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| North American beaver | |
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| A male North American beaver | |
Scientific classification 
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| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Mammalia |
| Order: | Rodentia |
| Family: | Castoridae |
| Genus: | Castor |
| Species: | C. canadensis
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| Binomial name | |
| Castor canadensis Kuhl, 1820[3]
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| Subspecies[23][24][25][26][27] | |
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List
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| Distribution of the North American beaver (dark green – native, light green – introduced) | |
| Synonyms | |
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Castor fiber canadensis | |
The North American beaver (Castor canadensis) is one of two extant beaver species, along with the Eurasian beaver (Castor fiber).[23] 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 official national wildlife of Canada symbols and is the official state mammal of Oregon and New York.[28] 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[23] and Canadian beaver,[29] distinguish this species from the other extant beaver species, Castor fiber, which is native to Eurasia.
Taxonomy
Evolution
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 beavers in North America are of two beaver teeth near Dayville, Oregon, and are 7 million years old.[30]
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 (which provides authoritative[31] taxonomic information on plants, animals, fungi, and microbes of North America and the world) 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 (formerly recognized) subspecies, which perhaps are now best thought of as populations with some distinct physical characteristics, are C. c. acadicus (New England beaver), C. c. canadensis (Canadian beaver), C. c. carolinensis (Carolina beaver), and C. c. missouriensis (Missouri River beaver).[32] The Canadian beaver originally inhabited almost all of the forested area of Canada,[33] 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 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.[34][35][36] 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).[37] The American beaver is slightly smaller in average body mass than the Eurasian species.[35] 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) (higher than the maximum known for the Eurasian beaver).[38][39][40][41]
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.[42] 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.[43] 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 (see Double coat). 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.[42] 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 (or constantly) 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.[44] Nonetheless, the beaver is remarkably specialized for the efficient digestion of its lignocellulose-heavy diet.[45]
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 the gray squirrel.[43]
Distribution
Before their near-extirpation by trapping in North America, 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.[46][47][48] 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.[49][50][51]
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.[52] 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.[53][54][55]
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.[56] 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.[57] 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.[58] The inner bark, twigs, shoots, and leaves of such trees are also an important part of the beaver's diet.[59] 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.[60]
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.[61] The largest beaver dam is 2,790 ft (850 m) in length—more than half a mile long—and was discovered via satellite imagery in 2007.[62] 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 1,244 ft (379 m).[63][64]
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.[42] To form the cache, beavers collect food in late fall in the form of tree branches, storing them under water (usually by sticking the sharp chewed base of the branches into the mud on the pond bottom), 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.[65] 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.[42]
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.[66]
Canals
Another component to the beaver's habitat is the canal. Canals are used to float logs to a pond, and dams may also be used to maintain the water levels in these canals. Several land trails can extend from the canals.[67] Despite being widespread in some beaver-inhabited areas, beaver canals and their environmental effects are much less studied than beaver dams. Beaver primarily develop canals to increase accessibility of river resources, facilitate transport of acquired resources, and to decrease the risk of predation. Beaver canals can be over 0.5 km in length.[68] Beavers build canals by pushing through soil and vegetation using their forelimbs.
It has been hypothesized that beavers' canals are not only transportation routes to extend foraging, but also an extension of their "central place" around the lodge and/or food cache. A 2012 study of beavers' mark on the landscape found that cut stumps were negatively related to distance from beaver canals, but not to the central body of water. This finding suggested that beavers may consider the canals to be part of their "central place" as far as foraging activity is concerned.[69]
Social behavior
Communication is highly developed in beaver, including scent marking, vocalization, and tail slapping. Beaver deposit castoreum on piles of debris and mud called scent mounds, which are usually placed on or near lodges, dams, and trails less than a meter from water. Over 100 of such mounds can be constructed within one territory.[42] Beaver colonies with close neighbors constructed more "scent mounds" than did isolated colonies, and the number of scent mounds at each active lodge is correlated with the distance to the nearest occupied lodge.[70]
Although seven vocal sounds have been described for beaver, most zoologists recognize only three: a whine, hiss, and growl. Vocalizations and tail slapping may be used to beg for food, signal to family members to warn of predators, or to drive away or elicit a response from predators.
Beavers usually mate for life, forming familial colonies. Beaver "kits" are born precocious and with a developed coat. The young beaver "kits" typically remain with their parents up to two years. Kits express some adult behaviors, but require a long period in the family to develop their dam construction skills, and other abilities required for independent life.[42]
Diet
Beavers are herbivorous generalists with sophisticated foraging preferences. Beavers consume a mix of herbaceous and woody plants, which varies considerably in both composition and species diversity by region and season.[42] They prefer aspen and other poplars, but also take birch, maple, willow, alder, black cherry, red oak, beech, ash, hornbeam, and occasionally pine and spruce.[65] They also eat cattails, water lilies, and other aquatic vegetation, especially in the early spring. Contrary to widespread belief, they do not eat fish.[71]
Beavers select food based on taste, coarse physical shape, and odor. Beavers feed on wood, bark, cambium,[72] branches, twigs, roots, buds,[72] leaves, stems, sprouts, and in some cases, the sap and storax of