Beaver is primarily a nocturnal and semi-aquatic rodent. It belongs to the genus Castor and is presently represented by two species commonly known as the North American Beaver and the European Beaver. They are known for building dams, canals and lodges. They are the second largest rodents known all over the world. They live in colonies and make dams which are deep in water and protect them from the predators. The population of the North American beaver has declined rapidly because of excessive hunting. They are killed for fur and the glands are used as a source of medicine and perfumes. They are known for their natural trait of making dams in the ponds in which they live. They have very sharp and powerful front teeth which are used for cutting trees for building their homes as well as for feeding. They are known for their alarm signals. When frightened the beaver dives rapidly and slaps water with its broad tail. This sound is audible from large distances both above and below water. This behaviour acts as a warning signal for other beaver present inside water. Once warned by the alarm call the other beavers dive into water and do not reemerge for some time. They walk slowly on land but are effective swimmers and can remain under water for about 15 minutes.

They are herbivorous in habit and prefer to feed on the wood of birch, maple, cherry, cottonwood, willow and alder trees. They also feed on sedges, pondweed and water lilies. They do not undergo hibernation but are known to store logs in a pile under water and eat the under bark. Some piles also float over water and may be covered with snow in the winter season. The insulation of the snow prevents the surrounding water from freezing in and around the log and also provides fresh air to the beavers to breathe. The fossil remains of beavers have been obtained from the peat and other superficial deposits of Britain and the continent of Europe. They have webbed hind feet and broad, scaly tail. The eye sight is somewhat poorly developed but the senses of hearing, smell and touch is very well developed. Their teeth grow continuously so they feed on wood regularly. They bear 4 incisors which are covered with a hard layer of orange coloured enamel in front and a soft dentine at the back. Incisors are chisel-like.

Beavers grow continuously throughout their lives. Adults may attain a weight of about 25 kg. Females are larger than males a feature uncommon among the mammals. They live up to 24 years. Their name is of Proto-Indo-European origin. They are very close to the squirrels bearing resemblance with the lower jaw and the skull. There are two species of beaver known at present both are although similar but there are certain features which distinguish them. The European beavers are somewhat bigger with larger, less rounded heads, larger and narrower muzzles. The tails are less oval and shorter shin bones are present which serve for efficient bipedal locomotion. They have longer nasal bones. The foramen magnum is rounded in the European beavers but it is triangular in the North American species. The anal glands of the European beaver are larger and thin walled enclosing great internal volume as compared to the North American ones. The fur colour is also different in both the species. 66% of all the European beavers have pale brown fur, 20% have reddish fur while 8% have brown coats and the rest 4% have black fur. Among the North American one 50% beavers are provided with pale brown fur, 25% with reddish fur, 20% with brown and 6% have black coats.

Both the species are not genetically compatible. North American beavers have 40 chromosomes while the European beavers possess 48 chromosomes. North American beavers are also known as Canadian beavers. They feed on water lily. Beavers are common hosts of Giardia lamblia which causes giardiasis. The beaver testicles and castoreum, a bitter secretion present in the castor glands of male or female beaver is used commercially for making medicine. Castoreum is also used in perfume preparation. They are national animals of Canada. They have entered the category of endangered because of habitat destruction and mass killing.