Tag Archives: Beavers in Alaska

The Common or True Beaver (Castor canadensis)

, True Beaver, Beavers in Alaska, Beavers on Kodiak Island, Wildlife of Kodiak Island, Mammals introduced to Kodiak Island

The common beaver or true beaver (Castor canadensis) is native to North America and is found in most areas from the Arctic to northern Mexico. C. canadensis is absent from Southern California, Nevada, most of Florida, and parts of Alaska. Beavers are not native to Kodiak Island but were introduced to Kodiak in 1925.


The true beaver is the largest rodent in North America and the second-largest rodent in the world. The South American capybara is the largest. Beavers continue to grow  throughout their lives and may reach a length of 3 to 4 feet (0.9-1.2 m), including the tail. Most adults weigh between 40 and 70 lbs. (17-32 kg), but they can weigh as much as 100 lbs. (45 kg).

A beaver has a heavy, thick, chestnut-brown coat and a warm, soft underfur that keeps it comfortable in all temperatures on land and in the water. This warm, soft fur was in such high demand by humans in the late 1800s and early 1900s that by 1930, beavers had been trapped nearly to extinction in many areas.

The beaver’s scientific name is derived from castoreum, a strong smelling, oily substance secreted by castor glands at the base its tail. A beaver uses its feet to comb this oil through its fur to provide a waterproof barrier that keep its skin dry underwater. It also has a thick layer of fat beneath its skin to insulate it in cold water.

A beaver’s body is adapted to a life spent mostly in the water. In addition to the castoreum which keeps its skin dry underwater, it has nictitating membranes that protect its eyes underwater and nose and ear valves that close when it submerges. It can cut and carry wood underwater without getting water in its mouth by tightly closing its loose lips behind its protruding front teeth. A beaver has large, webbed feet and a broad, black tail that is approximately 10 inches (25 cm) long and 6 inches (15 cm) wide that it uses as a rudder when swimming. It also uses its tail as a warning signal when it slaps it against the water. When it stands on its hind legs to cut down a tree, the tail serves as a fifth leg and helps to balance the animal. A beaver can stay underwater for fifteen minutes, and it can swim up to 5 mph (8 km/h). It propels itself through the water with its webbed hind feet while holding its front feet against its body. When swimming at the surface, only its head is visible above water. On land, a beaver moves with a waddling gait and can run between 6 and 8 mph (9.7-12.9 km/h).


Beavers are second only to humans in their ability to alter the environment for their own needs. Beavers spend part of their time on land of the rest of their time in the water. They must build their dens in an area that provides 2 to 3 feet (0.6-0.9 m) of water year-round to ensure that ice does not block the underwater entrance to their lodge or den in the winter. In areas where the natural water level is less than 2 ft. (0.6 m), beavers construct dams to increase the height of the water level. The height of an average dam is 6 ft. (1.8 m), producing an average water depth of 4 to 6 ft. (1.2 to 1.8 m). Beavers construct the dam by first diverting the stream to reduce the flow of water at their building site. Next, they drive branches and logs into the mud of the stream bed for the base. They then use whatever they can find, including sticks, bark, rocks, mud, and grasses to build the superstructure of the dam. They even include spillways and passageways in the dam structure to allow excess water to drain off the dam without damaging it. A beaver can move building materials that weigh as much as it does, and it can cut down a tree with a 6-inch-diameter (15.2 cm) trunk in 20 minutes. It may work alone to build a dam, or several family members may work together on the project.


Beavers build two different styles of dens, depending on the current and water level of the river. Both types of dens have at least one exit to deep water that will remain free of ice in the winter. In a swiftly flowing river with a year-round water level deep enough so that a dam is not required, beavers build simple bank dens into the side of the riverbank. They cover the top of a bank den with sticks, mud, and other debris. Bank dens usually have several entry tunnels. The interior of a bank den has one chamber that is approximately 2 ft. (.61 m) wide by 3 ft. (.9 m) long by 3 ft. (.9 m) high.

The second type of beaver den is called a “lodge.” A lodge is dome-shaped and is about 10 ft. (3 m) high by 19.6 ft. (6 m) wide at the base. Beavers build lodges in slow-moving water and use the same construction techniques they employ when building a dam. The lodge consists of an entry tunnel and two chambers. The entry tunnel is below the water level, and the floor of the first chamber is a few inches above the water line. The first chamber is used as an eating area and a place to dry off after getting out of the water. The floor of the second chamber is above the first chamber, and this area is used for sleeping and caring for the kits. Beavers cover the floor of this chamber with shredded wood or vegetation. A single family group, including an adult pair, this year’s kits, and the young from the previous year, occupy a lodge. The older offspring assist their parents in maintaining the lodge and the dam.

Beavers use the same lodge for many years, and since they add material to the lodge every year, the lodge grows over time. Beavers do no hibernate in the winter but stay active in their lodges. Since the only entrance to the lodge is underwater, they are frozen in the lodge or under the surface of the water if the river develops an ice layer. To prepare for winter, they store enough food to make it until spring. The walls of the lodge freeze in the winter, helping to insulate the interior and protecting them from predators.


Beavers do not eat wood, they eat the cambium, a soft tissue near the surface of the tree from which new wood and bark grow. They also eat aquatic plants and roots and grasses. During the winter, they mainly depend on woody material such and shrubs and branches, which they plant underwater close to the lodge entrance. When they deplete the food supply near their den, they must forage further from home for food, and this increases their risk from predators. When they no longer have a good supply of food near their den, they migrate to a new home.


Beavers are mostly nocturnal. They sleep during the day and forage for food and build their dams and dens at night. They communicate mainly by posturing and scent marking. They use their paddle-like tail to slap the water as a warning and to communicate other emotions. They build small, dome-shaped mounds and rub castoreum, the oily substance secreted by their castor glands, on the mounds to mark their territory. Young beavers make a sound like a quacking duck, and adult beavers sometimes grunt when they work.


There are no separate names for male and female beavers, but babies are called kits. Beavers are monogamous and mate for life, but if one of the mates dies, its partner may find another mate. They breed in January and February, and females give birth to between one and nine (two to four are average) kits between late April to June, after a gestation of 105 days. Kits are covered with a soft fur at birth. They can swim when they are four-days-old, and they can dive by the time they are two-months-old. Young beavers live with their parents until they are two-years-old, and they reach adulthood in their second winter. Beavers survive in the wild for approximately 10 to 12 years, but in captivity, they have been known to live as long as 19 years.

Impact on Environment

When beavers colonize an area, they can impact the environment in both positive and negative ways. When they build a dam on a river or stream, they create ponds by flooding the area near the dam, and this increases the water table in the area and recharges aquifers. Beaver dams can also create wetlands where there were none before, providing a habitat for birds, invertebrates, fish, and mammals.   On the negative side, beaver dams can cause flooding of crops and roads, damage dams and roads built by humans, harm forests and landscaping, negatively impact plants and animals, dam irrigation canals and prevent necessary water from flowing to farmlands, and spread diseases such as Giardia to the drinking water supply. In Alaska, beaver dams sometimes cause floods during the spring ice break-up on rivers and streams, and dams that block a stream can disrupt the salmon migrating to spawn in that stream.