Beaver

Beavers Fossil range: Late Miocene - Recent
American Beaver
Scientific classification
Kingdom: Animalia Phylum: Chordata Class: Mammalia Order: Rodentia Family: Castoridae Genus: Castor Linnaeus, 1758
Species
American beaver

Beavers are large semi-aquatic rodents. They are best known for their construction behavior, especially the building of dams. The American beaver (Castor canadensis) is native to North America from northern Canada to northern Mexico. The European beaver (Castor fiber) is native to northern Europe and northwestern Asia.

North American giant beaver (Castoroides ohioensis) was one of largest rodents that ever evolved. It disappeared, with other large mammals in the Holocene extinction event, which began about 13,000 years ago.

The mountain beaver (Aplodontia rufa) is not closely related to beavers and is a member of the rodent family Aplodontiidae.

General characteristics

A beaver skull

The two beaver species are very similar, being chiefly distinguished by the form of the nasal bones of the skull. They are the world's second largest rodents, after the capybara of South America (Hydrochoerus hydrochaeris), with adults weighing from 14 kg (30 lbs) to as much as 40 kg (88 lbs) (Nowak 1983). Like all rodents, beavers have two large incisors on both the upper and lower jaws. These grow constantly and must be sharpened and kept from growing too long by frequent gnawing. (The word "rodent" comes from the Latin word rodere, meaning "to gnaw.") The incisors of beavers are bright orange.

Beavers make their homes in or near bodies of fresh water such as lakes, streams, and ponds and spend much of their time in the water. They are protected from the water and the cold by their thick fur. This consists of two kinds of hair—the one close-set, silky and of a grayish color, the other much coarser and longer, and of a reddish brown.

Beavers have webbed hind-feet, which they use in swimming. They can stay submerged up to 15 minutes and can swim underwater at speeds up to 135 meters per minute (5 miles per hour). Their eyes are protected underwater by special transparent eyelids and their noses and ears can be shut (Voelker 1986).

The beaver's tail is large and flat and is used for balance and propulsion in the water, for support when the beaver stands on its hind feet, and to make a warning sound when it is slapped on the water's surface.

Beavers, as far as it is known, eat only plant foods. In summer they seem to prefer soft water plants and berries and sometimes eat algae. They also eat the bark and soft wood of trees. Tree branches are stored underwater to be eaten during the winter when other plant foods are not available (Buyukmichi 1967).

Social behavior

Beavers live in family groups usually consisting of one male and female pair and their offspring. The young, which are born in late spring, are born fully furred and can swim soon after birth. They live with their parents for about two years, during which they learn and help with the family's building and food storage activities. After two years they are driven off by their parents and have to set up new homes (Nowak 1983 Voelker 1986).

Building activities

Beavers are sociable animals, living in streams, where, so as to render the water of sufficient depth, they build dams of mud and of the stems and boughs of trees felled by their powerful incisor teeth. In the pond created by the dam they make their "lodges," which are roomy chambers, with the entrance beneath the water.

Beaver dams are created both as a protection against predators, such as coyotes, wolves and bears, and to provide easy access to food during winter. Destroying a beaver dam without removing the beavers takes a lot of effort, especially if the dam is downstream of an active lodge. Beavers can rebuild such primary dams overnight, though they may not defend secondary dams as vigorously.

Beavers have been known to build very large dams. The largest known was discovered near Three Forks, Montana, and was 2,140 feet (650 meters) long, 14 feet (4 meters) high, and 23 feet (7 meters) thick at the base.

The ponds created by well-maintained dams help isolate the beavers' home, their lodge, which is also created from severed branches and mud. The lodge has underwater entrances to make entry nearly impossible for predators. A very small amount of the lodge is actually used as a living area. Beavers dig out their den with an underwater entrance after they finish building the dam and lodge structure. There are typically two dens within the lodge, one for drying off after exiting the water, and another, drier one where the family actually lives.

Role in nature

The beaver works as a keystone species in an ecosystem by creating wetlands that are utilized by many other species. Next to humans, no other extant animal does more to shape its landscape. Such wetland benefits include flood control downstream, biodiversity (by providing habitat for many rare as well as common species), and water cleansing, both by the breakdown of toxins such as pesticides and the retention of silt by beaver dams. Over the eons, this collection of silt produces the rich bottom land so sought after by farmers. Beaver dams also reduces erosion as well as decreasing the turbidity that is a limiting factor for much aquatic life.

