Beaver, semiaquatic mammal noted for the building of dams. One species of beaver occurs in North America, the other in Eurasia. The two species differ chiefly in the shape of the nasal bones and are so much alike that some authorities consider them to be varieties of the same species. They are large rodents; the average adult beaver weighs about 16 kg (about 35 lb), but specimens as heavy as 40 kg (90 lb) have been found, and some extinct beavers were almost bearlike in size. is usually about 76 cm (about 30 in) long and stands less than 30 c ....