name = Beavers
fossil_range = Late
image_caption = American Beaver
image_width = 300px
phylum = Chordata
genus = "Castor"
genus_authority = Linnaeus, 1758
subdivision = "C. canadensis"
Beavers are two primarily nocturnal, semi-aquatic species of
rodent, one native to North America and one to Europe. They are known for building dams, canals, and lodges (homes). They are the second-largest rodent in the world (after the capybara). Their colonies create one or more dams to provide still, deep water to protect against predators, and to float food and building material. The North American population was 60-400 million, but as of 1988, 6-12 million, due to being hunted for fur, for their glands used as medicine and perfume, and because their harvesting of trees and flooding of waterways may interfere with other land uses. [Nowak, Ronald M. 1991. pp. 364-367. "Walker's Mammals of the World" Fifth Edition, vol. I. John Hopkins University Press, Baltimore.]
Beavers are known for their natural trait of building dams in rivers and streams, and building their homes (known as beaver lodges) in the resulting pond. Beavers also build canals to float build materials that are difficult to haul over land. They use powerful front teeth to cut trees and plants that they use for building and for food.
They are known for their "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 beavers in the area. Once a beaver has made this danger signal, nearby beavers dive and may not reemerge for some time. Beavers are slow on land, but good swimmers that can stay under water for as long as 15 minutes. (Wilson, 1971) Rarely, a frightened beaver attacks a human. [ [http://www.thelocal.se/11544/20080505/ Matilda, 4, bitten by beaver] ]
Beavers do not hybernate, but store sticks and logs underwater to feed on during the winter.
Fossil remains of beavers are found in the peat and other superficial deposits of England and the continent of Europe; while in the
Pleistoceneformations of England and Siberia occur remains of a giant extinct beaver, "Trogontherium cuvieri", representing a genus by itself.
Beavers have webbed hind-feet, and a broad, scaly tail. They have poor eyesight, but keen senses of hearing, smell, and touch.
Beavers continue to grow throughout life. Adult specimens weighing over convert|25|kg|lb|abbr=on are not uncommon. Females are as large as or larger than males of the same age, which is uncommon among
The word is descended from the Proto-Indo-European 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."Encyclopedia Brittanica 11th Edn]
They are the only extant members of the family
Castoridae, which contains a single genus, "Castor". Genetic researchhas shown the European and North American beaver populations to be distinct species and that hybridization is unlikely. Beavers are closely related to 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 habitats 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 their habitats are aquatic.
European and American beavers grow to about convert|2|ft|abbr=on long (plus convert|10|in|abbr=on of tail). They are essentially aquatic in their habits, never traveling by land unless driven by necessity. They are crepuscular rather than nocturnal, which means they are active at dawn and dusk, [cite web|url=http://www.ctaudubon.org/conserv/nature/beaver.htm|title=Beaver |accessdate=2008-05-24] and subsist chiefly on bark and twigs or the roots of water plants. They have been known to eat grasses on the banks of rivers and streams.
The European Beaver ("Castor fiber") was hunted almost to
extinctionin 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. A thriving community lives in northeast Poland, and the European Beaver also returned to the Morava Riverbanks in Slovakiaand the Czech Republic. They have been reintroduced in Scotland [cite web |url = http://www.guardian.co.uk/environment/2008/may/26/endangeredspecies.conservation| |title = Beavers returning to UK after 400 years] , Bavaria, Austria, The Netherlandsand Serbia( Zasavicabog) and are spreading to new locations.The beaver became extinct in Great Britain in the sixteenth century: Giraldus Cambrensisreported in 1188 ("Itinerarium" ii.iii) that it was to be found only in the Teifiin Wales and in one river in Scotland, though his observations are clearly first hand. In October 2005, six European beavers were reintroduced to Britain in Lower Mill Estate in Gloucestershire; in July 2007 a colony of four European beavers was established at Martin Merein Lancashire, [cite news |url=http://news.bbc.co.uk/1/hi/england/lancashire/6291260.stm |title=Beavers are back after 500 years |date=2007-07-11 |publisher= BBC News] and there are plans for re-introductions in Scotland and Wales. [ [http://www.msn.co.uk/htx/returnofthebeaver/ Return of the Beavers] at MSN.co.uk] [cite web |url = http://news.bbc.co.uk/1/hi/scotland/7158210.stm| publisher = BBC|title = Beavers could be released in 2009| date = 2007-12-24]
The American Beaver "(Castor canadensis)", also called the Canadian Beaver (which is also the name of a subspecies), or simply Beaver in North America, is native to Canada, much of the United States and parts of northern Mexico. The chief feature distinguishing "C. canadensis" from " C. fiber" is the form of the nasal bones of the skull. This species was introduced to the Argentine and Chilean Tierra del Fuego, as well as Finland, France, Poland and Russia.