A beaver dam has a certain amount of freeboard above the water level. When heavy rains occur, the pond fills up and the dam gradually releases the extra stored water. Often this is all that is necessary to reduce the height of the flood wave moving down the river, and will reduce or eliminate damage to human structures. Flood control is achieved in other ways as well. The surface of any stream intersects the surrounding water table. By raising the stream level, the gradient of the surface of the water table above the beaver dam is reduced, and water near the beaver dam flows more slowly into the stream. This helps in reducing flood waves, and increases water flow when there is no rain. Beaver dams also smooth out water flow by increasing the area wetted by the stream. This allows more water to seep into the ground where its flow is slowed. This water eventually finds its way back to the stream. Rivers with beaver dams in their head waters have lower high water and higher low water levels.

Drained Beaver Dam. Allegheny State Park

If a beaver pond becomes too shallow due to the settling of sediment, or if the tree supply is depleted, beavers will abandon the site. Eventually the dam will be breached and the water will drain out. The rich thick layer of silt, branches, and dead leaves behind the old dam is the ideal habitat for wetland species. Many of them will have been on the fringes of the pond.

As the wetland fills and dries out, pasture species colonize it and it becomes a meadow suitable for grazing. In an area with nothing but forest down to the stream edge, this provides a valuable niche for many animals which otherwise would be excluded. Finally the meadow will be colonized by riverine trees, typically aspens, willows and such species which are favoured by the beaver. Beavers are then likely to recolonize the area, and the cycle begins again. Each time this process repeats itself another layer of rich organic soil is added to the bottom of the valley. The valley slowly fills and the flat area at the bottom gets wider.

Species

The European Beaver (Castor fiber) was hunted almost to extinction in Europe, both for fur and for castoreum, a secretion of its scent gland believed to have medicinal properties. However, the beaver is now being re-introduced throughout Europe. Several thousand live on the Elbe, the Rhone and in parts of Scandinavia. In northeast Poland there is a thriving community of Castor fiber. They have been reintroduced in Bavaria, The Netherlands and Serbia (Zasavica bog) and are tending to spread to new locations. The beaver finally became extinct in Great Britain in the sixteenth century: Giraldus Cambrensis reported in 1188 (Itinerarium ii.iii) that it was to be found only in the Teifi in Wales and in one river in Scotland, though his observations are clearly first hand.

In October 2005, six European beavers were re-introduced to Britain in Lower Mill Estate in Gloucestershire, and there are plans for re-introductions in Scotland and Wales.

Habitat

File:117-1715 IMG.JPG

Canoeists try unsuccesfully to run a beaver dam in Algonquin Park. The dam is about 1 m high.

The habitat of the beaver is the riparian zone inclusive of stream bed. The habit of the beaver for hundreds of thousands of years in the Northern Hemisphere has been to keep these watery systems healthy and in good repair, although to a human observer, seeing all of the downed trees, it might sometimes seem that the beavers are doing just the opposite. The beaver works as a keystone species in an ecosystem by creating wetlands that are utilized by many other species. Next to humans, no other extant animal does more to shape its landscape. Introduced to an area without its natural predators, as in Tierra del Fuego, beavers have flooded thousands of acres of land and are considered an unstoppable plague. One notable difference in Tierra del Fuego from most of North America is that the trees found in Tierra del Fuego do not coppice as do willows, poplars, aspens, and other North American trees. Thus the "damage" by the beavers seems more severe. Oddly enough, the beaver is not as expert a forester as one would expect; the leading cause of death for beavers is being struck by the very trees that they fell.^{[citation needed]}

Dams

When objectionable beaver flooding occurs, modern water level control devices can be installed for a cost-effective and environmentally sound solution. Unwanted damage to trees can be prevented by wrapping chicken wire or sheet metal around the base of trees.

Trees, up to 250 mm (10 inches) in diameter, felled by beavers in one night.

Beaver dams can be disruptive; the flooding can cause extensive property damage, and when the flooding occurs next to a railroad roadbed, it can cause derailments by washing-out under the tracks, or when a beaver dam bursts and the resulting flash flood overwhelms a culvert. This disruption is not limited to human geography; beavers can destroy nesting habitat for endangered species, and often destroy mature trees for which they have no use.

On the other hand, dam building is extremely beneficial in restoring wetlands, the land's most beneficial ecosystem.

While beavers can create damage, part of the problem is one of perception and time scale. Such damage as the undermining of a roadway or the drowning of some trees is very visible shortly after the beginning of beavers' activity in an area. The benefits, mentioned below, are long-term and not easily seen except by someone who is monitoring a catchment and realizes the beneficial effects of beaver dams.

