Kingdom: Animalia
Phylum: Chordata
Subphylum: Vertebrata
Class: Mammalia
Order: Rodentia
Suborder: Sciurognathi
Family: Castoridae
Genus: Castor
Species: Castor canadensis or Castor fiber
Physical description
The beaver is one of the largest rodents in the world. It comes in two varieties: Castor fiber, the European (sometimes called Eurasian) beaver, and Castor canadensis, the North American beaver. The two species are distinct from one another, and though they are similar in appearance and behaviour research has shown that they are unlikely to interbreed, even if they were to inhabit the same continent.
Both kinds of beaver are roughly 2.6-3.6 feet in length, including their tails. (The tail by itself generally measures about ten inches in length, and is flat, broad, and hairless, covered in blackish scales.) They stand about twelve inches in height, measured from the ground to the top of the shoulder; but their hind legs are longer than their forelegs, making a beaver look like it's tilted forward when it walks. Their body build is stocky, and there is little sexual dimorphism: both male and female beavers can range in weight from 37-70 pounds, or 17-32 kilograms.
Their skulls and their teeth are very, very large, disproportionately so in comparison with the rest of their bodies. Their upper incisors are particularly prominent, half a centimetre wide by over two centimetres long (and also bright orange, as if having teeth that stick out isn't bad enough); these are important for felling trees and cutting through branches and logs, all of which are necessary tasks in the building of the dams and lodges where beavers live.
Beavers are primarily aquatic creatures; their hind paws have webs between the toes to facilitate swimming, and they can use both of their forepaws like hands. But so far as their value to humans goes, the most important parts of a beaver are its pelt and its castor glands. The pelt is waterproof and glossy, dark reddish brown with a rich silky undercoat covered over with coarser guard hairs, used for centuries to make fashionable clothing and fur hats; the glands are located on either side of the base of the tail, and the castoreum that they secrete was used in both perfume-making and for medicinal purposes.
Habitat
Both kinds of beavers live in lakes and rivers, generally near forests (as they need the trees to build their dams). Castor fiber was at one point found all across Europe, as far north as Siberia and Scandinavia, south to France, and in Great Britain; Castor canadensis is spread all throughout North America, except for the northernmost reaches of Canada, Mexico, parts of Florida, and the deserts of the southwestern United States.
On the shores of lakes, the banks of ponds, and sometimes on small islands somewhere in the middle of a body of water, beavers live in lodges built from sticks, moss, and grass, plastered with mud to keep it together and surrounded by water. If the water depth is insufficient, a beaver will dam a nearby stream with wood and mud and stones until the area around the lodge has become flooded to surround it appropriately. The single room inside the lodge might measure as much as eight feet wide, by up to three feet high; this is subject to change over the years as the lodge is repaired and sometimes expanded when it suffers damage from the elements. The floor itself doesn't touch the water; it is slightly above water level, and blanketed with wood chips, moss, or grass.
Coming in contact with beaver faeces can result in an infection called Giardiasis (also called beaver fever), so you would do well to take care when in areas populated by beavers. The disease is water-borne; drinking contaminated water from a pond or lake with beavers in it is probably one of the quickest ways to bring a camping or hiking trip to a premature and very unpleasant end. Boiling one's drinking water to purify it is a simple and intuitive way to avoid beaver fever.
Due to the demand for beaver pelts in Europe in the eighteenth and nineteenth centuries, Castor fiber was hunted nearly to extinction by 1860. Today there is a movement afoot to re-introduce the beaver to the areas it used to inhabit; this has happened with success in the Netherlands and Bavaria, and there is increasing interest in re-introducing it to Great Britain, where it became extinct in the sixteenth century.
Behaviour
Beavers are very social creatures; they live in family colonies of up to eight, which usually consist of an older breeding pair plus kits from the most recent litters. Beavers are born in litters of one to five, and the kits are generally weaned from their mother at three months, reaching sexual maturity in about three years. They are monogamous and they mate for life, though if one half of a breeding pair dies the survivor will seek out a new mate. The lifespan of a beaver in the wild is about ten to twenty years, sometimes cut short by disease or by their predators, namely coyotes and wolves but sometimes humans, as well. In captivity they have been known to live as long as thirty-five years.
Beavers are very well-adapted for swimming and diving, with their water-repellent coats and their webbed feet. Swimming they stay underwater for about three minutes at a time, but are capable of diving and remaining underwater for up to fifteen minutes if the need is pressing.
