The beaver is a primarily nocturnal, large, semi-aquatic rodent. Castor includes two extant species, the North American beaver (Castor canadensis) and Eurasian beaver (Castor fiber) . Beavers are known for building dams, canals, and lodges. 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. Source: Wikipedia.