Beaver lodge, approx. 20-foot diameter. Ontario, Canada

Wetland creation

Research is sparse on this topic, but it seems likely that much of the fabled bottom land in North America was created, or at least added to, by the efforts of the generations of beavers that lived there.

Nutrient removal

The removal of nutrients from the stream flow by beaver ponds is an interesting and very valuable process. Farming along the banks of rivers often increases the loads of phosphates, nitrates and other nutrients, causing problems downstream when this water is extracted for drinking. Besides silt, the beaver dam collects twigs and branches from the beavers' activity and leaves, notably in the fall. The main component of this material is cellulose, a polymer of β-glucose monomers (This creates a more crystalline structure than is found in starch, which is composed of α-glucose monomers. Cellulose is a type of polysaccharide.) Many bacteria produce cellulase which can split off the glucose and utilize it for energy. Just as algae get their energy from sunlight, these bacteria get their energy from cellulose, and they form the base of a very similar food chain. However, a source of energy is not enough for growth. These bacterial populations face serious shortages of nitrous and phosporous compounds, and and will absorb these nutrients as they pass by in the water stream. In this way, these and other nutrients are fixed into the beaver pond and the surrounding ecology, and are removed from the stream.

Pesticide and herbicide removal

Agriculture also introduces herbicides and pesticides into our streams. Bacteria are an extremely variable lot and some of these toxicants are metabolized and decomposed by the bacteria in the cellulose-rich bottom of a beaver dam.

Denitrification

Some scientist believe that the nitrate cascade, the production of far more fixed nitrogen than the natural cycles can turn back into nitrogen gas, may be as much of a problem to our ecology as carbon dioxide production. It is likely, but not proven, that beaver dams along a stream may contribute to denitrification (the removal of nitrogen). In sewage plants, denitrification is achieved by passing the water through successive aerobic and anaerobic stages. Under a beaver dam, as the water seeps down into the soil, the oxygen is used up by the fauna in the rich organic layer. At some point all the oxygen is used up and the soil becomes anaerobic. This water eventually finds its way into the stream and into another beaver dam. This aerobic, anaerobic cycle continues all the way down the stream and denitrification is a likely result.

Danger signal

When startled or frightened, a swimming beaver will rapidly dive while forcefully slapping the water with its broad tail. This creates a loud 'slap', audible over large distances above and below water. This noise serves as a warning to other beavers in the area. Once a beaver has made this danger signal, all nearby beavers will dive and may not reemerge for some time.

Fur trade

Beaver pelts were used for barter by Native Americans in the 17th century to gain European goods. They were then shipped back to Great Britain and France where they were made into clothing items. Widespread hunting and trapping of beavers led to their endangerment. Eventually, the fur trade fell apart due to declining demand in Europe and the takeover of trapping grounds to support the growing agriculture sector. A small resurgence in beaver trapping has occurred in some areas where there is an over-population of beaver; trapping is only done when the fur is of value, and normally the remainder of the animal is also utilized as animal feed.

Beavers in culture

File:CDN-5-Cents-Szmurlo.jpg

A Beaver on the Canadian 5 Cent Coin

Popular western culture typically depicts the animal positively, as a good-natured and industrious character.

• The North American Beaver (C. canadensis) is the national animal of Canada; it is depicted on the Canadian five-cent piece and was on the first Canadian postage stamp, the Three-Penny Beaver. As a national symbol, the animal is a favourite choice for depicting Canadians as furry characters and was chosen to be the mascot of 1976 Summer Olympics held in Montreal with the name "Amik" ("beaver" in Algonquin). It is also the symbol of many units and organizations within the Canadian Forces, such as on the cap badges of the Royal 22e Régiment and the Canadian Military Engineers. However, beavers are considered a pest by some people.

• Mr. and Mrs. Beaver who are important heroic characters in the classic fantasy novel, The Lion, the Witch and the Wardrobe.

• The beaver's habits, habitat and conservation status (as of 1908) are recurring themes in The Tent Dwellers, by Albert Bigelow Paine. Lillian Hoban's Charlie the Tramp is a children's book about a young beaver and his family.

• There is typically a Beaver Patrol in the Boy Scouts of America's Wood Badge adult-leadership training program.
  File:Beaver-Szmurlo.jpg

  A North American Beaver in Calgary, Alberta

• In the United States, Oregon is known as the "The Beaver State." The beaver is the state animal. It is also the mascot of Oregon State University. It is the state mammal of New York (after the historical emblem of New Netherland). It also appears on New York City's coat of arms, seal and flag because of the importance of the fur trade in initial settlement of the region.

