A large herbivorous rodent adapted for swimming with webbed feet and a scaly paddle-like tail. It lives in family groups and alters the flow of water in its range by digging canals and building dams.

Subspecies
24 subspecies are recognized.

Life span
10-15 years.

Statistics
80-120cm long with a 25-50cm long tail, standing 30-60cm at the shoulder and weighing 11-30kg.

Physical description
Beavers are large rodents with a broad, square muzzle, short limbs and a long, flattened scaly tail forming a paddle. They have a thick reddish brown coat that can vary from yellowish to almost black and a thick greyish undercoat. Their hind feet are webbed for swimming and they have a split second claw, which they use for grooming.

Distribution
North America from Alaska east to Labrador and south as far as North Florida and Mexico. They have been introduced to Argentina, Asia and Europe (where they cannot interbreed with the Eurasian beaver due to a different number of chromosomes).

Habitat
Wetland areas with woodland.

Diet
Non-woody plants in spring and summer, changing to trees and shrubs in the autumn and winter.

Behaviour
Although they move with an ungainly waddle on land, beavers can move surprisingly fast, and in water they are extremely good swimmers, paddling with their webbed hind feet and movements of the tail, which is also used for steering. They live in close family units of 4-8 containing an adult pair, which mate for life, and young from previous years (called kits).

The family live in a lodge, which is a large conical pile of branches in the middle of a lake averaging 3-4m diameter. There is an underwater entrance leading up into a dry living chamber 1-2m across, and sometimes an additional dining area nearer the water. They also dig burrow systems within their territory, which is often their primary residence. They then alter the water system around their home range, by digging canals to enable them to swim between feeding areas, and damming streams to provide large lakes that surround the lodge and provide safety from predators. When the pond silts up the beaver colony are forced to move on, and the area eventually becomes a water meadow. Most activity is at night, although during the winter when they remain in the lodge and active under the ice feeding on stored logs, they settle into a 29-hour day rhythmn.

Reproduction
Mating takes place in the winter - usually in water, but also in the lodge or burrow. After 105-107 days gestation, 2-3 kits (but up to 8) are born in the late spring inside the lodge. The kits' dense fur makes them very buoyant and so although they can swim within a few hours of birth, they are unable to swim down through the lodge's entrance. Weaning occurs after 6-8 weeks, although they start taking solid food before this time. They usually stay with the family, learning how to construct dams and lodges for two years before dispersing.

Conservation status
After extensive hunting for their meat, fur and oil - called castoreum (which is used as a medicine similar to aspirin and as a base for perfume) - beaver numbers declined. However, they are now making a come-back as the market for their products has declined, and North American beavers have been introduced to many new areas, particularly in Europe. Their effect on the landscape, however, does lead to conflict with humans.