The American beaver's preferred food is the water-lily ("
These animals are often trapped for their fur. During the early 19th century, trapping eliminated this animal from large portions of its original range. However, through trap and transfer and habitat conservation it made a nearly complete recovery by the 1940s. Beaver furs were used to make clothing and top-hats. Much of the early exploration of North America was driven by the quest for this animal's fur. Native peoples and early settlers also ate this animal's meat. The current beaver population has been estimated to be 10 to 15 million; one estimate claims that there may at one time have been as many as 90 million. [Seton-Thompson, cited in cite book
coauthors =Dietland Müller-Schwarze
title ="The Beaver: Natural History of a Wetlands Engineer"
publisher =Cornell University Press
isbn = 080144098X pp97-98; but note that to arrive at this figure he assumed a population density throughout the range equivalent to that in
The North American Giant beaver ("Castoroides ohioensis") was one of the largest rodents that ever evolved. About the size of a
Grizzly Bear, it disappeared along with other large mammals in the Holocene extinction event, which began about 13,000 years ago.
The habitat of the beaver is the
riparian zone, inclusive of stream bed. The actions of beavers for hundreds of thousands of years in the Northern Hemispherehave kept these watery systems healthy and in good repair, although a human observing all the downed trees might think that the beavers were doing just the opposite.
The beaver works as a
keystone speciesin an ecosystemby creating wetlandsthat are used by many other species. Next to humans, no other extant animal appears to do more to shape its landscapeFact|date=May 2007.
Effect of beaver on trees
Beavers fell trees for several reasons. They fell large mature trees, usually in strategic locations, to form the basis of a dam. They fell small trees, especially young second growth trees, for food. Ponds created by beavers also kill trees by drowning.
The dam's primary role: the beaver's home
Beaver dams are created as a protection against predators, such as coyotes, wolves and bears, and to provide easy access to food during winter. Beavers always work at night and are prolific builders, carrying mud and stones with their fore-paws and timber between their teeth. Because of this, destroying a beaver dam without removing the beavers is difficult, 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 may create a series of dams along a river.)
Shape of dam
A beaver shapes a dam according to the strength of the water's current. Relatively still water encourages dams that are almost straight; while dams in stronger currents are curved, bowed toward upstream. The beavers use
driftwood, green willows, birchand poplars; and they mix in mud and stones that contribute to the dam's strength.
Beavers are known to build very large dams. [ [http://abob.libs.uga.edu/bobk/bbeavdam.html Big Beaver Dam on Grand Island, Lake Superior ] ] The largest known was discovered by satellite imagery in Northern Alberta in 2007, approximately convert|2790|ft|abbr=on long [ [http://www.geostrategis.com/p_beavers-longestdam.htm National Geographic photos] ] , beating the previous record holder found near Three Forks, Montana, at convert|2140|ft|abbr=on long, convert|14|ft|abbr=on high, and convert|23|ft|abbr=on thick at the base. [ [http://home.earthlink.net/~scouters2/beaver.html The Longest Beaver Dam in the World] ] 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. [ [http://www.BeaversWW.org Beavers: Wetlands & Wildlife] ]
"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."Fact|date=November 2007
timulus for dam-building
It is primarily prolonged exposure to the sound of water in motion that stimulates the beavers to build. However, studies involving beaver habitual activities have indicated that beavers may respond to an array of stimuli, not just the sound of running water (such as seeing water movement). In two experiments Wilson (1971) and Richard (1967, 1980) demonstrate that, although beavers will pile material close to a loudspeaker emitting sounds of water running, [ [http://www.naturealmanac.com/archive/beaver_dams/beaver_dams.html 'Why Beavers Build Dams' at Natural Events Almanac] ] they only do so after a considerable period of time. Additionally the beavers, when faced with a pipe allowing water to pass through their dam, eventually stopped the flow of water by plugging the pipe with mud and sticks. The beavers were observed to do this even when the pipe extended several meters upstream and near the bottom of the stream and thus produced no sound of running water. Beavers normally repair damage to the dam and build it higher as long as the sound continues. However, in times of high water, they often allow spillways in the dam to flow freely.