• Due to its engineering capabilities, the beaver serves as the mascot of the Massachusetts Institute of Technology, California Institute of Technology, Oregon State University, and the University of Toronto. It is also an emblem for London School of Economics and the name of its student newspaper, The Beaver.

• In the 17th century, based on a question raised by the Bishop of Quebec, the Roman Catholic Church ruled that the beaver was a fish for purposes of dietary law. Therefore, the general prohibition on the consumption of meat on Fridays during Lent does not apply to beaver meat. The legal basis for the decision probably rests with the Summa Theologica of Thomas Aquinas, which bases animal classification as much on habit as anatomy.
• In the Cheese Shop sketch of Monty Python's Flying Circus, a famished customer asks the proprietor of a cheese shop for any one of dozens of different kinds of cheese, including the nonexistent Venezuelan Beaver Cheese. Venezuela has no indigenous beavers.

• Nickelodeon aired The Angry Beavers, a popular children's television show.

• Bell Canada also advertises using two animated beavers called Frank and Gordon.

• Happy Tree Friends has two characters named Handy and Toothy who are beavers.

• Pokemon Diamond and Pearl has 2 pokemon based on a beaver, Bipper, and its evolution.

• In computability theory, a Busy Beaver is a Turing machine which does as much work as possible given the size of its tape alphabet and underlying state machine.

1911 encyclopedia text

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Beaver tracks in snow, in Ontario. Hind paws approx 20 cm long.

The following text is taken from the 1911 Encyclopedia Brittannica.

Beaver, the largest European aquatic representative of the mammalian order RODENTIA, easily recognized by its large trowel-like, scaly tail, which is expanded in the horizontal direction.

The word is descended from the Aryan name of the animal, cf. Sanskrit babhru's, brown, the great ichneumon, Lat. fiber, Ger. Biber, Swed. bäver, Russ. bobr'; the root bhru has given "brown," and, through Romanic, "bronze" and "burnish."

The true beaver (Castor fiber) is a native of Europe and northern Asia, but it is represented in North America by a closely-allied species (C. canadensis), chiefly distinguished by the form of the nasal bones of the skull.

File:Beaver dam in Yellowstone.jpg

Yellowstone National Park.

Beavers are nearly allied to the squirrels (Sciuridae), agreeing in certain structural peculiarities of the lower jaw and skull. In the Sciuridae the two main bones (tibia and fibula) of the lower half of the leg are quite separate, the tail is round and hairy, and the habits are arboreal and terrestrial. In the beavers or Castoridae these bones are in close contact at their lower ends, the tail is depressed, expanded and scaly, and the habits are aquatic.

File:Beaver dam in Tierra del Fuego.jpg

Tierra del Fuego.

Beavers have webbed hind-feet, and the claw of the second hind-toe double. They have poor eyesight, but a keen sense of hearing, smell, and touch.

In length beavers—European and American—measure about 2 ft. exclusive of the tail, which is about 10 inches long. They are covered with a fur to which they owe their chief commercial value; this consists of two kinds of hair—the one close-set, silky and of a greyish colour, the other much coarser and longer, and of a reddish brown.

Beavers are essentially aquatic in their habits, never travelling by land unless driven by necessity. Formerly common in England, the European beaver has not only been exterminated there, but likewise in most of the countries of the continent, although a few remain on the Elbe, the Rhone and in parts of Scandinavia. The American species is also greatly diminished in numbers from incessant pursuit for the sake of its valuable fur.

The mud is plastered down by the fore-feet, and not, as often supposed, by the tail, which is employed solely as a rudder. 

They are mainly nocturnal, and subsist chiefly on bark and twigs or the roots of water plants.

File:Beaver dam in Fossil Butte NM-750px.JPG

Fossil Butte National Monument.

The dam differs in shape according to the nature of particular localities. Where the water has little motion it is almost straight; where the current is considerable it is curved, with its convexity towards the stream. The materials made use of are driftwood, green willows, birch and poplars; also mud and stones intermixed in such a manner as contributes to the strength of the dam; but there is no particular method observed, except that the work is carried on with a regular sweep, and that all the parts are made of equal strength.

"In places," writes Hearne, "which have been long frequented by beavers undisturbed, their dams, by frequent repairing, become a solid bank, capable of resisting a great force both of ice and water; and as the willow, poplar and birch generally take root and shoot up, they by degrees form a kind of regular planted hedge, which I have seen in some places so tall that birds have built their nests among the branches."

