae continue into and almost to the end of the tail.

Size

North American beavers are the largest rodents in North America and the second largest in the world (capybaras of South America being the heaviest). They weigh between 35 and 65 pounds (16 to 30 kilograms), with 110 pounds (50 kilograms) the heaviest on record. Beavers are 3 to 4 feet (1 to 1.2 meters) long and one to one and a half feet (0.3 to 0.5 meters) tall.

Native Habitat

Beavers are found throughout North America with the exception of the California and Nevada deserts and parts of Utah and Arizona. They live in ponds, lakes, rivers, marshes, streams and adjacent wetland areas.

Beavers are one of the few animals that actually change their habitat; they build watertight dams of sticks woven with reeds, branches, and saplings, which are caulked with mud. Dams reduce stream erosion by forming slow-moving ponds. These ponds serve as habitat for a wide range of small aquatic life as well as providing water and food for much larger animals. By building dams, beavers create new habitats that can support an incredibly diverse biological community.

Beavers also build dome-like lodges that rise 6.5 feet (2 meters) or more and can reach widths of 39 feet (12 meters). A lodge can have one or more underwater entrances and living quarters are located in the top of the lodge above the water line. Often built away from the shore, these lodges form islands that can only be entered from underwater. The lodge chamber may be four feet (1.2 meters) wide and two feet (0.6 meters) high, insulated by walls one-third of a meter thick and ventilated by a small air hole in the roof called a "chimney." Typically, the floor is covered in wood shavings to absorb excess moisture and provide bedding. Beavers spend the summer and fall building dams and gathering and storing food for the winter.

Communication

Beavers employ a variety of communication signals. One method is slapping their tails on the water to communicate an alarm signal. Usually performed by an adult, it alerts others in the area to seek refuge in deep water and may also frighten the predator.

Beavers also communicate with beavers outside the family unit by depositing scents around the edges of their territory. Beavers are unique among rodents in that they actually build scent mounds which are heaps of mud, sticks, and grass up to a third of a meter high and about a meter wide on which they deposit scents from their anal glands. Within the lodge, beavers employ various vocalizations (the voice box is quite rudimentary though) and postures to communicate with family members. At the Zoo, they have been heard occasionally hissing if they are unhappy.

Beavers have important castor and oil glands near the anus. Castor, a very pungent, thick liquid, is produced for scent marking and is vital in leaving a long lasting odor. The oil glands produce the oil used to waterproof a beaver's fur. The oil is slightly different between the sexes and is used in reproductive communication.

Food/Eating Habits

Beavers are herbivores, eating leaves, woody stems and aquatic plants. Their chief building materials are also their preferred foods: poplar, aspen, willow, birch and maple. In cold climates, they spend the winter inside their lodge chamber, feeding on branches they have stored on the muddy pond floor as a winter food supply. The water acts as a refrigerator, keeping the stems cold and preserving the nutritional value.

At the Smithsonian's National Zoo, they are fed once a day a diet of mixed vegetables, rodent chow and a healthy supply of wood.

Social Structure

Beavers form strong family bonds. They are social animals and each group is made up of one breeding pair, the kits of the year, and the surviving offspring from the previous year, called yearlings. There also may be one or more sub-adults, 2 years or older, of either sex from previous breeding seasons which generally do not breed. In winter, they live together in their lodge and share food from the common larder (stored food supply). Their family life is exceptionally stable and is based on a hierarchy in which adults dominate yearlings and yearlings dominate kits. Physical aggression is rare and vocalizations, gestures and postures are used within the lodge to communicate issues of dominance status within the group.

Reproduction and Development

Beavers pair for life. They breed in the water from January to late February and females give birth in spring to kits that weigh about one pound (0.5 kg). Kits are born with their eyes open and are completely covered with fur. They take to the water inside the lodge within one half hour after birth. They are skillful swimmers within a week but are too buoyant to dive. Kits typically stay close to their mother in the lodge for the first few weeks, nursing frequently and gaining considerable weight. Females have four nipples and sometimes sit upright to nurse. There is evidence of teat sharing among kits, which may explain the high survival rate of all members of a large litter. Kits nurse for about six weeks and all members of the family share in bringing solid food to them. On land, mothers often carry kits on their broad tails, sometimes even walking erect and holding them in their paws. In the water, kits may rest upon their mother's back. The young remain with their parents for two years, helping with lodge maintenance and raising kits until they are, usually, driven away just before the birth of a new litter.

Sleep Habits

Beavers are mainly nocturnal throughout their range. However, in regions where ponds freeze over throughout the winter season, beavers may stay in their lodges or under the ice using their fat reserves and feeding off the cache they've gathered. In the lodge and underwater, light levels remain constant and low during the 24-hour day, so that sunrise and sunset are not apparent. In the absence of solar "cues," activity is not synchronized with the solar day. The circadian rhythm, or regular day cycle, breaks down, and beaver "days" become longer, varying in length from 26 to 29 hours. At the Zoo, beavers usually wake up in the early evening, around 4pm

Lifespan

North American beavers typically live 10 to 15 years. The oldest on record is 30 years in human care.

Beavers are a classic story of American conservation at its worst and at its best. During the early 19th Century, the single most valuable commodity in most of North America was beaver pelt. Much of the exploration of the New World developed from the fur trade and wars were even fought over access to beaver trapping areas, including the French and Indian War. Beaver fur was in constant demand for robes, coats, clothing trim, and top hats (called beavers). Beaver fur was fashionable in European capitals and some of America's great financial empires and real estate holdings were founded on the profits of beaver fur. With trapping totally unregulated well into the 20th Century, beavers all but disappeared from most of their original range.

Now protected from over exploitation, beavers have reestablished themselves over most of the continent, and have even become an agricultural pest in some regions. Beaver dams can change a farmer's stream into mud or swamp. Dams may also flood stands of commercial timber, highways, and croplands and may block the upstream run of spawning salmon. However, beaver dams are also important. They maintain an ecological balance by controlling run off, erosion and floods and keep the water table high enough to support rich vegetation. Beavers have also supplied people with information on swimming techniques, dam building, engineering, natural fortification, and flood control.

Beavers are still harvested for their fur and meat, but are actively managed throughout most of their range. The beaver's main predator, besides humans, is the wolf. Other predators include coyotes, wolverines, bears, foxes and lynxes.

Three North American beavers live on American Trail—two males named Chipper and Birch, and a female named Chloe.