Beaver

Beavers are semiaquatic mammals who live on ponds, lakes, or streams. Most live in groups called colonies, which together erect a home, called a lodge, constructed of mud and sticks. So-called bank beavers inhabit cavities in the sides of stream banks rather than lodges. Abeaver colony consists of a family comprising the breeding pair and their offspring from the previous two years. Typical colonies consist of four to eight individual beavers. Beavers appear to be monogamous, and females are dominant over males and juveniles, at least during parts of the year. Members of the colony work together to maintain the lodge, build dams, and defend their territory against other beavers. Beaver colonies occur in typical densities of 0.4 to 0.8 colonies per square kilometer, though they can occur at higher densities when suitable habitat is plentiful.

The Beaver Life Cycle
Beavers have a single breeding season, in winter, during which mating takes place. Females experience a period of estrus that lasts for about two weeks, during which they are receptive for ten to twelve hours. Gestation lasts for about 110 days. Litter sizes range from zero to nine, with an average of two to four young born each year. Beavers are born in a relatively well-developed state; their eyes are open and they are fully furred. Young beavers nurse from their mother for two or three months, though beavers begin eating some solid food at only a few weeks of age. Beavers reach sexual maturity at 1.5 to 2 years, at which time they are forced by their parents to leave the colony. Beavers are herbivorous and eat the woody parts of trees and shrubs. Favorite foods include aspen and alder trees. Beavers will chew down large trees, not to gain access to the large trunks, which are impossible for beavers to move, but rather to gain access to smaller branches growing at the top of the tree, which the beavers then eat. Active year round, beavers must plan ahead to provide food for themselves during the winter. During the warmer months, the beavers will group branches together in the water near their lodges. During winter, when the pond or stream ices over, beavers swim under the ice to gain access to these stick piles as a source of food. Abeaver lodge is constructed with an underwater entrance. Thus, during the winter, beavers can enter and exit the lodge without ever coming up above the ice. Beavers are known as important habitat modifiers due to their dam construction behavior. Beavers chew down trees and use the tree trunks and their associated branches to build dams along streams. Dams cause water pooling and slow the water flow behind the dam, increasing water depth and creating suitable areas for building lodges. Furthermore, beaver dams cause localized flooding that can dramatically alter landscapes. Once beavers have used all of the surrounding vegetation, either in dam construction or as food, they will relocate to a new area. The abandoned beaver pond eventually fills in to become a "beaver meadow" which supports a variety of plants that would not otherwise occur in that location.

Physical Characteristics of Beavers
Beavers are one of the largest rodents, second in size only to the capybara. The two species in the family Castoridae are similar in many aspects. Both species show little sexual dimorphism and a normal range in body mass from twelve to twenty-five kilograms. Both male and female beavers have castor glands, which produce a substance called castoreum. Beavers deposit castoreum on piles of mud outlining the boundaries of their territories, in essence creating a message board for other beavers that pass by. Beavers also have broad, flat tails which are used not so much for packing mud, as is commonly believed, but instead as a means of communication. Beavers smack the water surface with their tails, producing a loud noise that can easily be heard by other animals. Beaver tails are also used for directional control while swimming. Beavers are dark brownin color, and their thick fur coats contain a dense underfur that traps air and thus insulates the beaver. Long guard hairs provide a protective layer over the underfur. Their webbed hind feet function as excellent paddles when swimming. Forefeet are not webbed and thus are more capable of manipulating objects. Split claws on the first and second digit of the forefeet are used to groom the dense fur, thus maintaining its water repelling and insulating properties. In North America, beavers have made a dramatic comeback in numbers after being hunted almost to extinction in the late nineteenth and early twentieth centuries. In Europe, beavers have fared less well and in some areas are listed as threatened or endangered, due mostly to destruction of suitable habitat.