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Summit Outside: Busy beavers: nature’s engineers and builders

Special to the Daily/US Fish & Wildlife Service
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Editor’s note: This is the second in a three-part series examining the beaver.The “busy beaver” is a term used for someone who is industrious. The English have a verb “to beaver” which means to work hard and constantly. Beavers are prolific engineers and builders, and prefer to work mostly at night and are amazingly well adapted for what they do. Their specially adapted incisor teeth and powerful lower jaw muscles allow them to chew down trees. Their teeth never stop growing, and their four front teeth are self-sharpening. Beavers carry mud and stones with their fore-paws and timber between their teeth and have been seen to work as a team to carry a large piece of timber. The beavers’ large webbed hind feet are ideal for swimming and their hand-like front paws allow them to manipulate objects with great dexterity. When swimming underwater a protective transparent membrane will cover their eyes, and flaps close to keep water out of their nostrils and ears. Behind their incisors they have inner lips that allow them to carry sticks in their mouths so they can chew while swimming without inhaling water. Beavers stand on their hind legs and cut down trees, while balancing on their tail. A colony of beavers can cut down as many as 200 trees a year. When a beaver cuts down a tree it makes two cuts in the trunk, one above the other and then tears off the piece of wood between the cuts working their way around the trunk until the tree breaks. Then the beaver usually runs into the water while the tree falls, then listens to make sure the noise has attracted no predators before re-emerging. The beaver removes the branches then rolls the trunk into the river. Some of the wood may be stored in deep water, while other pieces are used for construction of a dam or lodge. The beavers start their building by holding large sticks in their mouths and driving them straight into the river bottoms or ponds. Then almost anything that a beaver can find goes into the lodges and dams: sticks, grass, rocks, or even old shoes or whatever else they can find. The beaver lodge is an amazing piece of architecture. The lodges can be 15-40 feet across at the base and 3-8 feet above the water line. First they build their lodges, which look like six-foot heaps of branches and mud. They gnaw two to five tunnels to be used as entrances and exits. Only a small section of the lodge is actually used as a living area. Beavers actually dig out their dens with underwater entrances after they finish building the dams and lodge structures. There are typically two dens or rooms within the lodge, one for drying off after exiting the water, and the second, a drier, inner chamber is where the beaver family actually lives. The inner chamber has one or two vent holes in the roof. If you visit a beaver lodge on a very cold winter day, you may see the beaver’s breath escaping from this chimney-like peak, or even hear the murmurs of the beaver family inside!The inner chamber is lined with grasses or shredded bark which absorbs moisture. Beddings of grasses, reeds and wood chips are changed regularly. Even if the outside temperature is -40 F, the inside of the lodge remains above freezing. The beavers cover their lodges in the fall with fresh mud, which freezes and hardens when the frost sets in. Some of the pile is generally above water and accumulates insulating snow in the winter. Beavers do not hibernate, but store sticks and logs in a pile in their ponds, eating the under bark. This insulation of snow often keeps the water from freezing in and around the food pile, providing a location where beavers can breathe when outside their lodge. In cold climates each fall beavers will stockpile sticks underwater. They live on these sticks because once their pond freezes they will no longer have access to trees on land. Beavers remain inside their lodge all winter except when they swim under the ice to their food cache for a stick to nibble on. The main construction materials of aspen, willow, cottonwood and maple are also preferred foods. The lodge’s underwater entrances make entry nearly impossible for any other animal, however muskrats and otters have been seen sharing beaver lodges. Beavers can make a lodge in a matter of days. There are two main types of lodges, the conical lodge and the bank lodge. The most recognizable type is the conical shaped dwelling surrounded by water. It is made from sticks, mud and rocks. While the large conical lodge in the middle of a large pond might seem to be the main beaver habitation, they actually prefer lodges on the edges of ponds. The conical lodge may be built while water is rising behind a dam. When the pond is large, they might move to and build a bank lodge in the shade and maintain the shade by not cutting the overhanging trees. This bank lodge allows them to build burrows into the earth along the shore of the pond. There is a large bank lodge in “Pirates Cove” and at the edge of Fish Hook Island on Dillon Reservoir.Beavers build a dam to raise the water level, which causes water to back up, form ponds and cover the lodge entrances and exits. Beavers can build a 35-foot long dam in just one week. They also build canals to float building materials that are difficult to haul over land. The dams are built by first putting in place vertical poles, then filling between the poles with a crisscross of horizontally placed branches. They fill in the gaps between the branches with a combination of weeds and mud until the dam traps sufficient water to surround the lodge. The dam is built like a layer cake held together by mud, which is smeared on with their paws and noses. Beavers can rebuild such primary dams overnight. Beavers have a significant effect on their immediate environment. By damming streams and forming shallow ponds, beavers create wetlands. Wetlands control downstream flooding by storing and slowly releasing floodwater and also improve water quality by removing excess nutrients, trapping silt, binding and removing toxic chemicals. These wetlands provide habitat for a diversity of plants, and wildlife, such as deer, bats, otter, herons, waterfowl, songbirds, raptors, salamanders, turtles, frogs and fish. Breckenridge resident Dr. Joanne Stolen is retired from teaching microbiology Rutgers University, and has taught classes at CMC. She is now pursuing a career in art, specializing in nature and many of the animals she writes about. Her work can be seen locally.


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