The beaver (Castor canadensis,) is the largest North American rodent. Most adults weigh from 35 to 50 pounds (15.8 to 22.5 kg), with some occasionally reaching 70 to 85 pounds (31.5 to 38.3 kg). Individuals have been known to reach over 100 pounds (45 kg). The beaver is a stocky rodent adapted for aquatic environments. Many of the beaver’s features enable it to remain submerged for long periods of time.

It has a valvular nose and ears, and lips that close behind the four large incisor teeth. Each of the four feet have five digits, with the hind feet webbed between digits and a split second claw on each hind foot. The front feet are small in comparison to the hind feet .The under fur is dense and generally gray in color, whereas the guard hair is long, coarse and ranging in color from yellowish brown to black, with reddish brown the most common coloration.

The prominent tail is flattened dorsoventrally, scaled, and almost hairless. It is used as a prop while the beaver is sitting upright and for a rudder when swimming. Beavers also use their tail to warn others of danger by abruptly slapping the surface of the water. The beaver’s large front (incisor) teeth, bright orange on the front, grow continuously throughout its life. These incisors are beveled so that they are continuously sharpened as the beaver gnaws and chews while feeding, girdling, and cutting trees. The only way to externally distinguish the sex of a beaver, unless the female is lactating, is to feel for the presence of a baculum (a bone in the penis) in males and its absence in females.

Range

Beavers are found throughout North America, except for the arctic tundra, most of peninsular Florida, and the southwestern desert areas. The species may be locally abundant wherever aquatic habitats are found.

Habitat

Beaver habitat is almost anywhere there is a year-round source of water, such as streams, lakes, farm ponds, swamps, wetland areas, roadside ditches, drainage ditches, canals, mine pits, oxbows, railroad rights-of-way, drains from sewage disposal ponds, and below natural springs or artesian wells. Beavers build dams to modify the environment more to their liking. Dam building is often stimulated by running water.

The length or height of a dam generally depends upon what is necessary to slow the flow of water and create a pond. In areas of flat topography, the dam may not be over 36 inches (0.9 m) high but as much as 1/4 miles (0.4 km) long. In hilly or mountainous country, the dam may be 10 feet (3 m) high and only 50 feet (15 m) long. Beavers are adaptable and will use whatever materials are available to construct dams — fencing materials, bridge planking, crossties, rocks, wire, and other metal, wood, and fiber materials.

Therefore, about the only available aquatic habitat beavers avoid are those systems lacking acceptable foods, lodge or denning sites, or a suit-able dam site. Some of the surrounding timber is cut down or girdled by beavers to form dams. Subsequent flooding of growing timber causes it to die, and aquatic vegetation soon be-gins growing. Other pioneer species (for example, willow, sweetgum, and buttonbush) soon grow around the edges of the flooded area, adding to the available food supply. The beaver thus helps create its own habitat.

Food Habits

Beavers prefer certain trees and woody species, such as aspen, cotton-wood, willow, sweetgum, blackgum, black cherry, tulip poplar, and pine, depending on availability. However, they can and will eat the leaves, twigs, and bark of most species of woody plants that grow near the water, as well as a wide variety of herbaceous and aquatic plants. Beavers often travel 100 yards (90 m) or more from a pond or stream to get to corn fields, soybean fields, and other growing crops, where they cut the plants off at ground level and drag them back to the water. They eat parts of these plants and often use the remainder as construction material in the dam.

General Biology, Reproduction, and Behavior

Beavers are active for approximately 12 hours each night except on the coldest of winter nights. The phrase “busy as a beaver” is appropriate. It is not uncommon, however, to see beavers during daylight hours, particularly in larger reservoirs. Beavers are generally monogamous; copulation may take place either in the water or in the lodge or bank den. After a gestation period of about 128 days, the female beaver generally gives birth to 3 or 4 kittens between March and June, and nurses them for 6 weeks to 3 months. The kittens are born fully furred with their eyes partially opened and incisors erupted through the gums. They generally become sexually mature by the age of 1 1/2 years.

Beaver communicate by vocalizations, posture, tail slapping, and scent posts or mud mounds placed around the bank and dam. The beaver’s castor glands secrete a substance that is deposited on mud mounds to mark territorial boundaries. These scent posts are found more frequently at certain seasons, but are found year-round in active ponds.

Beavers have a relatively long life span, with individuals known to have lived to 21 years. Most, however, do not live beyond 10 years. The beaver is unparalleled at dam building and can build dams on fast-moving streams as well as slow-moving ones. They also build lodges and bank dens, depending on the available habitat. All lodges and bank dens have at least two en-trances and may have four or  more.

The lodge or bank den is used primarily for raising young, sleeping, and food storage during severe weather. The size and species of trees the beaver cuts is highly variable — from a 1-inch (2.5-cm) diameter at breast height (DBH) softwood to a 6-foot (1.8-m) DBH hardwood. In some areas beavers usually cut down trees up to about 10 inches (25 cm) DBH and merely girdle or partially cut larger ones, although they often cut down much larger trees. Some beavers seem to like to girdle large pines and sweet-gums.

They like the gum or storax that seeps out of the girdled area of sweet-gum and other species. An important factor about beavers is their territoriality. A colony generally consists of four to eight related beavers, who resist additions or outsiders to the colony or the pond. Young beavers are commonly displaced from the colony shortly after they become sexually mature, at about 2 years old. They often move to another area to begin a new pond and colony. However, some become solitary hermits inhabiting old abandoned ponds or farm ponds if available.

Beavers have only a few natural predators aside from humans, including coyotes, bobcats, river otters, and mink, who prey on young kittens. In other areas, bears, mountain lions, wolves, and wolverines may prey on beavers. Beavers are hosts for several ectoparasites and internal parasites including nematodes, trematodes, and coccidians. Giardia lamblia is a patho-genic intestinal parasite transmitted by beavers, which has caused human health problems in water supply systems.

The Centers for Disease Control have recorded at least 41 outbreaks of waterborne Giardiasis, affecting more than 15,000 people. For more information about Giardiasis, see von Oettingen (1982).