Disruption by dams
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 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.Introduced to an area without its natural predators, as in
Tierra del Fuego, beavers have flooded thousands of acres of land and are considered a plague. One notable difference in Tierra del Fuego from most of North America is that the trees in Tierra del Fuego do not coppiceFact|date=July 2008 as do willows, poplars, aspens, and other North American trees. Thus the "damage" by the beavers seems more severe.
Benefits of dams compared to disruption
On the other hand, dam building can be beneficial in restoring wetlands. 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 reduce erosion as well as decrease the turbidity that is a limiting factor for much aquatic life. 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 may be long-term and unnoticed except, for example, by someone monitoring a catchment.
The dam's role in the stream's life cycle
If a beaver pond becomes too shallow due to sediment accumulation, 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 may have been on the fringes of the pond. Wetlands have significant environmental benefits.
The grazing meadow (vega and valle)
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.
The riverine forest
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.
The dam's beneficial ecological effects
Each time the stream life cycle 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. Research is sparse, 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.Fact|date=July 2008
A beaver dam has a certain amount of
freeboardabove 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 further 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.
Beaver ponds can cause the removal of nutrients from the stream flow. 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 polymerof β-glucose monomers(This creates a more crystalline structure than is found in starch, which is composed of α-glucosemonomers. Cellulose is a type of polysaccharide.) Many bacteria produce cellulase which can split off the glucoseand use 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 phosphorous compounds, 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.
Some scientists 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,Fact|date=July 2008 but not proven, that beaver dams along a stream may contribute to
denitrification(the conversion of nitrogen compounds back into 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 consumed 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 back into the aerobic stream and into another beaver dam. This aerobic, anaerobic cycle continues all the way down the stream and denitrification is a likely result.
Beavers and salmon
Beaver dams are a nursery for salmon. An early indication of this was seen following the 1818 agreement between the British government of Canada and the government of America allowing Americans access to the Columbia watershed. The Hudson's Bay Company, in a fit of pique sent word to its trappers to extirpate the fur-bearing animals in the area. The beaver was the first to go. Salmon runs fell precipitously in the following years even though, at that time, none of the factors were extant that we associate with the decline of salmon runs. [ [http://www.nwcouncil.org/history/Beavers.asp 'Beavers' at The Northwest Power and Conservation Council] ]
The functions of beaver dams in increasing salmon runs are many. They provide deep enough water for the juvenile salmon to hide from predatory wading birds. They trap nutrients in their ecology and notably the huge nutrient pulse represented by the migration of the adult salmon upstream. These nutrients help feed the juveniles after they finish their yolk. They provide quiet water so that the young salmon can put energy into growth rather than into fighting currents and larger smolt with a food reserve have a better chance when they reach the sea. And beaver dams keep the water clear which favours all the salmonoids, trout included.
The ponds created by well-maintained dams help isolate the beavers' home, their "lodge", which is also created from severed branches and mud. The beavers cover their lodges late every autumn with fresh mud which freezes when the frost sets in. The mud becomes almost as hard as stone, so that neither wolves nor
wolverines can get in.
The lodge has underwater entrances to make entry nearly impossible for any other animal (however, muskrats have been seen living inside beaver lodges with the beavers who made it). A very small amount of the lodge is actually used as a living area. Contrary to popular belief, beavers actually 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.
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. Sometimes 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.
When the ice breaks up in spring they always leave their embankments, and rove about until a little before fall, 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.
Beavers can create serious damage when spread outside of their natural environment, and are therefore considered in some places as pests or
invasive species. For example, in the 1940s, beavers were brought to the island of Tierra Del Fuegoin southern Argentina, for commercial fur production. However, the project failed and the beavers were released into the wild. Having no natural predators in their new environment, they quickly spread throughout the island, reaching a number of 100,000 individuals within just 50 years (when released into the wild there were only a few pairs). They are now considered a serious invasive species on the island, due to their massive destruction of forest trees, and efforts are being made for their eradication. [ [http://edition.cnn.com/NATURE/9907/09/argentina.beaver/ CNN - Argentina eager to rid island of beavers - July 9, 1999 ] ]
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
Francewhere they were made into clothing items. Widespread hunting and trapping of beavers led to their endangerment. Eventually, the fur tradedeclined 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 done when the fur is of value, the remainder of the animal may be used as feed. In the 1976/1977 season, 500,000 beaver pelts were harvested in North America. [Nowak, Ronald M. 1991. pp. 638. "Walker's Mammals of the World" Fifth Edition, vol. I. John Hopkins University Press, Baltimore.]
