Documentary: Educational: Leave It To Beavers. Animal Engineers The fascinating story of beavers in North America - their history, their near extinction, and their current comeback, as a growing number of scientists, conservationists and grass-roots environmentalists have come to regard beavers as overlooked tools when it comes to reversing the disastrous effects of global warming and world-wide water shortages. Once valued for their fur or hunted as pests, these industrious rodents are seen in a new light through the eyes of this novel assembly of beaver enthusiasts and "employers" who reveal the ways in which the presence of beavers can transform and revive landscapes. Using their skills as natural builders and brilliant hydro-engineers, beavers are being recruited to accomplish everything from re-establishing water sources in bone-dry deserts to supporting whole communities of wildlife drawn to the revitalizing aquatic ecosystems their ponds provide. Beavers are large, semiaquatic rodents in the genus Castor native to the temperate Northern Hemisphere. There are two extant species: the North American beaver (Castor canadensis) and the Eurasian beaver (C. fiber). Beavers are the second-largest living rodents after the capybaras. Beavers build dams and lodges using tree branches, vegetation, rocks and mud; they chew down trees for building material. Dams impound water and lodges serve as shelters. Their infrastructure creates wetlands used by many other species, and because of their effect on other organisms in the ecosystem, they are considered a keystone species. Adult males and females live in monogamous pairs with their offspring. When they are old enough, the young will help their parents repair dams and lodges and may also help raise newly born offspring. Beavers hold territories and mark them using scent mounds made of mud, debris and castoreum, a liquid substance excreted through the beaver's castor sacs. Beavers can also recognize their kin by their anal gland secretions and are more likely to tolerate them as neighbors. Historically, beavers have been hunted for their fur, meat and castoreum. Castoreum has been used in medicine, perfume and food flavoring, while beaver pelts have been a major driver of the fur trade.