Distribution:

The whole of North America except for the extreme northern regions of Canada, the deserts of the southwest United States, Mexico and Florida.

Habitat:

Almost entirely aquatic. While beavers are most home in water, they are quite capable of traveling on land, and do so regularly to reach nearby trees. Beavers are comparatively slow and awkward on land though, and are not able to move as gracefully. A single pond will consist of a family group living in burrows in stream banks or in lodges of mud, stones and tree branches. These lodges are built by the beavers themselves.

Access to both burrows and lodges is sub-surface for protection from predators. Due to the construction of the lodge, many predators are unwilling to take the time to dig through the walls to get at the beavers safe within. In the rare cases when this has happened, the beavers have usually escaped into the pond. To assure adequate water depth, streams are dammed using branches, logs, mud and stones. Lodges are usually between 8 and 12 feet in diameter (2.4 to 3.5 meters) and have heights of around 3 feet (1 meter), but can be found as large as 16ft by 6 1/2ft high (5 by 2 meters).

Food:

Despite the large front teeth of the beaver, these animals are herbivores. Meaning they eat plants only. Most commonly they eat the bark of certain hardwoods such as poplar, aspen, birch, cherry, willow, maple and alder. Aspen and pond weeds are their favorite natural foods. Among "domesticated" or beavers that have been raised by humans, they have been known to eat a number of fruits such as apples.

Description:

Length of head and body: 22 to 27 inches for an adult beaver (0.55 to 0.70 meters) , Length of tail: 12 to 16 inches (0.3 to 0.4 meters), Weight: 30 to 68 lbs (13.5 to 31 kg).

The beaver is the largest rodent in North America and second only to the Capybara in the American continents. Beavers are relatively short and squat, with are large rump. Each foot has five toes, with the hind feet being webbed to help the beaver swim. The second toe on each hind foot also has an extra claw (a double claw) to help the beaver groom. Beavers have small, short ears and have been known to stand using their hind feet only. Typically the tail is used to help balance the beaver while they perform this manuever.

Beavers have two different types of hair, a long tough hair (guard hair) that grows through a shorter softer layer of hair. This "underlayer" of fur helps insulate the beaver in colder or wet weather. Beaver's outer hair is a dark chestnut brown. The "underlayer" is usually a softer, reddish tinted brown. Usually their heads and underparts are slightly lighter than the rest of the body. This is not the case with the black subspecies of beaver.

The two incisors in each jaw grow continuously as they are worn away. Four rootless molars are selenodontic (having crescent ridges of hard enamel on the crowns which are used for cutting the fibres of their wood diet - compare to the smooth crowns of the molars of ruminants). Dentition is 1 0 1 3 indicated in the absence of cuspids and canines to tear and cut flesh. A Beaver's chewing force is 80 kg (176 lbs) compared to man's 40 kg. (88 lbs).

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.

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. 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. Both males and females have scent glands which exude oily musk (catoreum) which is used for trail marking, to attract mates and to waterproof fur. Their efficient digestive system allows for utilization of a large cellulose intake.

The beaver's large incisors are kept sharp by having hard enamel faces and softer dentine backs. The dentine wears away faster than the enamel as the animal gnaws, leaving a keen biting edge of enamel at all times. These incisors never stop growing, so the beaver has to chew or grind its teeth continually to keep them ground down to a manageable length. If a beaver breaks a tooth or otherwise distorts its bite, the incisors elongate, force open the mouth permanently and cause the animal to starve.

Threats to Survival:

The young are threatened by bears, wolves, wolverines, lynx, fishers and otter. Despite the dangers posed by these natural predators, man has proven to be far more dangerous to beavers. Due to their dependence on water and forests, beavers are very susceptible to many forms of pollution and deforestation. Pollution may cause wound infections, or even poison beavers. Silting in streams damages the stream and pond ecosystem, and deforestation presents a lack of food. Beavers are seen as a nuisance by many construction and developments as they require large areas of relatively flat land for their dams. Most cases nowadays start with the dam being repeatedly damaged in order to make the beaver "choose" to leave the desired area. Should this technique fail the beavers are often caught and transported to another region.

Status:

Common. The danger of extinction now seems remote, after the depredations of the 19th century which were halted by legislation. In 1600 there were 60 to 100 million beaver in North America. Whitemen exported 50,000 skins annually until by 1800, the beaver was extinct east of the Mississippi. Legislation has protected the beaver and today 175,000 pelts are harvested annually. In some places the beaver has re-established itself to nuisance proportions.

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 curved against the current but it is doubtful if this is by design. 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.

Effect on the Environment:

In terms of the scale with which animals affect the overall environment beavers are second only to man. Their massive transformation of woodland into ponds, some of impressive size, is impressive to say the least. Unfortunately, the activities of beavers in changing water levels and cutting timber are often regarded by man as nuisances. Beavers have found creative uses for many waterworks. Storm run-offs and drainage systems often provide ready-made streams. A bridge with debris grids built into it (designed to prevent large pieces of wood from floating further downstream) can be used as an extremely strong base for a dam. Pond areas created by a beaver might be considered a hazard for nearby developments, so will be slowly released to restore the "natural stream".

The beneficial effects are described by Bruce Wright in Wildlife Sketches - Near and Far; he calls the beaver "a key to wildlife abundance". Young beavers separate from their families at about two years. They find an area where tender young poplars grow. They start to build a dam. Upstream they build a lodge; as the pond fills up, they collect a raft of poplar branches for winter feed. As the pond grows, sediments falling from the trees and silt carried by the stream settle to form a fertile muck; pond weeds and lilies flourish. As years pass the beavers, having exhausted the supply of poplars move on. Water lilies and pond weeds run riot. The pond now provides range for moose, waterfowl and other wildlife. As the dam decays, the water level falls, a sedge meadow appears and white-tailed deer replace the moose. As coarse grass takes over, woodcock move in. Finally the dam gives way and the fringe of alders spreads to deposit rich leaf mould.

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