Beavers

Did you know…
Beavers are built for underwater work. Their noses and ears have valves that close when beavers submerge. The beaver's large front teeth-or incisors-protrude in front of their lips, enabling them to cut and chew submerged wood without getting water in their mouths.

Did you know…
With their strong jaws and teeth, beavers can chew through a six-inch tree in 15 minutes. A single beaver can chew down hundreds of trees each year. An orthodontist's nightmare, a beaver's front teeth never stop growing; beavers must gnaw, chew, and chop nearly all the time. So by keeping up their homes, beavers are also keeping down their dental bills.

History

According to the Canadian Wildlife Service, no other animal has influenced a nation's history to the extent that the beaver has influenced the history of Canada. When Europeans began to settle in northern North America, beaver trapping was one of the primary activities, with The Hudson's Bay Company actually being founded in 1670 chiefly to trade for beaver fur. Despite their celebrated status, trapping drove beaver populations to the brink of extinction by the late 19th century. Canada was home to an estimated six million beavers before the fur trade began. With up to 200,000 pelts being shipped to Europe each year during the peak of the fur trade it was only the introduction of legislation that stopped trappers from wiping out the entire species in North America. Further help came in the form of the beaver conservation movement which began in the late 1930s with the writings and lectures of Grey Owl. Present day populations have rebounded to a certain extent, but trapping is still prevalent in most parts of Canada.

Habitat Range

Beavers are found throughout Canada, all the way north to the mouths of the Mackenzie and Coppermine rivers on the Arctic Ocean. Most common in forested areas, beavers also expand into unforested habitats where there are watercourses bordered by deciduous (broad-leaved) trees or shrubs. Thus, in western Canada, they are found along streams on the dry prairie. Even in the tundra, beavers occasionally colonize shrubby water edges where water is deep enough to allow for food storage and access to the den under the winter ice.

Frontline Ecologists

Beavers are more than intriguing animals with flat tails and lustrous fur. American Indians called the beaver the "sacred center" of the land because this species creates rich habitats for other mammals, fish, turtles, frogs, birds and ducks. Since beavers prefer to dam streams in shallow valleys, much of the flooded area becomes wetlands. Such wetlands are cradles of life with biodiversity that can rival tropical rain forests. Almost half of endangered and threatened species in North America rely upon wetlands.

"As the dam is not an absolute necessity to the beaver for the maintenance of his life, his normal habitation being rather natural ponds and rivers, and burrows in their banks, it is, in itself considered, a remarkable fact that he should have voluntarily transferred himself, by means of dams and ponds of his own construction, from a natural to an artificial mode of life"
Lewis H. Morgan, The American Beaver and his Works, New York 1868

Besides being a keystone species, beavers reliably and economically maintain wetlands that can sponge up floodwaters (the several dams built by each colony also slows the flow of floodwaters), prevent erosion, raise the water table and act as the "earth's kidneys" to purify water. The latter occurs because several feet of silt collect upstream of older beaver dams, and toxics, such as pesticides, are broken down in the wetlands that beavers create. Thus, water downstream of dams is cleaner and requires less treatment.

General

Beavers are active usually in morning and evening. Their main activities are cutting trees for building or repairing lodges and dams, or for winter food. They can fell large trees (8 cm diameter in 5 minutes). They use their upper incisors to cut a ring around the trunk at the height of their mouth when standing on their hind feet and tail. They sink their lower incisors into the trunk below the ring and lever out a large chip. Trees are trimmed and either dragged to the building site, cut into logs, or even floated in a canal dug by the beaver. The site for a lodge must be in water deep enough to provide room for storage of food and to keep the entrances under water. To ensure water depth in a stream, a dam is built. First untrimmed trees are laid down in a narrow part of the steam so that they dig into the stream bottom. Then a layer of mud and stones is laid, much of which seeps into gaps between branches. Layers of mud and branches follow. The upstream face of the dam is waterproofed with mud. Dams have been found 1640ft long by 13ft high (500 x 4 metres). Lodges are built by piling logs and branches held down by rocks and mud until the mass reaches 3.3 to 6.6ft (1 - 2 metres) above the water level. The beaver then submerges and gnaws its way into the mass, making access tunnels and chambers with raised sleeping platforms. If the water level rises and enters the chamber, the beaver gnaws away the roof and the detritus raises the floor. The outside of the lodge is plastered with mud leaving a ventilation hole at the top.

"...consciousness is an inseparable and essential quality of the mental principle. When a beaver stands for a moment and looks upon his work, evidently to see whether it is right, and whether anything else is needed, he shows himself capable of holding his thoughts before his beaver mind; in other words, he is conscious of his own mental processes."
Lewis H. Morgan, The American Beaver and his Works, New York 1868.

Reproduction and Development

Beavers are monogamous usually but if one mate dies, the other will "remarry". Family groups consist of two adults, several two year olds and the young of the current year. They mate first at about three years old. Gestation is 128 days. Litters of 2 to 6 are born in April and May. Kits can swim when a few hours old; weaned at one month. The mother carries the kits in her mouth supported on her front legs while walking upright on her hind legs and tail. Young leave or are forced out of the colony by two years of age. Large lodges may have several family groups. Family life is cooperative, all help with the hard work of gathering food, building and repair.
Beavers that are forced to leave the family pond will often travel downstream from the original pond. There they may start a new pond and a life of their own. This can cause a chain of ponds leading down a stream as successive generations of beavers build their way down. Trapping is still the most common source of mortality.

"The use of the curve in beaver dams is of very common occurrence, and it has always been regarded as a striking evidence of the intelligence of its builders."
[...]
"It is generally asserted that the introduction of a curve, with its convexity up stream, was the result of intelligence and design on the part of the architects; and that its use at the precise point where the pressure of the water is the greatest, affords conclusive evidence that the beavers understood its mechanical advantages.Whether these curves were the result of accident or of design is a question."
Lewis H. Morgan, The American Beaver and his Works, New York 1868

Adaptations

Very efficient swimmers, using hind feet and tail. Physiological adaptations to their aquatic life allow them to stay under water for up to fifteen minutes. They have valves in the nose and ears which close automatically on submerging, the mouth closes behind the incisors so that they can chew underwater. The eyes have membranes which can be drawn over the eyeball. They have an oversized liver to deal rapidly with byproducts, and enlarged lung capacity and high tolerance to CO2. Their blood supply can be diverted from the paws to ensure supply to the brain and their metabolism can slow down to conserve blood supply.