GRE Reading Comprehension: JiJing 352-GRE阅读机经352篇 - 5Y55MMXLK717NU5NK

Early naturalists believed two species of beaver lived in North America: dam beavers and bank beavers. The bank species was thought to resemble the muskrat in behavior, living in burrows or lodges and unable to build dams. In fact, dams are primarily a strategy for dealing with annual variations in water levels. If water levels fall in summer, as they do in most of North America, then beavers lodge entrances may be exposed. With stabilized water levels, their homes are much safer. Along deep rivers, where bank beavers are found, this problem seldom arises. But these beavers do know how to build dams, and do so if the need arises, as may occur if they are forced to relocate after felling and consuming all nearby trees.