North American beaver
The North American beaver is one of two extant beaver species, along with the Eurasian beaver. It is native to North America and has been introduced in South America and Europe. 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.
Taxonomy
Evolution
The first fossil records of beavers are 10 to 12 million years old in Germany, and they are thought to have migrated to North America across the Bering land bridge. During the mid to late Pleistocene epoch, giant beavers weighing up to 200 pounds thrived in central North America. These large early beavers were not yet known to fell trees and build dams. The oldest fossil record of beavers in North America are of two beaver teeth near Dayville, Oregon, and are 7 million years old. Early beavers had diverse ways of life, taking on terrestrial and semi aquatic forms. Dam building originated over 10 million years ago by a semi-aquatic variation of beavers.Subspecies
At one time, 25 subspecies of beavers were identified in North America, with distinctions based primarily on slight morphological differences and geographical isolation at the time of discovery. However, modern techniques generally use genetics rather than morphology to distinguish between subspecies, and currently the Integrated Taxonomic Information System does not recognize any subspecies of C. canadensis, though a definitive genetic analysis has not been performed. Such an analysis would be complicated by the fact that substantial genetic mixing of populations has occurred because of the numerous reintroduction efforts intended to help the species recover following extirpation from many regions.The most widespread subspecies, which perhaps are now best thought of as populations with some distinct physical characteristics, are C. c. acadicus, C. c. canadensis, C. c. carolinensis, and C. c. missouriensis. The Canadian beaver originally inhabited almost all of the forested area of Canada, and because of its more valued fur, was often selected for reintroductions elsewhere. The Carolina beaver is found in the southeastern United States; the Missouri River beaver, as its name suggests, is found in the Missouri River and its tributaries; and C. c. acadicus is found throughout the New England area in the northeastern United States.
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, with being typical. In New York, the average weight of adult male beavers was, while non-native females in Finland averaged. However, adults of both sexes averaged 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. 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, with the tail adding a further. Very old individuals can exceptionally exceed normal sizes, weighing more than or even as much as .Like the capybara, the beaver is semiaquatic. The beaver has many traits suited to this lifestyle. It has a large, flat, paddle-shaped tail and large, webbed hind feet. The unwebbed front paws are smaller, with claws. The forepaws are highly dextrous, and are used both for digging, and to fold individual leaves into their mouth and to rotate small, pencil-sized stems as they gnaw off bark. The eyes are covered by a nictitating membrane which allows the beaver to see under water. The nostrils and ears are sealed while submerged. Their lips can be closed behind their front teeth so that they can continue to gnaw underwater. A thick layer of fat under its skin insulates the beaver from its coldwater environment.
The beaver's fur consists of long, coarse outer hairs and short, fine inner hairs. The fur has a range of colors, 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. There is also another set of oil glands producing unique chemical identifiers in the form of waxy esters and fatty acids. The lush, workable fur was made into a number of products, most notably hats. Demand for furs for hats drove beavers nearly to the point of extinction, and the North American species was saved principally by a sudden change in style.
The beaver possesses continuously growing incisors, and is a hindgut fermenter whose cecum, populated by symbiotic bacteria, helps to digest plant-based material. These traits are not unique to beavers, and are in fact present among all rodents. Nonetheless, the beaver is remarkably specialized for the efficient digestion of its lignocellulose-heavy diet.
Brain anatomy of the beaver is not particularly specialized for its semiaquatic life history. The brain masses of a beaver weighing 11.7 and 17 kg are 41 and 45 g respectively. C. canadensis has an encephalization quotient of 0.9 compared to other rodents; this is intermediate between similar terrestrial rodents and arboreal squirrels, and higher than similar aquatic terrestrial rodents, the muskrats and nutria. The cerebrum is well developed, and the neocortex comparatively large. Larger areas of the beaver's somatosensory cortex are dedicated to the processing of stimuli from the lips and the hands, more so than the tail and whiskers, which play a relatively minor role. The visual area of the brain is smaller than that of the gray squirrel.
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 near-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.
Dam-building
The purpose of the dam is to create a deepwater refuge enabling the beaver to escape from predators. When deep water is already present in lakes, rivers, or larger streams, the beaver may dwell in a bank burrow and bank lodge with an underwater entrance. The beaver dam is constructed using branches from trees the beavers cut down, as well as rocks, grass, and mud. Where naturally occurring woody material is limiting, beavers may build their dams largely of rocks. 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. 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 do not repair breaches in the dam made by otters, and sometimes breach the dam themselves and lower the water level in the pond 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 tested when a recording of running water was played in a field near a beaver pond. Although it was on dry land, the beaver covered the tape player with branches and mud. The largest beaver dam is 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 more than twice the width of the Hoover Dam which spans.
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. In colder climates where their pond freezes over, beavers also build a food cache from this food resource. To form the cache, beavers collect food in late fall in the form of tree branches, storing them under water, 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. The frozen combination of branches and ice is known as a cap, sealing the food cache. Beavers often maintain an underwater entrance to their dam, and they can access their food cache from their lodge by swimming under the ice. In warmer climes, a winter food store is less common.
Muskrats have been thought to steal food from beaver lodges, but seemingly cooperative relationships exist, with beavers allowing muskrats to reside in their lodge if they gather fresh reeds.