Beavers—famously industrious aquatic rodents who eat the bark of maple, linden, birch, and poplar trees—live gregariously in colonies that thrived for centuries before the animals’ habitats were radically altered by humankind. Master builders, beavers protect the entrance to their burrows in the banks of streams, and create food storage areas by constructing remarkably sturdy dams out of trees they cut down, some as wide as eighteen inches in diameter—great, tangled, well-anchored, sloping heaps that hold back large ponds, where they then erect impregnable lodges. Beaver dams and ponds can help prevent spring ﬂoods, and they preserve water during dry spells. Systematically hunted for their fur (beaver skins once served as currency in northern North America), beavers were vanquished from most areas but are now being reintroduced in the Northeast. Oregon, where the animal was hunted to near extinction, has retained its nickname, the Beaver State.