North American beaver

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.

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 pine and sweetgum.^[42]

When herbaceous plants are actively growing, they make up much of the beaver's diet. In the winter, beavers switch to woody plants and the food they have stored over the winter. The protein to calorie ratio of a beaver's diet is 40 mg/calorie in summer and 8 mg/calorie for the rest of the year. In northern latitudes, the water lilies Nymphaea and Nuphar are the most important herbaceous component. The rhizomes are stored in the food cache and remain actively growing.^[42]

Willow is an important protein source and is likely to be available for the longest period of time in a beaver's habitat especially in the far north. When available, aspen and poplar are preferred over willow. Conifers are also cut or gnawed by beavers, and used for food and/or building material.

Beavers do not necessarily use the same trees as construction material and as food. Inedible material is more likely to be used as the cap of a beaver family's food cache, the upper part which is frozen in the ice, while the cache itself is composed of edible, high quality branches, which remain unfrozen and accessible.^[42]

Beavers avoid red maple, which can be the only tree left standing at the edges of some beaver ponds.^[42][73]

The beaver's gut microbiome is complex and specialized for a wood-heavy diet, sharing a number of similarities with other mammalian herbivores. However the microbial community in the beaver shows less taxonomic diversity than the "typical" mammalian gut. The major operational taxonomic units (OTU) are Bacteroidota and Bacillota.^[45]

Predators

Common natural predators include coyotes, wolves, and mountain lions.^[74] American black bears may also prey on beavers if the opportunity arises, often by smashing their paws into the beavers' lodges.^[75][76][77] Perhaps due to differing habitat preferences, grizzly bears were not known to hunt beavers in Denali National Park, Alaska.^[78] Less significant predators include wolverines, which may attack a rare beaver of up to adult size, and Canada lynx, bobcats, and foxes (predators of kits or very sick or injured animals, rather than full-grown beavers due to their increasingly smaller size). American alligators, which only minimally co-exist in the wild with beavers, also seldom threaten them. Both golden eagles (Aquila chrysaetos) and bald eagles (Haliaeetus leucocephalus) may on occasion prey on a beaver, most likely only small kits.^[79] Despite repeated claims, no evidence shows that North American river otters are typically predators of beavers but anecdotally may take a rare beaver kit.^[80]

As introduced non-native species

In the 1940s, beavers were brought to Tierra del Fuego in southern Chile and Argentina for commercial fur production and introduced near Fagnano Lake. Although the fur enterprise failed, 25 mating pairs of beavers were released into the wild. Having no natural predators in their new environment, they quickly spread throughout the main island, and to other islands in the archipelago, reaching a number of 100,000 individuals within just 50 years. Although they have been considered an invasive species, it has been more recently shown that the beaver have some beneficial ecological effects on native fish and should not be considered wholly detrimental.^[96] Although the dominant Lenga beech (Nothofagus pumilio) forest can regenerate from stumps, most of the newly created beaver wetlands are being colonized by the rarer native Antarctic beech (Nothofagus antarctica). It is not known whether the shrubbier Antarctic beech will be succeeded by the originally dominant and larger Lengo beech, however, and the beaver wetlands are readily colonized by non-native plant species.^[96] In contrast, areas with introduced beaver were associated with increased populations of the native catadromous puye fish (Galaxias maculatus).^[97][98] Furthermore, the beavers did not seem to have a highly beneficial impact on the exotic brook trout (Salvelinus fontinalis) and rainbow trout (Oncorhynchus mykiss) which have negative impacts on native stream fishes in the Cape Horn Biosphere Reserve, Chile.^[98] They have also been found to cross saltwater to islands northward; and reached the Chilean mainland in the 1990s.^[99] On balance, because of their landscape-wide modifications to the Fuegian environment and because biologists want to preserve the unique biota of the region, most favor their removal.^[100]

North American beavers were released in Finland in 1937, before it was realized that they formed a separate species; following this, 7 beavers expanded to a population of 12,000 within 64 years.^[101] Eurasian beavers had earlier been extirpated from the region, so the release was intended as a reintroduction project.^[102] By 1999, it was estimated that 90% of beavers in Finland were the American species. However, the species is not always considered invasive, as in Europe it has a similar keystone effect to European beavers, which have not recolonized the area. The beaver population has been controlled by issuing hunting licenses.^[103] A report in 2010 concluded that while the current population of American beavers was not problematic, as the species has larger litters than European beavers and builds somewhat larger dams, it could become a problem if its range continues expanding into Russia, but this does not seem to be taking place.^[104]

In Europe, significant invasive populations of Canadian beaver are only present in Finland and Karelia, as the boundary between species has somewhat stabilized, but smaller occurrences have been detected elsewhere.^[102] Ephemeral populations of C. canadensis in Germany and Poland were found from the 1950s to 1970s. Zoo escapes in 2006 created a small population of invasive C. canadensis in Luxembourg, Rhineland-Palatinate and Belgium.^[101] American beavers have not been detected in Sweden, Norway, or Denmark.^[104]

As food

Beaver meat is similar tasting to lean beef, but care must be taken to prevent contamination from the animal's strong castor (musk) gland. It is usually slow-cooked in a broth, and was a valuable food source to Native Americans.^{[citation needed]} Early French Canadian Catholics considered beaver to be "four-legged fish" that could be eaten at Lent.^[105]

Symbolism

As one of the national symbols of Canada,^[106] the North American beaver is depicted on the Canadian nickel.^[106] This beaver was also featured on the first Canadian postage stamp, the Three Penny Beaver, which is considered the first postage stamp to show an animal instead of a head of state.^[107] It is also the state animal of Oregon and New York of the United States, and a common school emblem for engineering schools, including the California Institute of Technology, the Massachusetts Institute of Technology, and the University of Alberta as well as the mascot for Oregon State University, Babson College, and the City College of New York. A beaver is featured prominently on the stamp and seal issued to Professional Engineers and Geoscientists by APEGA. It also appears on the back on the state flag of Oregon. The beaver also appears in the coats of arms of the Hudson's Bay Company,^[108] University of Toronto, Wilfrid Laurier University, and the London School of Economics.

The beaver is also the symbol of the Royal Canadian Engineers both combat and civil.

Busy beaver is a term in theoretical computer science which refers to a terminating program of a given size that produces the most output possible.

Much of the early economy of New Netherland was based on the beaver fur trade. As such, the seal of New Netherland featured the beaver; likewise, the coats of arms of Albany, New York and New York City included the beaver.