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Beaver are rarely seen during daylight hours. Here at Gunpowder Falls, Maryland
Common Name
: North American Beaver, American Beaver, Canadian Beaver – As the beaver originated in Eurasia, the etymology is of ancient origin and consequently arcane. One of the more compelling attributions is the Sanskrit word babhru meaning reddish-brown which evolved to the Norse bjorr, German bibar, Old English beofor and Middle English bever to which an ‘a’ was added to accentuate the long ‘e’ sound. The toponym distinguishes the New World species it from the Old World Eurasian beaver.


Scientific Name: Castor canadensis – Castor is the Latin word for beaver and is logically assigned as its genus. Castor is the name one of the twin brothers, the other being Pollux, which are mythologically ascribed to the two brightest stars of the constellation Gemini. There is no known association between the god Castor and beaver, the former was the god of horses and the latter is a rodent. The species name establishes Canadian provenance.

Beavers are the largest rodents in North America, and, save for the capybara of South America, the largest rodents in the world. The order Rodentia is the largest subgroup of the Class Mammalia in the Phylum Chordata; about 50 percent of all mammals are rodents. Their evolutionary niche is that of gnawing herbivores with opposing upper and lower incisors which grow throughout their lives at a rate of 2 – 5 millimeters per week and must consequently be ground down to an equal and opposite extent by abrasion; calcium is consequently a very important part of the diet. Rodents have two to five sets of rear grinding teeth that are separated from the front buckteeth by a gap with specialized musculature so that chewing to effect comminution for digestion is possible without interference. Rodents must expend appreciable energies in grinding hard and therefore abrasive objects as a matter of quotidian dental maintenance – “bucky” beaver is their epitome.
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Beaver in its preferred habitat


Beavers are the rodent masters of the riparian habitat which they adapt to suit their semi-aquatic lifestyle. According to the fossil record, the proto-beaver originated in Eurasia in the Miocene Epoch about 10 million years ago and crossed to North America over the Beringia land bridge exposed during one of the recurrent ice ages. The Eurasian species (Castor fiber – the species name fiber is an alternative Latin name for beaver so this means literally beaver-beaver) persists mostly due to recent reintroduction into northern Europe and central Asia after having been nearly extirpated by burgeoning human populations. Like their American cousin C. canadensis, they are highly adapted to living in and around water with rudder-like tails, webbed propelling back feet, goggle-covered eyes, valved ears and nostrils, a skin flap to seal their mouth so that they can use the front teeth, and a diving suit of fat and fur that insulates them against hypothermia. The two species are not sexually compatible as evolution has altered their DNA to the extent that the American beaver has lost 8 of the original 48 chromosomes of its ancestral Eurasian predecessor.


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Beaver Lodge
Beavers are nature’s engineers, building dams in streams to create the water reservoirs that are their adaptive habitat - a palladium against predation. Dam construction requires a combination of skills: lumberjack, mason and diver. The process starts with felling trees that form the dam’s lumber foundation on the bank of a properly sized watercourse, a task which must be done mostly at night due to the dangers of diurnal exposure in an area typically monitored by carnivores. From dusk to dawn, a single adult beaver can cut down a 4 inch tree and cut it into 6 foot sections and drag them to the water for dam placement. Dam construction ensues as logs are interspersed with twigs and branches to form a layer that is alternated with gravel and mud consolidation until the structure achieves the necessary girth, height and length to stop and then retain sufficient water for the planned demesne. The top of the dam is finished off with a layer of mud that is exhumed during repeated dives into the upstream side as the pond inexorably expands its hydration area. Beaver dams can be surprisingly large – one notable example was reported in Montana measuring 2,140 feet in length, 14 feet in height and 23 feet in girth. In contrast, the largest human-made embankment type dam located on the Missouri River at Fort Peck, Montana is 21,026 feet long and 250 feet high. However, the Fort Peck dam took over 10,000 workers 7 years to build at a cost of 100 million dollars; the beaver dam was free though likely the resultant effort of several generations of beavers of the same family colony. Beaver dams require considerable maintenance due to the impermanence of the primarily earthen construction materials. The tocsin for dam degradation is the sound of running water which triggers a Pavlovian conditioned response; this was demonstrated convincingly when a beaver responded to a tape recorder playback of water seepage within audible range of its pond by covering it with mud and twigs. Dam repair requires continuous diligence and beavers do eventually move on; their ponds silt up to form boggy meadows in sylvan glades.


