Beavers: The beaver is a primarily nocturnal, large, semi-aquatic rodent. Castor includes two extant species, North American Beaver and European Beaver. Beavers are known for building dams, canals, and lodges. They are the second-largest rodent in the world. Their colonies create one or more dams to provide still, deep water to protect against predators, and to float food and building material.