The beaver (genus Castor) is a primarily nocturnal, large, semi-aquatic rodent.

Castor includes two extant species, North American Beaver (Castor canadensis) (native to North America) and Eurasian Beaver (Castor fiber) (Eurasia). Beavers are known for building dams, canals, and lodges (homes).

They are the second-largest rodent in the world (after the capybara).

Their colonies create one or more dams to provide still, deep water to protect against predators, and to float food and building material.

The North American beaver population was once more than 60 million, but as of 1988 was 6–12 million.

This population decline is due to extensive hunting for fur, for glands used as medicine and perfume, and because their harvesting of trees and flooding of waterways may interfere with other land uses.

Beavers are known for their natural trait of building dams on rivers and streams, and building their homes (known as "lodges”) in the resulting pond.

Beavers also build canals to float building materials that are difficult to haul over land. They use powerful front teeth to cut trees and other plants that they use both for building and for food.

In the absence of existing ponds, beavers must construct dams before building their lodges. First they place vertical poles, and then fill between the poles with a crisscross of horizontally placed branches.

They fill in the gaps between the branches with a combination of weeds and mud until the dam impounds sufficient water to surround the lodge.

They are known for their alarm signal: when startled or frightened, a swimming beaver will rapidly dive while forcefully slapping the water with its broad tail, easy to hear over great distances above and below water. This serves as a warning to beavers in the area.

Once a beaver has sounded the alarm, nearby beavers will dive and may not reemerge for some time. Beavers are slow on land, but are good swimmers, and can stay under water for as long as 15 minutes.

Beavers are herbivores, and prefer the wood of quaking aspen, cottonwood, willow, alder, birch, maple and cherry trees.

They also eat sedges, pondweed, and water lilies.
Beavers do not hibernate, but store sticks and logs in a pile in their ponds, eating the underbark. Some of the pile is generally above water and accumulates snow in the winter.

This insulation of snow often keeps the water from freezing in and around the food pile, providing a location where beavers can breathe when outside their lodge.

Beavers have webbed hind-feet, and a broad, scaly tail. They have poor eyesight, but keen senses of hearing, smell, and touch.

A beaver’s teeth grow continuously so that they will not be worn down by chewing on wood.

Their four incisors are composed of hard orange enamel on the front and a softer dentin on the back.

The chisel-like ends of incisors are maintained by their self-sharpening wear pattern.

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