North American Beaver

Appearance

This beaver is the largest rodent in North America and the second or third largest rodent in the world, after the South American capybara. The species' Eurasian counterpart, the European beaver, reaches similarly large sizes. Adults usually weigh from 11 to 32 kg, with 20 kg being a typical mass. The head-and-body length is 74–90 cm, with the tail adding a further 20–35 cm. Very old individuals can exceptionally exceed normal sizes, weighing more than 40 kg or even as much as 50 kg.

Like the capybara, the beaver is semi-aquatic. The beaver has many traits suited to this lifestyle. It has a large flat paddle-shaped tail and large, webbed hind feet reminiscent of a human diver's swimfins. The unwebbed front paws are smaller, with claws. The eyes are covered by a nictitating membrane which allows the beaver to see underwater. The nostrils and ears are sealed while submerged. A thick layer of fat under its skin insulates the beaver from its cold water environment.

The beaver's fur consists of long, coarse outer hairs and short, fine inner hairs. The fur has a range of colours but usually is dark brown. Scent glands near the genitals secrete an oily substance known as castoreum, which the beaver uses to waterproof its fur.

Naming

In the United States and Canada, the species is often referred to simply as "beaver", though this causes some confusion because another distantly related rodent, "Aplodontia rufa", is often called the "mountain beaver".

The first fossil records of beaver are 10 to 12 million years old in Germany, and they are thought to have migrated to North America across the Bering Strait. The oldest fossil record of beaver in North America are of two beaver teeth near Dayville, Oregon and are 7 million years old.

Behavior

Beavers are mainly active at night. They are excellent swimmers and may remain submerged for up to 15 minutes. More vulnerable on land, they tend to remain in the water as much as possible. They use their flat, scaly tail both to signal danger by slapping the surface of the water and as a location for fat storage.

They construct their homes, or "lodges," out of sticks, twigs, rocks and mud in lakes, streams, and tidal river deltas. These lodges may be surrounded by water, or touching land, including burrows dug into river banks. They are well known for building dams across streams and constructing their lodge in the artificial pond which forms. When building in a pond, the beavers first make a pile of sticks and then eat out one or more underwater entrances and two platforms above the water surface inside the pile. The first is used for drying off. Towards winter, the lodge is often plastered with mud which when it freezes has the consistency of concrete. A small air hole is left in the top of the lodge.

The dam is constructed using logs from trees the beavers cut down, as well as rocks, grass and mud. The inner bark, twigs, shoots and leaves of such trees are also an important part of the beaver's diet. The trees are cut down using their strong incisor teeth. Their front paws are used for digging and carrying and placing materials. Some researchers have shown that the sound of running water dictates when and where a beaver builds its dam. Besides providing a safe home for the beaver, beaver ponds also provide habitat for waterfowl, fish, and other aquatic animals. Their dams help reduce soil erosion and can help reduce flooding. However, beaver dams are not permanent and depend on the beavers' continued presence for their maintenance. Beavers generally concentrate on building and repairing dams in the fall in preparation for the coming winter.

In northern areas they often don't repair breaches in the dam made by otters, and sometimes breach the dam themselves and lower the water level in the pond in order to create more breathing space under the ice or get easier access to trees below the dam. In a 1988 study in Alberta, Canada, no beavers repaired "sites of water loss" during the winter. Of 178 sites of water loss, beavers repaired 78 when water was opened, and did not repair 68. The rest were partially repaired.

Beavers are best known for their dam-building. They maintain their pond-habitat by reacting quickly to the sound of running water, and damming it up with tree branches and mud. Early ecologists believed that this dam-building was an amazing feat of architectural planning, indicative of the beaver's high intellect. This theory was questioned when a recording of running water was played in a field near a beaver pond. Despite the fact that it was on dry land, the beaver covered the tape player with branches and mud. The largest beaver dam is 2,790 ft in length—more than half a mile long—and was discovered via satellite imagery in 2007. It is located on the southern edge of Wood Buffalo National Park in northern Alberta and is twice the width of the Hoover Dam which spans 1,244 ft.

Normally, the purpose of the dam is to provide water around their lodges that is deep enough that it does not freeze solid in winter. The dams also flood areas of surrounding forest, giving the beaver safe access to an important food supply, which is the leaves, buds, and inner bark of growing trees. They prefer aspen and poplar, but will also take birch, maple, willow, alder, black cherry, red oak, beech, ash, hornbeam and occasionally pine and spruce. They will also eat cattails, water lilies and other aquatic vegetation, especially in the early spring. In areas where their pond freezes over, beavers collect food in late fall in the form of tree branches, storing them underwater, where they can be accessed through the winter. Often the pile of food branches projects above the pond and collects snow. This insulates the water below it and keeps the pond open at that location.

