Beavers are semi-aquatic rodents native to North
America and Europe. They are the only living members of the family Castoridae, which contains a single genus, Castor. Genetic research
has shown the European and North American beaver populations to be distinct species and that
hybridization is unlikely.
General
Beavers are best known for their natural trait of building dams in rivers and streams, and building their homes (known as beaver lodges) in
the eventual pond. They are the second-largest rodent in the world (after the capybara).
Beavers continue to grow throughout life. Adult specimens weighing over 25 kg (55 lb) are not uncommon. Females are as large as or larger than males of the same age, which is uncommon
among mammals.
Species
European Beaver
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. A thriving community lives in north east Poland, and the European Beaver also returned to the Morava River banks in
Slovakia and the Czech Republic. They have been
reintroduced in Bavaria, The Netherlands and Serbia (Zasavica bog) and are spreading to new locations.
The beaver 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.[1]
American Beaver
The American Beaver (Castor canadensis) or simply Beaver in North America is native
to Canada, much of the United States and parts of northern Mexico. The beavers were introduced to the Argentine and Chilean
Tierra del Fuego, as well as Finland, France, Poland and Russia.
These animals are often trapped for their fur. During the early 19th century, trapping eliminated this animal from most of its
original range. The 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; there may have originally been ten times that many beavers in North America
before the days of the fur trade.
Giant beaver
The 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.
Habitat
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
used by many other species. Next to humans, no other extant animal does more to shape its landscape.[citation needed] 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.
Dams
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.[2] The
largest known was discovered near Three Forks, Montana, and was 2,140 feet long, 14 feet high, and 23 feet thick at the
base.[3] 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.[4]
It is primarily 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. 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,[5] 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.
Trees, up to 250 mm (10 inches) in diameter, felled by beavers in one night.
A tree abandoned by beavers, presumably too large
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. 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 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, 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.
Flood control
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.
Wetland creation
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.
Wetlands have significant environmental benefits.
The grazing meadow (vega)
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.
Bottom land
As related above, 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. 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 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
phosporous 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.
Denitrification
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, 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.
Lodges
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 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.
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.
Uses
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.
The only fur in North America which surpassed the beaver's in commercial value was that of the silver morph red fox, which was said to be forty times more valuable.[6]
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.[7] Beaver
testicles were exported from Levant (a region centered on Israel) from the tenth to nineteenth century.[8] Claudius Aelianus comically described beavers chewing off
their testicles to preserve themselves from hunters.[9]
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 Codex for use in dysmenorrhea and hysterical conditions (i.e. pertaining to the womb), for
raising blood pressure and increasing cardiac
output. The activity of castoreum has been credited to the accumulation of salicin from
willow trees in the beaver's diet, which is transformed to salicylic acid and has an action very similar to aspirin.[10] Castoreum continues to be used in perfume production.
Beavers in culture
A Beaver on the Canadian 5 Cent Coin
Popular western culture typically depicts the animal positively, as a good-natured and industrious character.
- The importance of the North American Beaver (C. canadensis) in the economic,
social, and political development of Canada through the fur
trade led to its designation as the national animal of that country. 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
1976 Summer Olympics held in Montreal
with the name "Amik" ("beaver" in Algonquin). 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.
- 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.
- Canadian children between 5 and 7 can join "Beavers", the youngest organization in the Scouts Canada movement. During
meetings, children gather at the "pond" which is the site of their "colony." Within the colongy, the children are divided into
"lodges."[1]
- 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. The beaver also
appears on the state flag of Oregon. 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.
- In the book After Man: A Zoology of the Future by
Dougal Dixon, the modern-day beaver still exists 50 million years from now. However, its
back legs and tail have fused together to make one large paddle for better movement in water.
1911 encyclopedia text
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.
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.
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.
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 neighbourhood they make their "lodges," which
are roomy chambers, with the entrance beneath the water. 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.
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.
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.
Wikimedia Commons has media related to:
Notes
- ^ Return of the Beavers on MSN.co.uk
- ^ http://abob.libs.uga.edu/bobk/bbeavdam.html
- ^ http://home.earthlink.net/~scouters2/beaver.html
- ^ http://www.BeaversWW.org
- ^ http://www.naturealmanac.com/archive/beaver_dams/beaver_dams.html
- ^ Morton, Thomas (1972).
New English Canaan: Or, New Canaan (Research Library of Colonial Americana), pp.188. ISBN 0405033095.
- ^ Yupik
Eskimo home remedies. Retrieved on 2007-06-17.
- ^ PMID 12576209
- ^ Claudius Aelianus: Beaver Testicles and Red-Hot Feline Semen.
- ^ Stephen Pincock (2005-03-28). The quest for pain relief: how much have we
improved on the past?. Retrieved on 2007-06-17.
- ^ http://www.chowdc.org/Papers/Saunders%202001.html
- ^ http://www.jimmyakin.org/2005/02/lent_roundup.html
- ^ (French)Lacoursière, 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 Summa Theologica of St. Thomas Aquinas II. 147:8 provides legal foundation upon which theologians
argued in favour of beaver being like fish.
References
This article incorporates text from the Encyclopædia
Britannica Eleventh Edition, a publication now in the public
domain.
External links
hsb:Bobrnrm:Bièvre
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