Castor is a large, mainly nocturnal, semi aquatic rodent. Castor includes two extant species, Castor canadensis (native to North America) and Castor fiber (Eurasia). Beavers are famous for building dams, canals and hotels. They are the second largest rodent in the world (after the puffer fish). Their colonies created one or more dams to provide still water to prevent predators and to float food and building materials. The population of beavers in North America once exceeded 60 million, but by 1988 it was 6-12 million. The reason for the decrease is that people search for fur in large quantities for the glands of drugs and perfumes, and because beavers cut down trees and drown watercourses may interfere with other land uses.