They are herbivores, eating plants that grow in the water and on riverbanks like cat-tails and water lilies, but they are most well-known for eating trees. A beaver prefers aspen, but will also resort to other softwoods like poplar and hazel; hardwoods are only used for construction purposes, in dams and lodges. Since they are so low to the ground, beavers can't reach the soft buds and branches higher up in trees, so they gnaw on tree-trunks until the trees fall. This is the image of the beaver that most people are familiar with: standing upright using its tail flat on the ground to balance and support its weight, gnawing a trunk into an hourglass shape until it is slender enough to fall under its own weight. They are mostly nocturnal creatures, and a single beaver is capable of felling a tree with a diameter of sixteen inches in a single night.
Beavers communicate with each other using scents, in castoreum secreted from the castor glands; posture, like groundhogs; sounds, like high-pitched whistles and whines; and by slapping their tails against the water as they dive to warn of danger.
Their dams and lodges are sometimes detrimental to their habitat; river traffic can be blocked by dams, and as colonies of beavers move to new places, the areas around the streams that they inhabit can be severely damaged by flooding. Conversely, their dams also create new micro-environments where other creatures and plants can thrive. Also by stopping the flow of running water, they help to slow erosion.
We Are the Beaver: the fur trade and the making of a national symbol
The North American variety of beaver, Castor canadensis, was one of the first natural resources to be exploited by European explorers in the New World. In fact, the fur trade was perhaps the best reason for establishing colonies in what is now Canada, after it was determined with absolute certainty that this new world was, indeed, not the Orient. Their dreams of finding a Northwest Passage to the Pacific shattered, the explorers and settlers of the new colonies needed an excuse for sticking around: that reason was the fur trade, which proved to be very lucrative indeed, even in its earliest days.
In the late seventeenth and early eighteenth centuries, high fashion in Europe called for fur hats made from beaver pelts -- and as the European beaver was hunted to extinction, the North American beaver population conveniently numbered about six million. Under Henry IV, New France became a fur-trading empire in its own right, with entrepreneurs ranging further and further into the continent's interior searching for friendly First Nations peoples who might be inclined to trade beaver pelts for European goods. Traders licensed by the competing French and English governments and unlicensed traders (coureurs de bois) alike were soon selling pelts back in Europe for over twenty times what they had paid for them in North America. During the peak of the fur trade, over a hundred thousand pelts were being shipped overseas annually.
In 1670 the Hudson's Bay Company was founded to facilitate the fur trade. The success that it saw motivated its founders to incorporate the beaver into its coat of arms in 1678, and there it remains: in the middle of the coat of arms, a St. George's Cross divides a shield into four quadrants, each of which contains a beaver. Shortly thereafter a coin was minted, which was equal in value to a single beaver pelt; this is why to this day the Canadian nickel depicts a beaver on its obverse.
Again because of its importance to the French colony's economy, in 1678 the governor of New France suggested that it be named New France's official emblem. During the French and Indian Wars, a number of English assaults on Montreal and Quebec took place; after a final failed attack on Quebec in 1690, the French crown struck a commemorative medal which also had a beaver on it -- representing Canada, the beaver was depicted at the feet of a seated woman, who represented France.
Even after New France was ceded to England after the Seven Years War, the beaver retained its symbolic importance. It was incorporated into the armorial bearings of Montreal, and it even appeared on Canada's first ever (three-penny) postage stamp in 1833, at the behest of Sir Sandford Fleming.
The fur trade fell off as all things eventually must when European fashion turned away from fur hats, but the humble beaver wasn't forgotten: its fate was sealed on 24 March 1975, when an "act to provide for the recognition of the beaver (castor canadensis) as a symbol of the sovereignty of Canada" was passed by Parliament. This is not only a nod to the importance of the fur trade in Canadian history, but also to the character of the beaver itself: it is often anthropomorphised and thought of as being a hard-working and honest creature, which traits also characterise the stereotypical Canadian. Most fortunately, unlike the European beaver, the North American beaver managed to survive the fur trade; the beaver population across the nation is still flourishing.
The beaver as a symbol isn't just limited to Canada; it is also the state animal for Oregon and the state mammal for New York, as well as a mascot and emblem of schools like the California Institute of Technology, Oregon State University, and the Massachusetts Institute of Technology.
Rebecca Anderson. "Castor canadensis". University of Michigan Museum of Zoology. (10 April 2005) http://animaldiversity.ummz.umich.edu/site/accounts/information/Castor_canadensis.html
N. Harris. "Castor fiber". University of Michigan Museum of Zoology. (10 April 2005) http://animaldiversity.ummz.umich.edu/site/accounts/information/Castor_fiber.html
"Beaver". Wikipedia. (10 April 2005) http://en.wikipedia.org/wiki/Beaver
"The beaver". Heritage Canada. (10 April 2005) http://www.pch.gc.ca/progs/cpsc-ccsp/sc-cs/o1_e.cfm
See drawings of beaver tracks at http://www.bear-tracker.com/beaver.html | 
