Beavers have large, webbed hind feet and a flat, leathery tail that serves as an aid for swimming.
Illustration courtesy of Nancy Haver
They also use their tails for temperature regulation, fat storage, as prop while standing upright, and for communication (beavers slap their tails on the water when alarmed). In the water it is easy to confuse beavers with muskrats (another aquatic mammal common in Massachusetts). Muskrats, weighing only two to three pounds, are much smaller than adult beavers and have narrow, vertically flattened tails (as opposed to the wide horizontally-flattened tails of beavers). Both species have small eyes and ears, but muskrat ears are generally less noticeable than those of beavers.
Distribution
Beavers are distributed throughout most of North America from northern Mexico to northern Canada. They are both common and abundant throughout most of Massachusetts. They are absent from southeastern Massachusetts, Cape Cod, and the Islands.
An important requirement for beavers is water deep enough to provide aquatic habitat beneath winter ice. As a result they are generally associated with rivers, ponds and lakes, or areas that can be converted to beaver ponds. Although beavers are found in areas with steep slopes, they generally prefer fertile valleys with flat terrain and perennial streams that can be dammed to create ponds. These areas also produce an abundance of food preferred by beavers.
Food
Beavers do not eat fish; they are strict vegetarians. As such, they feed on a variety of aquatic plants (especially water lilies) and the shoots, twigs, leaves, roots, and bark of woody plants. In particular, the bark and inner bark of trees and shrubs are important foods, expecially in winter. Aspen, birch, alder, and willow are favored food plants.
BEAVER MEAL OF BARK - YUM-YUM!
Trees and shrubs are felled by beavers to gain access to twigs, leaves, and bark. Bark and leaves may be stripped where they fall or transported back to the safety of water.
TREES FELLED BY BEAVERS ALONG THE CONNECTICUT RIVER
Well-used beaver trails typically lead from a beaver pond to upland stands of important food trees. Trails near the pond often fill with water forming canals that are used by beavers to float sticks and logs from uplands to the pond. As winter approaches, branches are stockpiled on the pond bottom near the lodge. Beavers rely on this cache of food to see them through until spring. Once stripped of leaves and bark, branches and logs are often used as construction material for dams or lodges.
Life Cycle
Beavers stay with the same mate for life. They mate in winter (January to March) and females give birth in a lodge sometime between April and June. A single litter each year usually contains four kits (but may have as many as nine). Young kits spend most of their time in the lodge where they are relatively safe from predators. Although they are weaned by three months of age, young beavers will stay with their parents through two winters before dispersing the following spring.
A single family unit of beavers is typically made up of two adults, that year's kits, and young from the previous year. Such a group is called a colony and usually contains six to eight individuals in areas where harvest pressure is low. Most beavers become sexually mature in their third year at which time they leave, or are driven out by the parents, to seek mates and territories of their own. Adult beavers have few predators, and they may live up to twenty years or more in the wild.Dam and Lodge Building
Beavers are renowned for ther ability to modify their surroundings to meet their needs. They accomplish this by damming up small rivers and streams to form ponds. These ponds are areas of still, deep water that provide access to food, protection from terrestrial predators and shelter in winter.
Dams are impressive structures made of sticks and mud. The base of a dam is made up of mud and stones. Upon this, beavers pile branches and sticks, oriented with the butt ends upstream. Mud, stones and aquatic vegetation are used as plaster. Despite what you may have learned from cartoons, beavers do not pack the mud down with their tails. Instead, they scoop it up with their forepaws and apply it to the dam with their feet and snouts.
Although most dams are less than 100 feet long, some have been recorded at over 1,500 feet in length. Several dams may be constructed close together along a brook, creating a series of terraces with standing water. Beavers are constantly on the look-out for leaks or breaches in their dams. Tipped off by the sound of escaping water, beavers will act quickly to plug any leak with mud sticks.
Ponds created by beavers provide habitat for may of their favorite food plants (water lilies, cottonwood, willow, and alder). Water also means security for these agile swimmers because most of their predators live on land.
Within ponds created by dams, beavers construct lodges out of sticks and mud. Lodges may be fifteen to forty feet across at the base and protrude three to six feet above the water. Within each lodge, a single internal chamber is situated above the water line and is lined with dry plant material. A vent leading fromt his chamber to the surface of the lodge provides to air circulation. One or more underwater entrances offer the only access into the lodge. Thick walls of sticks and mud provide substantial protection from predators and the elements.
Illustration courtesy of Nancy Haver
In winter, the pond offers sufficient aquatic habitat beneath the ice that the beavers are active year-round. Once the pond freezes over, beavers are confined to the pond until ice-out in the spring. Tree branches, cached on the bottom of the pond, provide winter food and are brought into the lodge to be eaten.
Beavers will also use bank dens for shelter, usually on rivers or lakes.
Illustration courtesy of Nancy Haver
Underwater entrances lead to tunnels that extend back as far as thirty feet, ending in a den situated above water. Over time many of these bank dens take on the appearance of lodges as sticks and mud are piled around the entrance.
Eventually these ponds are abondoned, either when preferred food plants become scarce or when silt accumulation makes them too shallow. Lacking repairs, dams break and the ponds drain. In the nutrient rich silt, herbaceous plants flourish, forming beaver meadows. Over time, shrubs and trees eventually come to dominate these areas, setting the stage for the beavers' return.
This article is excerpted from Beavers in Massachusetts by Scott Jackson and Thomas Decker, University of Massachusetts Cooperative Extension System, Massachusetts Division of Fisheries and Wildlife.
