Beavers Are Nature's Builders

Enlarge Image Image Gallery Since their incisors grow constantly, beavers actually have a need to gnaw. Yva Momatiuk & John Eastcott/Minden Pictures

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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.

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