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Beavers Are Nature’s Builders

Author Photo
By John Marshall | Aug 14, 2009

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Since their incisors grow constantly, beavers actually have a need to gnaw.
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This lodge houses an entire family of beavers and may have several entrances.
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If need be, a beaver can stay under water for up to 20 minutes.
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Patient and persistent, gnawing beavers are capable of felling large trees.
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Beaver dam nestled in Grand Teton National Park.
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This industrious rodent causes some costly damage with flooding.

Perhaps no other wild animal has played a more important role in the history and economy of North America than the beaver (Castor canadensis). The demand for beaver fur in Europe in the early 1800s helped drive the exploration and later the white settlement of much of the United States and Canada. Widespread and uncontrolled trapping nearly wiped out the beaver from much of its original range. Today, beaver populations have recovered to the point where they are considered pests in many areas. 

Basic biology

The beaver is North America’s largest rodent, with adults reaching anywhere from 30 to 80 pounds, and occasionally as much as 100 pounds. They are found throughout most of North America; the exceptions are parts of the southwestern deserts, the arctic tundra and most of the Florida peninsula. Beavers mate for life, although they will find a new mate if one of the pair dies. They live in an extended family group known as a colony, with all members of the colony being related. A typical colony consists of an adult pair, offspring from the previous year that have yet to reach sexual maturity, and the current offspring (called kits). Upon reaching sexual maturity, the younger beavers are driven out of the colony. Members of the colony will defend their territory from other beavers that try to move in.

Part of the beaver’s scientific name (Castor) comes from the castor glands located near the anus. These glands secrete a strong musk that beavers use for marking their territory, depositing the scent on mounds of grass and mud near the water’s edge.

Beavers are highly adapted to an aquatic environment, possessing thick, waterproof fur, webbed hind feet for swimming, and nose and ear valves that close underwater. They can slow their heart rates while under water and use oxygen stored in their livers, which allows them to stay under water for as long as 20 minutes. The beaver’s most notable characteristic is its flattened, muscular tail. Beavers use their tails as rudders when swimming, as a balance when sitting on land (such as when they are gnawing on a tree) and to slap the water as a warning of an approaching predator.

Like all rodents, beavers have large (in their case, very large) front teeth called incisors. Also like all rodents, their incisors grow continuously, which leads to the constant need to gnaw on things to keep uncontrolled tooth growth in check. Beavers can also close their lips behind their incisors so they can chew underwater without getting water in their mouths. For beavers, trees are the favorite objects of their sizeable incisors. Tree bark is chewed off for food in a process called “girdling,” in which all the bark is removed around the circumference of the tree. Since food and water move through the inner bark, girdling almost always results in the tree’s death.

Contrary to some popular beliefs, beavers do not eat fish or other animal matter. They are strict vegetarians, feeding mostly on the bark of a wide range of tree and shrub species, such as cottonwoods, willows and alders. When available, beavers actually prefer to feed heavily on aquatic herbaceous plants, like duck potato, pond weed and water lilies. Herbaceous plants make up most of the beaver’s diet during the spring and summer, with bark from woody plants being consumed primarily in the late fall and winter. When woody plants are eaten, beavers usually go for the small diameter trees (usually less than 5 inches) and branches. Much of this is stored in food caches inside their dens for use in the winter. This is especially important in northern parts of their range where lakes and ponds may be frozen for several months a year.

The beaver colony will stay in the same area as long as there is sufficient food. Once the food supply has been exhausted (most of the trees cut down), the beaver colony will abandon the area in search of new habitat. During this migration phase, they become more vulnerable to predators.

Because of their large size and aquatic nature, adult beavers have relatively few natural predators. These include bears, mountain lions, wolves and alligators. In addition to these predators, beaver kits and juveniles may also fall victim to otters and mink. The major predators of beavers are, and probably have been for centuries, humans. 

