Click to Enlarge This young beaver uses its flat tail for balance while it gnaws on sticks. In the water, the tail is used as a rudder and as a warning signal. Beaver dams provide habitat for many wetland species. A large tree is no match for a beaver’s tough teeth, as shown in the skull above. Fresh gnaw marks around a tree trunk is a sure sign that beaver are nearby.   Nature's Furry Wetland Engineers by Joan McKee These four-legged, chisel-toothed semi-aquatic mammals are... If beaver don’t like where they live, they just relandscape. No shovel, backhoe or bulldozer required. Sharp teeth, super lung power and special underwater adaptations give beaver the tools they need to build dams and to make their own deep wetland pools. Underwater is where beaver start their dam-building project. First they find a narrow spot in a small stream. Beaver use their hand-like front paws to place sticks in the opening. Then they scoop up mud and pack it against the sticks. The water rises. They add more layers of sticks, rocks and mud. Water backs up behind the dam. In this quiet deeper pool, the beaver can store the freshly cut branches they will nibble on during the winter. In the deeper water, they dig cozy dens in the river bank. Beaver are semi-aquatic mammals; they can move on both land and water. They venture on land to cut trees and plants for food. Sometimes they travel short distances on land to other streams. On land, they move slowly. Their flat tails help them keep their balance while they cut down trees. Beaver are the largest rodents in North America. Adults usually weigh between 30 to 50 pounds—with some reaching more than 100 pounds. Like all rodents, their front teeth continue to grow throughout their lives. They have to gnaw on trees and sticks to keep them filed down. The chisel-like teeth are sharp enough to cut down large trees. Beaver also use their teeth to carefully remove the bark from the twigs. Underneath the bark is the cambium, the inner growing layer of the tree that beaver eat in the winter. But it’s in the water where these large rodents are the swiftest and where they are safer from predators. They slap their tails against the surface of the water to warn other beaver of danger. If they need to swim away quickly, they use their large webbed hind feet to propel them through the water. Their tail is the rudder. Their eyes, up high on their forehead, let them see above the water while most of their body remains hidden underwater. When they dive, a membrane closes over their eyes. This allows them to see better underwater than above it. Also, valves in the ears and nose close to prevent water from entering. Beaver can gnaw sticks underwater because their lips meet behind their teeth—keeping them from swallowing water. Most other mammals—you included—can only spend about three minutes underwater before they run out of oxygen and must come to the surface to breathe. Beaver, however, can stay underwater for up to 15 minutes. Their lungs hold a large supply of oxygen that their enlarged livers use to send oxygen throughout the body. Without these special abilities, beaver wouldn’t be able to stay underwater long enough to build dams, dig dens and store winter food. Beaver ponds create wetland habitat where fish can lay eggs and tadpoles can grow into frogs. Wetland plants can take root in the shallow waters and help stabilize the soil. If the beaver family decides to dig another den, muskrats, mink or otters may move into their abandoned home. Beaver are the hardworking wetland engineers, but many animals enjoy the benefits of their labor.