Beavers: Castoridae

Beavers are generally nocturnal, active at night. They are active year round. These animals are semiaquatic, living partly on land and partly in water, and are graceful moving about in water. They use their webbed feet and paddle-like tails to swim.

Beavers are hard workers and are considered the engineers of the animal kingdom because of the complex dams and lodges they build. Dams can be extensive, reaching over 10 feet (3 meters) high and stretching hundreds of feet long. A typical dam is 65 to 98 feet (20 to 30 meters) long. Mud and stones may set the foundation, base, for the dam. Brush and poles are added with the butt ends facing upstream, and mud, stones, and soggy vegetation are used as plaster on top of the poles. A dam is built higher than the water level. With maintenance and upkeep, dams are used by several generations of beavers.

Beavers may create multiple homes in their territory. Homes can take the form of a burrow, hole or tunnel, along a bank to make a den, or a wood lodge. Built of sticks and mud, the dome-shaped lodge is generally surrounded by the water backed up by the dam. The lodge may eventually reach more than 6.6 feet (2 meters) above the surface of the water. Each home may have several underwater entrances, which must reach below the winter ice. In some areas, especially near large rivers, beavers dig complex dens instead of building lodges. Burrows also may have underwater entrances that lead to the dry areas.

Beavers live in colonies, groups, of four to eight related individuals. Generally, the colony consists of a mated pair of adults and young that are less than two years old. There is usually only a single breeding female in a colony. A single beaver colony sometimes maintains several dams to control water flow. The oil that beavers' glands produce is used to mark their territory. This oil is also used to grease the beaver's fur coat to make it water repellent. Constant grooming and this oil keeps beavers' fur waterproof. It uses its second claw on its hind feet for grooming. Males and females display territorial behavior and will fight trespassing beavers. Communication is through postures, scent marking, tail slapping, and vocalizations, including a whistling or whining call.

In the winter, beavers anchor sticks and logs underwater to feed on during winter. If their pond freezes over, they swim beneath the ice and feed on previously stored food. The senses of hearing, smell, and touch are well developed.

Beavers usually mate for life and are monogamous, have one mate. If one of the pair dies, the beaver may then find another mate. Females are dominant. Mating takes place once a year from January to March. Gestation, pregnancy, is 100 to 110 days. Females generally have three to four offspring, called kits, but can have anywhere from one to nine. Offspring generally will nurse for two to three months.