Beavers usually mate for life. The young beaver "kits" typically remain with their parents for up to two years.

Habitat

The beaver was trapped out and almost extirpated in North America as its fur and castoreum were highly sought after. The beaver furs were used to make clothing and beaver hats. In the United States extensive trapping began in the early 17th century with more than 10,000 beaver per year taken for the fur trade in Connecticut and Massachusetts between 1620 and 1630. From 1630 to 1640, approximately 80,000 beaver were taken annually from the Hudson River and western New York. From 1670 onwards, the Hudson's Bay Company sent two or three trading ships into the bay every year to take furs to England from Canada. As eastern beaver populations were depleted, English, French and American trappers pushed west. Much of the westward expansion and exploration of North America was driven by the quest for this animal's fur. Before the 1849 California Gold Rush, there was an earlier 19th century California Fur Rush which drove the earliest American settlement in that State. During the approximately 30 years of the era of the Mountain Man, the West from Missouri to California and from Canada to Mexico was thoroughly explored and the beaver was brought to the brink of extinction.

With protection in the late 19th and early 20th centuries the current beaver population has rebounded to an estimated 10 to 15 million; this is a fraction of the originally estimated 100 to 200 million North American beaver before the days of the fur trade.

These animals are considered pests in parts of their range because their dams can cause flooding, or because their habit of felling trees can pose danger to people, as in Charlotte, North Carolina's Park Road Park. Because they are persistent in repairing damage to the dam, they were historically relocated or exterminated. Non-lethal methods of containing beaver-related flooding have been developed. One such flow device has been utilized by both the Canadian and U.S. governments, called "Beaver Deceivers," or levelers, invented and pioneered by wildlife biologist, Skip Lisle.

The beaver is a keystone species, increasing biodiversity in its territory through creation of beaver ponds and wetlands. As wetlands are formed and riparian habitats enlarged, aquatic plants colonize newly available watery habitat. Insect, invertebrate, fish, mammal, and bird diversity are also expanded.Beaver ponds have been shown to have a beneficial effect on trout and salmon populations. Many authors believe that the decline of salmonid fishes is related to the decline in beaver populations.

Research in the Stillaguamish River basin in Washington state, found that extensive loss of beaver ponds resulted in an 89% reduction in coho salmon smolt summer production and an almost equally detrimental 86% reduction in critical winter habitat carrying capacity. This study also found that beaver ponds increased smolt salmon production 80 times more than the placement of large woody debris. Swales and Leving had previously shown on the Coldwater River in British Columbia that off-channel beaver ponds were preferentially populated by coho salmon over other salmonids and provided overwintering protection, protection from high summer snowmelt flows and summer coho rearing habitat. The presence of beaver dams has also been shown to either increase the number of fish, their size, or both, in a study of brook trout, rainbow trout and brown trout in Sagehen Creek, which flows into the Little Truckee River at an altitude of 5,800 feet in the northern Sierra Nevada. These findings are consistent with a study of small streams in Sweden, that found that brown trout were larger in beaver ponds compared with those in riffle sections, and that beaver ponds provide habitat for larger trout in small streams during periods of drought. Similarly, brook trout, coho salmon and sockeye salmon were significantly larger in beaver ponds than those in un-impounded stream sections in Colorado and Alaska. In a recent study on a headwater Appalachian stream, brook trout were also larger in beaver ponds.

Reproduction

North American beaver have one litter per year, coming into estrus for only 12 to 24 hours, between late December and May but peaking in January. Unlike most other rodents, beaver pairs are monogamous, staying together for multiple breeding seasons. Gestation averages 128 days and they average two to three kits per litter with a range of two to six kits. Most beaver do not reproduce until they are three years of age, but about 20% of two-year-old females reproduce.

Food

Beaver meat is similar tasting to lean beef, but care must be taken to prevent contamination from the animal's strong castor gland. It is usually slow-cooked in a broth, and was a valuable food source to Native Americans. Early French Canadian Catholics considered beaver to be "four-legged fish" that could be eaten at Lent.

Despite their name, the fried pastries found in parts of Canada called beaver tails contain no beaver.

Predators

Common natural predators include gray wolves, coyotes, and mountain lions. Beaver can be particularly important food for lone wolves. American black bears may also prey on beavers if the opportunity arises, often by smashing their paws into the beaver's lodges. Perhaps due to differing habitat preferences, Brown bears were not known to hunt beavers in Denali National Park. Less significant predators include wolverines, Canadian lynx, bobcats, and fox, which are increasingly unlikely to take full-grown beavers due to their smaller size, and American alligators, which only minimally co-exist with beavers. Both Golden Eagles and Bald Eagles may on occasion predate a beaver, most likely only small kits. Despite repeated claims there is no evidence that river otters are predators of beavers.