Beavers are also nature’s architects, constructing custom-built lodges, which, like Frank Lloyd Wright’s Falling Water, are both in and above the water. The lodge is a sequel to the erection of the dam that creates the pond, the moat surrounding the beaver’s near impregnable castle. In stream habitats with adequate depth so that a dam is not needed, the lodge is built into the stream bank. Lodge building is similar to dam building, as the construction materials must of necessity be the same. The frame of logs is anchored and then more loosely filled with detritus to afford ventilation access to the living chambers, which normally consist of lower feeding and upper sleeping levels. Access to the domicile is through a tunnel portal located underwater, which must be carefully dug out once all else is in place and relatively solid. This requires extensive underwater operations without the benefit of a caisson. Evolution accordingly favored underwater speed and endurance; a beaver can rapidly accelerate to a speed of 5 knots (5 nautical miles an hour or about 5.5 mph) and can stay underwater for 15 minutes – about the same amount of time as many marine mammals. Underwater endurance is achieved not by greater lung capacity but by better programming. Sympathetic nerves constrict the blood vessels directed to the extremities so that more oxygenated blood is directed to the brain and lactic acid which is produced by the muscles in the absence of oxygen accumulates without ill effect during the dive – the resultant lactate salt is oxygenated and excreted once surfaced. The beaver is a veritable aquanaut.


Beavers eat the tender inner bark and buds of trees, they do not eat fish. During the warmer months, nocturnal prandial excursions suffice for sustenance; one can readily localize beaver habitats by
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Beavers eat mostly the inner bark of trees
the characteristic gnawed circumferential lathe-like carvings that girdle trees near their lodge. Girdling kills the tree; the arboreal vascular system conducts water and minerals from the roots (and associated mycorrhizal fungi) with capillary force upward in the inner woody xylem and the nutrients of photosynthesis return by gravity to the roots in the outer phloem which is destroyed by beaver. In preparation for winter when the pond ice may be frozen, beavers harvest bark-bearing tree logs in lengths of two to eight feet and embed them in the mud at the bottom of their pond to be readily and safely accessible. While their favorite tree is the aspen, they can make do with most trees, notably those that are common in riparian habitats like birch, alder and willow. Unless the tree is to be used in dam or lodge construction or for consumed as winter fare under the ice, there is no size restriction other than perhaps appetite; beavers can fell trees up to 46 inches in diameter, a challenge to Paul Bunyan’s reputation as the lumberjack nonpareil.



Beaver were hunted almost to extinction in North America primarily for their distinctive fur but also for the aromatic and medicinal castoreum extracted from their scent glands. The fur trade started as simple barter between Native Americans and the early colonists which introduced beaver coats to Europe. The luxuriant fur was ideally suited to felt used in haberdashery and the top hat became the sine qua non of the British toff in the 17th Century. As demand burgeoned and prices rose, the pelt rush was on. French colonization began with the founding of Quebec in 1608 by Samuel de Champlain to initially dominate the fur trade as coureurs des bois or runners of the woods penetrated the interior along the Saint Lawrence River and the Great Lakes. Likewise the Dutch and later the English after the second Anglo-Dutch War in 1667 moved up the Hudson River Valley in the pursuit of the increasingly rare beaver. To some extent, the quest for beaver pelts is the fons et origio of the colonization of North America. Beaver is the national animal of Canada and the state animal of Oregon and New York in recognition of its foundational role. It is a tribute to the adaptability of the beaver that it survived extirpation due to human depredation of up to one hundred thousand pelts a year.