Their houses are formed of the same materials as the dams, with little order or regularity of structure, and seldom contain more than four old, and six or eight young beavers. It not unfrequently happens that some of the larger houses have one or more partitions, but these are only posts of the main building left by the builders to support the roof, for the apartments have usually no communication with each other except by water.

Lassen Volcanic National Park.

The beavers carry the mud and stones with their fore-paws and the timber between their teeth. They always work in the night and with great expedition. They cover their houses late every autumn with fresh mud, which, freezing when the frost sets in, becomes almost as hard as stone, so that neither wolves nor wolverines can disturb their repose.

The favourite food of the American beaver is the water-lily (Nuphar luteum), which bears a resemblance to a cabbage-stalk, and grows at the bottom of lakes and rivers. Beavers also gnaw the bark of birch, poplar and willow trees; but during the summer a more varied herbage, with the addition of berries, is consumed.

When the ice breaks up in spring they always leave their embankments, and rove about until a little before the fall of the leaf, when they return to their old habitations, and lay in their winter stock of wood. They seldom begin to repair the houses till the frost sets in, and never finish the outer coating till the cold becomes severe. When they erect a new habitation they fell the wood early in summer, but seldom begin building till towards the end of August.

Castoreum is a substance contained in two pear-shaped pouches situated near the organs of reproduction, of a bitter taste and slightly foetid odour, at one time largely employed as a medicine, but now used only in perfumery.

Fossil remains of beavers are found in the peat and other superficial deposits of England and the continent of Europe; while in the Pleistocene formations of England and Siberia occur remains of a giant extinct beaver, Trogontherium cuvieri, representing a genus by itself.

Mountain beaver

The mountain beaver (Aplodontia rufa) is a primitive rodent unrelated to beavers and not usually found in mountainous areas. It has several common names including Aplodontia, boomer, ground bear, and giant mole. The name sewellel or suwellel is a First Nations/Native American name . This species is the only member of its genus, Aplodontia, and family, Aplodontiidae.

Mountain beavers are brown in color, but fur can range from slightly more reddish or blackish depending on subspecies. There is a light patch under each ear. The animals have distinctively short tails. Adults weigh between about 500-900 g with a few specimens topping 1,000 g. Total length is about 30-50 cm with a tail length equal to 1-4 cm.

Mountain beavers cannot produce concentrated urine. They are thought to be physiologically restricted to the temperate rain forest regions of the North American Pacific coast due to their inability to obtain sufficient water in more arid environments. Their karyotype is 2n=46.

Mountain beavers are found in coastal areas of the Pacific Northwest of North America. These are usually low elevation regions, but they can occasionally be seen as high as treeline. They can be found in both deciduous and coniferous forests, but appear to prefer the former. These animals appear to be physiologically limited to moist regions with minimal snowfall and cool winters. They do not appear to be able to conserve body heat or warmth as efficiently as other rodents. They do not hibernate.

References

ISBN links support NWE through referral fees

• Buyukmihci, H. S. 1971. The Hour of the Beaver. New York : Rand McNally & Company
• Caras, R. A. 1967. North American Mammals. New York : Galahad Books
• Invasive Species Specialist Group (ISSG). 2007. Global invasive species database: Castor canadensis. Invasive Species Specialist Group Website. Retrieved April 11, 2007.
• IUCN Species Survival Commission (IUCN). 2007. .2007 ICUN Red List of Threatened Species: Castor fiber. International Union for Conservation of Nature and Natural Resources. Retrieved April 11, 2007.
• Nowak, R. M., and J. L. Paradiso. 1983. Walker's Mammals of the World. Baltimore, Maryland: The Johns Hopkins University Press. ISBN 0801825253.
• Voelker, W. 1986. The Natural History of Living Mammals. Medford, New Jersey: Plexus Publishing, Inc. ISBN 0937548081.

External links

• Beavers: Wetlands & Wildlife website
• Bob Arnebeck's beaver observation site
• Ecology of the Beaver
• The Romance of the Beaver; being the history of the beaver in the western hemisphere, by A. Radclyffe Dugmore. Illustrated with photographs from life and drawings by the author. Publisher: Philadelphia, J.B. Lippincott company; London, W. Heinemann 1914 (a searchable facsimile at the University of Georgia Libraries - View DjVu with Plug in, open source viewer, or Sun Java - View Layered PDF with Adobe Reader 7)'
• Aigas Field Centre's beaver diary
• Hinterland Who's Who - Beaver

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