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Beaver Natural History
Distribution and habitat --Beavers favor habitat containing shrubs and softwood trees, flat terrain, and perennial streams that can be dammed to create ponds. Beaver are generally associated with rivers, ponds, lakes, and areas that can be converted to beaver ponds. The water must be deep enough to provide suitable aquatic habitat under winter ice. Our forests continue to provide excellent beaver habitat, and by now beaver have been fully restored to the Commonwealth. Beaver are both common and abundant throughout most of Massachusetts. They are still uncommon in southeastern Massachusetts, and absent from Cape Cod and the islands.
Description --Beavers are North America's largest native rodents, weighing between 35 and 80 pounds as adults. They can be 2-3 feet in length, with an additional 10-18 inches for the tail. Males and females are similar in size. Beavers typically have a dark brown to reddish brown fur coat. They are muscular animals with large bones, well-developed incisors, and a massive skull that supports strong chewing muscles. In order to move swiftly in water, beavers have large, webbed hind feet and a flat, wide leathery tail. Their front feet are small and dexterous, which allows the beaver to carry construction material such as stones and sticks. An animal often mistaken for the beaver is the much smaller muskrat, which is another aquatic rodent. However, muskrats only weigh 2-3 pounds and have narrow, rat-like tail.
Life History -- Adult beavers have few predators and may live up to 20 years or more. Beavers stay with the same mate for life and breed during winter (January through March). The females give birth to 1-9 kits (4 kits is the average) inside a lodge between April and June. The kits spend most of their time in the lodge, where they can develop and stay relatively safe from predators. Within a week of being born, the kits learn to swim, and by three months of age, the kits are weaned. The kits stay with their parents through two winters before dispersing the following spring. A single family unit is called a colony, and is typically made up of 6-8 individuals that consists of two adults, that year's kits, and the young from the previous year. Beavers are semi-aquatic mammals spending approximately 80% of their time in water. They are unique among mammals in that they alter their habitat to meet their needs, primarily by damming up small rivers and streams to form ponds. These ponds allow beavers to have access to food, protection from terrestrial predators, and shelter in winter. Dams are structures built out of sticks and mud, with the base of the dam consisting of mud and stones. Beavers are constantly on the look-out for leaks or breaches in the dam; they are tipped off by the sound of escaping water. Lodges, like dams, are also built out of sticks and mud, and are usually built in the deepest part of the newly formed pond. They can be 15-40 feet across at the base and 3-8 feet above the water line. The inside of the lodge is kept clean and provides a warm shelter for the beaver during the winter months. Studies have shown that even if the outside temperature is -40° F, the inside of the lodge remains above freezing. The lodge also provides a secure place for the female to give birth to her kits.
Foods -- Beavers are strict vegetarians, also known as herbivores. They feed on a variety of aquatic plants (especially water lilies) and the shoots, twigs, leaves, roots, and bark of woody plants. The bark, particularly the inner bark, of trees and shrubs are important foods, especially in winter. Aspen, birch, alder, and willow are favored food plants.
Beneficial Aspects -- While many people think about beaver only when they are causing problems, it is important to remember the beneficial aspects of beavers. Since European settlement, more than half of the wetlands in the lower 48 states have been lost. By damming streams and forming shallow ponds, beavers create wetlands. These wetlands provide habitat for a tremendous diversity of plants, invertebrates, and wildlife, such as deer, bats, otter, herons, waterfowl, songbirds, raptors, salamanders, turtles, frogs, and fish. But it is not just wildlife that benefits from beaver-created wetlands; people benefit too. Wetlands control downstream flooding by storing and slowly releasing floodwater. They also improve water quality by removing or transforming excess nutrients, trapping silt, binding and removing toxic chemicals, and removing sediment. Flooded areas can also recharge groundwater.
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