Beavers have castor sacs located near the genital area that secrete castoreum which is applied to the fur as a waterproofing lubricant and as a pheromone to establish territorial scent boundaries. The discovery and use of castoreum is a matter of antiquity, as the Eurasian beaver have been trapped and skinned for millennia. It is not unlikely that the Doctrine of Signatures was the impetus for its medicinal use. Popularized by Paracelsus in a world of religious absolutism, the doctrine provided that God placed things on earth for mankind with a signature, or sign of its intended use. The syllogistic reasoning for castoreum was: if the pungent scent sacs were near the genital area; then they must be good for genital problems. It was listed in an 1893 handbook on clinical gynecology as a remedy for dysmenorrhea, painful menstruation. Castoreum has also historically been used for a wide variety of treatments including headaches, fever and as a general analgesic; it was listed in the 1820 edition of the United States Pharmacopoeia. One postulate is that some castoreum is comprised at least in part of salicylic acid from the beaver’s consumption of willow bark and would therefore have anti-inflammatory properties similar to aspirin. The use of castoreum in perfumes as a musky animal constituent is also of ancient provenance; archaeological evidence of large scale perfumeries dating to the Bronze Age and the myrrh of the Magi attest to this. In the complex alchemy of 20th Century olfactory stimulation, animal scents, which include ambergris from sperm whales, musk from male musk deer and civet from civet cats in addition to castoreum from beavers, were used as erogenous base tones in most perfumes - Guerlain’s Shalimar for example. The implementation of national and international laws such as the 1975 Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES) gradually suppressed most wild animal depredation and led to the synthetization of chemical formulated equivalents. Research on castoreum revealed over 60 different constituents depending to some extent on the food preference of the beaver, stark testimony to the complexity of the ecological balance of the soils, trees and animals that coexist in nature.


Beavers are a boon or a bane depending on conflicting anthropocentric criteria. Dams and the resultant ponds are a boon for the societal domains of ground water management and forest ecology. There have been several studies over the last several decades undertaken to better understand the complex effects of beaver activity on forest ecology, primarily the retention of water in a series of ponds along the course of a stream. A survey of the Taylor Creek watershed in California found that beaver ponds reduced phosphorous, one of the primary constituents of fertilizer, from 170 to 70 micrograms per liter. The availability of water habitat in winter is also important for other fauna such as migrating birds and spawning fish. Declines in beaver populations in Washington State resulted in about 90 percent less salmon, attributed to the die off of juvenile salmonids due to inadequate water deep enough to remain ice-free in winter. While there have been several beaver reintroduction efforts over the last several decades, the most ambitious has been the Washington State Cascade Watershed repopulation which was initiated to evaluate the effectiveness of beaver dam flood control in lieu of an estimated $10 billion traditional dam remediation. Over a ten year period, 240 beaver were released in about fifty sites resulting in 170 ponds; efficacy evaluation is on-going. Outside their native forest habitats, beaver can be a nuisance. Beavers are frequently a bane from the individual landowner’s pe
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MIT Brass Rat
rspective when land is rendered no longer arable due to flooding or when expensive ornamental trees are girdled for food and possibly dismembered for dam or lodge construction. In 1999, a beaver severely damaged one of the nation’s treasured Japanese cherry trees that surround the tidal basin in Washington D.C.

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Ipana's Bucky Beaver


Beavers figure prominently in cultural applications where engineering and dentation are involved. The MIT class ring is heralded as the “brass rat” for its depiction of the beaver whose skills instantiate engineering excellence. Bucky Beaver was an animated character created by Disney as the mascot for Ipana brand toothpaste in the 1950’s; the ‘brusha, brusha” jingle was a mainstay of Saturday morning cartoon shows of that era.