Beaver activities

Beavers also use their teeth to cut down trees for use in dams and to construct dens called lodges. Their habit of constructing dams makes beavers one of the few animals besides humans that actively modify their landscape. It is also the trait that most often brings the animal into conflict with humans. Dams are usually constructed on small to medium-sized streams, with sufficient flow to produce a pond. The ideal pond depth is 2 to 3 feet. This is deep enough to provide refuge from potential predators. The dams are built using a variety of trees, sticks, mud and leaves. Most trees used for dam construction are less than 1 foot in diameter, although there are records of beavers using trees as large as 5 feet in diameter and 150 feet long. If the dam is broken, or breached, beavers will set about to repair the damage in short order.

Dams are frequently constructed in areas of restricted water flow, such as culverts passing under roads and at bridges. This can produce flooding of roads and other areas used by humans.

Once a pond has been formed by the dam, beavers may build a lodge. The lodge is a dome-shaped den constructed of tree limbs and mud, and it is usually placed in the middle of the pond, although a lodge can sometimes be constructed against the bank. The lodge is a permanent residence in which beavers give birth, store food and live. It consists of a living chamber and at least two (sometimes more) entrances.

Beavers do not always build lodges. On larger streams and rivers, they frequently dig bank dens. Bank dens are similar to lodges in their construction, consisting of a living chamber and a food cache area. Also like lodges, bank dens have at least two entrances, both of which are under water to provide protection against potential predators. 

Problem areas

Beavers are both loved and hated for their activities. We marvel at their industrious nature and their ability to modify their environment, while complaining about the impact their actions have on us. The building of dams is a beaver activity that can be especially troublesome. One study in the southeastern United States estimated that flooding from beaver dams destroys more than $20 million worth of timber annually. Besides dam building, beaver foraging on timber trees, orchard groves, agricultural crops and ornamental plants can cause significant economic damage.

Beaver problems can be dealt with in a variety of ways, both lethal and non-lethal. First, let’s consider the non-lethal means. Beaver dams can be breached, and a pipe (called a beaver pipe) inserted into the dam to allow water to drain through. The upstream part of the pipe is surrounded by a wire mesh to prevent beavers from plugging it up. This does not eliminate flooding, but reduces its effects without eliminating the beavers. Dams may also be physically demolished or sometimes blown up. Of course, if the beavers are not dealt with first, they will simply rebuild the dam as fast as possible.

Trees and shrubs can be protected by a variety of means, such as the use of chemical repellants. Plants may also be physically protected from beavers by placing hardware cloth, metal flashing or even tiles around them. Although this may be effective on a limited basis, such as in a yard or park, physical exclusion from large areas, such as commercial timberland, orchards or agricultural fields, is not practical.

Live trapping and removal of beavers to new habitats is another possibility, but not always practical. Unless the beavers can be placed in an area far from human habitation, it is likely they will become a problem again. If beavers are placed in habitat currently occupied by other beavers, someone will have to move and find a new place to live. This new location may once again place the beavers in conflict with humans.

In many instances, lethal control is the only economical and practical alternative. In these cases, the beavers are typically trapped or shot. Of course, once they are removed from an area the now-available habitat may be re-colonized by beavers from surrounding areas. If a landowner feels she must resort to lethal means, she should always check with her state wildlife agency first concerning appropriate regulations.

While some landowners may simply want the beavers removed, others may want to keep them under control by trapping them for their fur. This can provide a supplemental source of income for the landowner without getting rid of the beavers entirely. Some states also maintain a list of private individuals who will trap nuisance animals for either a fee or sometimes for the fur. Once again, your state wildlife agency will have information on regulations concerning beaver trapping. In addition to state wildlife agencies, the local cooperative extension service office can provide a wealth of information about dealing with beaver (or any other wildlife) problems. 

Finding room

Of course, having beavers on your land is not necessarily a bad thing. Beaver ponds provide wildlife habitat for everything from fish and frogs to otters and ducks. Wood ducks are especially fond of beaver ponds, since the dead timber provides hollow trees in which to nest. A beaver pond can be stocked and managed for fishing just like any other pond, and the best thing is, the beavers will build it for free. During droughts, beaver ponds may provide a critical source of water for both livestock and wildlife, and during times of flooding, beaver dams can play a critical role in reducing downstream flooding.

In spite of the many problems they cause, beavers remain a fascinating part of our wildlife heritage. Making a place for them can enrich both the landowner and the land.

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