The North American beaver (Castor canadensis) is one of two extant beaver species, along with the Eurasian beaver (Castor fiber). It is native to North America and has been introduced in South America (Patagonia) and Europe (primarily Finland and Karelia). The North American beaver is one of the national symbols of Canada and the official state mammal of Oregon and New York. North American beavers are widespread across the continental United States, Canada, southern Alaska, and some parts of northern Mexico.
In Canada and the United States, the North American beaver is often referred to simply as "beaver", although this can cause some confusion because another distantly related rodent, Aplodontia rufa, is often called the "mountain beaver". Other vernacular names, including American beaver and Canadian beaver, distinguish this species from the other extant beaver species, Castor fiber, which is native to Eurasia.
Engineers entire wetlands one dam at a time.
Description
The beaver is the largest rodent in North America and competes with its Eurasian counterpart, the European beaver, for being the third-largest in the world, both following the South American capybara and lesser capybara. The European species is slightly larger on average but the American has a larger known maximum size. Adults usually weigh from 11 to 32 kg (24 to 71 lb), with 20 kg (44 lb) being typical. In New York, the average weight of adult male beavers was 18.9 kg (42 lb), while non-native females in Finland averaged 18.1 kg (40 lb). However, adults of both sexes averaged 16.8 kg (37 lb) in Ohio. The species seems to conform to Bergmann's rule, as northern animals appear to be larger. In the Northwest Territory, adults weighed a median of 20.5 kg (45 lb). The American beaver is slightly smaller in average body mass than the Eurasian species. The head-and-body length of adult North American beavers is 74–90 cm (29–35 in), with the tail adding a further 20–35 cm (7.9–13.8 in). Very old individuals can exceptionally exceed normal sizes, weighing more than 40 kg (88 lb) or even as much as 50 kg (110 lb).
Distribution
Beavers were practically ubiquitous and lived from south of the arctic tundra to the deserts of northern Mexico, and from the Atlantic to the Pacific Oceans, but were trapped across the continent for economic gain and political competition, leading to regional extirpation in many regions like the Great Plains. Certain areas saw near-extirpations and declining numbers, but not extirpation, partially due to various conservation measures were taken, with mixed results, by the Hudson's Bay Company. By the 1900s and 1910s, beavers existed largely in isolated pockets, but where those pockets existed is not well documented. However, despite these difficulties reintroduction and natural spread has restored the species back into most of it natural range, with innovations in management allowing sustainable harvest yields now exceeding previous record harvests. They are widely distributed in boreal and temperate ecoregions, where populations are rebounding from historic over-exploitation. Recently, beaver have been observed colonizing arctic tundra, likely as a result of climate-induced increases in riparian shrubs.
Physician naturalist Edgar Alexander Mearns' 1907 report of beaver on the Sonora River may be the earliest report on the southernmost range of this North American aquatic mammal. However, beavers have also been reported both historically and contemporarily in Mexico on the Colorado River, Bavispe River, and San Bernardino River in the Mexican states of Sonora and Chihuahua.
Behavior
Beavers are active mainly at night. They are excellent swimmers and may remain submerged 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. Beavers are well known for building dams across streams and constructing their lodges in the artificial ponds which form. 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.
Ecology
The beaver was trapped out and almost extirpated in North America because 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, around 80,000 beavers 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. Archaeological and historical evidence suggests that beaver ponds created "moth-hole like" habitats in the deciduous forest that dominated eastern North America. This nonforest habitat attracted both Native American and early colonial hunters to the abundant fish, waterfowl, and large game attracted to the riparian clearings created by these aquatic mammals. The first colonial farmers were also attracted to the fertile, flat bottomlands created by the accumulated silt and organic matter in beaver ponds.
Reproduction
North American beavers have one litter per year, coming into estrus for only 12 to 24 hours, between late December and May but peaking in January. Beaver reproduction occurs earlier in warmer southern climates, however in their northern range beavers may wait until late spring to reproduce. Depending on when beavers reproduce, their litters are typically born between April and June each year. Unlike most other rodents, beaver pairs are monogamous, staying together for multiple breeding seasons. Gestation averages 128 days and they have a range of three to six kits per litter (usually 4–5). Most beavers do not reproduce until they are three years of age, but about 20% of two-year-old females reproduce. Female beavers will prepare for birth by making a soft bed inside the lodge. After birth, young beavers stay sheltered in their lodges for up to a month before they begin to swim and eat solid foods. Juvenile beavers stay with their parents for up to 2 years before leaving to create their own homes.