The beaver is the largest North American rodent. A common furbearer, the beaver inhabits waterways of every North American state and Canadian province. A paddle-shaped tail distinguishes the species, and self-sharpening teeth allow beavers to mow down sizable trees. Beaver often alter the landscape with the construction of dams, canals, and lodges. Beaver are territorial as long as the habitat will support family groups called "colonies." Beaver are powerful animals both on land and in the water.
Beaver continue to grow in size throughout life, and weights in excess of 60 or 70 pounds do occur when foods are abundant and accessible during the entire year. Unlike many other species, females are as large as males of the same age, and they are sometimes larger. A paddle-shaped, leathery tail positively identifies the species. An adult's tail is usually about 10 inches long and 5 or 6 inches wide, with a thickness of 1/2 inch in the middle.
The hind feet of beaver are fully webbed and large. These feet often measure 6 inches in length, and the spread of the toes is equal to or greater than the length as the beaver swims. Five toes with strong nails are found on the hind feet, including a unique split toenail on one toe, which serves as a comb for grooming. The front feet seem small in contrast to the hind feet. These feet measure 2 1/2 to 3 inches in length and are not webbed at all. Beaver normally swim with their front feet held against the chest. The large webbed hind feet provide the propulsion, and the tail acts as a rudder.
Guard hairs in beaver fur are 2 inches in length, overlaying a soft, dense underfur about an inch deep. Colors vary from section to section and from blond to nearly black. Both male and female beaver have large glands called castors beneath the skin on their lower bellies. These glands produce an oil that the beaver combs into its fur to waterproof it. This oil is also deposited by the beaver at selected locations as a territorial marker or a mating attractant in the spring.
Beaver have transparent eyelids that cover the eyes as the beaver submerges, enabling the beaver to see well when submerged. The ears and nose have valves that close as the beaver submerges, preventing the entry of water. Two upper and two lower incisor teeth dominate the front of a beaver's mouth. The upper incisors overlap the lower incisors, and friction from chewing causes the teeth to self-sharpen.
Similar to birds and reptiles, beaver have a single lower body opening known as a cloaca. This single opening serves the urinary and bowel tracts and the secreted oil from the castor glands, and it covers the reproductive organs of both males and females.
Beaver usually live in family units consisting of the older mated pairs, young from the previous year, and young from the current season called kits. Breeding season takes place in late January or February in most states. Young from the previous year are about 22 months of age at this time, and they are evicted from the colony to relocate and seek mates of their own.
The gestation period of beaver is 107 days, and the adult male and kits usually take up temporary residence in a bank den while the new litter is being born in April, May, or June. The birthing process may take several days, and three to five kits is the typical litter size. Beaver kits are fully furred when born, their eyes are open, and the incisor teeth are visible. Newborn beaver kits take to the water easily, and they might be swimming before they're one day old. Most adult beaver are monogamous and stay with their mate throughout life.
Beaver require deep water for protection from their enemies, and they alter the landscape a great deal with dam building and flooding. Dams can be hundreds of feet in length and vary in height from only a few feet to 7 or 8 feet.
Permanent lodges are often constructed by piling layer after layer of sticks into a large conical form above the waterline. Two or more underwater tunnels are then chewed up into the pile, and an inner chamber is hollowed out to serve as living quarters. Finally, the outside of the lodge is plastered with mud and rocks, except for the peak, which is left porous enough to allow an air exchange to the inner chamber. There are two levels to the chamber. One level is near the waterline near the plunge holes, where the beaver shed water before climbing to the higher resting or nesting areas.
In areas prone to flooding, or where strong currents may be present, beaver usually construct bank dens by digging tunnels from underwater up into banks. Bank dens often have two or more submerged entrances. Many times, the beaver will construct a pile of sticks over the tops of the underground living chambers. These piles of sticks are sometimes called caps.
Shallow pockets are sometimes dug into banks near the waterline and these are known as feed pockets. In northern areas, beaver construct feed piles by submerging large amounts of small trees and limbs to serve as a food source after ice prevents the beaver from activity above the ice. These feed piles are usually constructed close to the den as a convenience to the kit beaver, who do not normally travel far from the den itself.
Weight: over 70 pounds
Mating season: late January and February
Gestation: 107 days
Litter size: 3-5 kits
Can swim within hours of birth
Can hold their breath up to 15 minutes
Life span: up to 12 years
At times, solitary beaver will be found living alone. These beaver are known as bachelors, whether they're male or female.
Adult beaver mark out their territories in early spring by dragging up mud and debris from the bottom and depositing the debris in mounds along the shores, where they also deposit oil from their castor glands. These castor mounds often leave a reddish stain on the bank, and the odors are powerful enough for a human to easily detect.
Beaver are territorial, and territories seldom overlap. Generations of beaver may continuously inhabit a choice area, even building canals to help float food from inland cutting sites. If and when food supplies are exhausted, they do relocate to a better area. Once beaver have determined to claim a territory, they are difficult to dissuade. If the activities of the beaver flood roads or damage property, the beaver usually have to be removed to prevent recurring damages.
Although beaver normally submerge for three or four minutes at a time, they are quite capable of holding their breath for 12 to 15 minutes. They exhale a little in spurts as they swim or work under water, and a large beaver is quite capable of traveling nearly 1/2 mile under the surface before it must resurface for more air.
Migrations of beaver usually occur with the breaking up of ice in late winter or early spring as the 22- or 23-month-old beaver are expelled just prior to birthing time for the new litter. These beaver may choose to go upstream or downstream. Although these beaver are capable of reproducing, they usually do not until the next season after a mate and a new territory have been established. Most new colonies are established within a few miles of the home colony.
Beaver are primarily vegetarians, though an occasional beaver may eat a dead fish. Preferred foods include the bark of aspen, willow, cottonwood, dogwood, and many other varieties of trees and shrubs. In early spring, beaver will often eat the bark and twigs of evergreens. In season, beaver will also eat water lilies, leaves, grasses, roots, and a variety of crops including corn, wheat, oats, carrots, potatoes, apples, clovers, and alfalfa.
Beaver live in every North American state and Canadian province and are harvested by trapping in 46 states.
Tracks and Scat
Beaver usually alter the habitat a great deal with the building of dams and the resulting flooding of lowlands. The deeper water behind dams creates a better habitat for muskrats and a variety of other wildlife species such as fish and waterfowl. Mink and otter hunt regularly around beaver dams. These locations provide suitable denning sites, as well, for these furbearers.
Dam building on trout streams can have an adverse effect on trout survival by slowing the water and allowing it to warm to temperatures higher than the trout can tolerate. Dams also serve as barriers to migrating trout and salmon. At times, beaver cause a significant amount of property damage by cutting trees and flooding large areas, killing the timber. Culvert plugging is common and often floods roads.
Beaver also host an internal parasite, giardiasis. Water reservoirs inhabited by beaver can become contaminated by the giardiasis cysts, which are too small to be filtered out of the drinking water. These cysts hatch in the small intestines of people who drink the contaminated water, resulting in diarrhea, nausea, and stomachaches.
Serious beaver predators include mountain lions, wolves, lynx, and bobcats. At times, bears kill mature beaver. Juvenile beaver are vulnerable to coyotes, eagles, and large owls, as well. Tularemia can be a devastating disease in beaver, wiping out entire populations when conditions are good for disease transmissions. Tularemia infects livers and is usually fatal to beaver of all ages.
A beaver is considered to be old at 12 years of age.