Both beaver testicles and
castoreum, a bitter-tasting secretion with a slightly fetid odor contained in dried preputial or vaginal follicles of male or female beaver, have been articles of trade for use in traditional medicine. Yupik( Eskimo) medicine used dried beaver testicles like willow bark to relieve pain. Beaver testicles were exported from Levant(a region centered on Israel) from the tenth to nineteenth century. [PMID 12576209] Claudius Aelianuscomically described beavers chewing off their testicles to preserve themselves from hunters. European beavers (" Castor fiber") were eventually hunted nearly to extinction in part for the production of castoreum, which as used as an analgesic, anti-inflammatory, and antipyretic. Castoreum was described in the 1911 British Pharmaceutical Codexfor use in dysmenorrheaand hystericalconditions (i.e. pertaining to the womb), for raising blood pressureand increasing cardiac output. The activity of castoreum has been credited to the accumulation of salicinfrom willow trees in the beaver's diet, which is transformed to salicylic acidand has an action very similar to aspirin. [cite web|url=http://www.the-scientist.com/2005/03/28/S31/1/|title=The quest for pain relief: how much have we improved on the past?|author=Stephen Pincock|date=2005-03-28|accessdate=2007-06-17] Castoreum continues to be used in perfumeproduction.
As a national emblem
The importance of the American Beaver in the development of Canada through the fur trade led to its designation as the national animal. It is depicted on the Canadian five-cent piece and was on the first postage stamp issued in the Canadian colonies in 1849 (the so-called "Three-Penny Beaver"). As a
national symbol, the beaver was chosen to be the mascot of the 1976 Summer Olympicsheld in Montrealwith the name "Amik" ("beaver" in Anishinaabemowin). The beaver 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.
In dietary law
In the 17th century, based on a question raised by the
Bishopof Quebec, the Roman Catholic Churchruled that the beaver was a fish(beaver flesh was a part of the indigenous peoples' diet, prior to the Europeans' arrival) for purposes of dietary law. Therefore, the general prohibition on the consumption of meaton Fridays during Lentdoes not apply to beaver meat. [http://www.chowdc.org/Papers/Saunders%202001.html] [ [http://www.jimmyakin.org/2005/02/lent_roundup.html JIMMY AKIN.ORG: Lenten Reader Roundup ] ] [fr iconLacoursière, Jacques. "Une histoire du Québec" ISBN 2-89448-050-4 Explains that Bishop François de Laval in the 17th century posed the question to the theologians of the Sorbonne, who ruled in favour of this decision.] 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. [ [http://www.newadvent.org/summa/314708.htm "The Summa Theologica of St. Thomas Aquinas"] II. 147:8 provides legal foundation upon which theologians argued in favour of beaver being like fish.]
* [http://www.itis.usda.gov/servlet/SingleRpt/SingleRpt?search_topic=TSN&search_value=180211 ITIS 180211] 2002-12-14
* "The American Beaver and His Works" by Lewis Henry Morgan, Published by J.B. Lippincott & Co., 1868. Some 1911 Britannica material appears to be copied from this source. [http://books.google.com/books?id=ET4AAAAAQAAJ]
*Rue, Leonard Lee, III.
**"The World of the Beaver", Lippincott Company, 1964.
**"Beavers", 2002. ISBN 0896585484
The Tent Dwellers" by Albert Bigelow Paine. Beavers' habits, habitat and conservation status (as of 1908) are recurring themes.
* [http://animal.discovery.com/mammals/beaver/ Beaver Facts & Pictures]
* [http://www.BeaversWW.org Beavers: Wetlands & Wildlife website]
* [http://purl.galileo.usg.edu/ugafax/QL737xR6xD84 The Romance of the Beaver] - history of the beaver in the western hemisphere by A. Radclyffe Dugmore.
* [http://www.aigas.co.uk/2007-Beaver-Diary-g.asp Aigas Field Centre Beaver Project] - history of a pair of European beavers released into a large enclosure in the Highlands of Scotland.
Wikimedia Foundation. 2010.