Beavers

Beaver Facts

Classification:

Kingdom: Animalia

Phylum: Chordata

Subphylum: Vertebrata

Class: Mammalia

Subclass: Theria

Infraclass: Eutheria

Order: Rodentia (rodents)

Suborder: Castorimorpha

Family: Castoridae (beavers)

Genus and species: Castor canadensis (North American beaver), C. fiber (Eurasian beaver)

Geographical location: North America (C. canadensis), Europe and Asia (C. fiber)

Habitat: Ponds, lakes, and streams, except in southern Florida and the deserts of the southwestern United States

Gestational period: Three months

Life span: Twenty-five to thirty years in captivity; ten to twelve years in the wild

Special anatomy: Castor glands in both males and females are used for intraspecific communication. Webbed hind feet and a broad, flat tail allow for excellent swimming ability. Their dense fur is water-repellent. Beavers are semiaquatic mammals that live in ponds, lakes, or streams. Most live in groups called colonies. Together, a colony will construct a habitation, called a lodge, constructed of mud and sticks. Bank beavers inhabit cavities in the sides of stream banks rather than lodges. Beavers typically build lodges in the middle of a pond or lake to reduce their vulnerability to larger predators such as wolves.

A beaver colony consists of a family comprising the breeding pair and their offspring from the previous two years. Typical colonies consist of two to eight individual beavers. Beavers appear to be monogamous, and females are dominant over males and juveniles, at least during parts of the year. Members of the colony work together to maintain the lodge, build dams, and defend their territory against other beavers. Beaver colonies occur in typical densities of 0.4 to 0.8 colonies per square kilometer, though they can occur at higher densities when suitable habitat is plentiful.

The Beaver Life Cycle

Beavers have a single breeding season, in winter, during which mating occurs. Females experience a period of estrus that lasts about two weeks, during which they are receptive for ten to twelve hours. Gestation lasts for about three months. Litter sizes range from three to six young born each year. Beavers are born in a relatively well-developed state; their eyes are open, and they are fully furred. Young beavers nurse from their mother for two or three months, though beavers begin eating some solid food at only two weeks of age. Beavers reach sexual maturity at three years, at which time they are forced by their parents to leave the colony.

Beavers are herbivorous and eat the woody parts of trees and shrubs. Favorite foods include aspen and alder trees. Beavers will chew down large trees, not to gain access to the large trunks, which are impossible for beavers to move, but rather to gain access to smaller branches growing at the top of the tree, which the beavers eat.

Active year-round, beavers must plan to provide food for themselves during the winter. During the warmer months, the beavers will group branches together in the water near their lodges. During winter, when the pond or stream ices over, beavers swim under the ice to gain access to these stick piles as a source of food. A beaver lodge is constructed with an underwater entrance. Thus, beavers can enter and exit the lodge during the winter without ever coming up above the ice.

Beavers are known as important habitat modifiers due to their dam construction behavior. Beavers fell trees by chewing them, and then they use the tree trunks and their branches to build dams along streams. Dams cause water pooling and slow the water flow behind the dam, increasing water depth and creating suitable areas for building lodges. Furthermore, beaver dams cause localized flooding that can dramatically alter landscapes. Once beavers have used all the surrounding vegetation in dam construction or as food, they will relocate to a new area. The abandoned beaver pond eventually fills in to become a “beaver meadow,” which supports a variety of plants that would not otherwise grow in that location.

Physical Characteristics of Beavers

Beavers are one of the largest rodents, second in size only to the capybara. The two species in the family Castoridae are similar in many aspects. Both species show little sexual dimorphism, or a difference in appearance among males and females. They have a normal range in body mass from 11 to 32 kilograms (24 to 70 pounds). Both male and female beavers have castor glands, which produce a substance called castoreum. Beavers deposit castoreum on piles of mud outlining the boundaries of their territories, in essence creating a message board for other beavers that pass by. Beavers also have broad, flat tails which are used not so much for packing mud, as is commonly believed, but instead as a means of communication. Beavers smack the water surface with their tails, producing a loud noise that can easily be heard by other animals. Beaver tails are also used for directional control while swimming.

Beavers are dark brown, and their thick fur coats contain a dense underfur that traps air and thus insulates the beaver. Long guard hairs provide a protective layer over the underfur. Their webbed hind feet function as excellent paddles when swimming. Forefeet are not webbed and thus are more capable of manipulating objects. Split claws on the first and second digit of the forefeet are used to groom the dense fur, therefore maintaining its water repelling and insulating properties.

Unlike other mammals, beavers do not hibernate during the winter. Because there will be less food availability during this period, beavers must stockpile much of what they consume beforehand. Beavers store much of their winter cache of food under the cold water, which helps it last longer. They will also be less physically active to conserve their energy and reduce their consumption needs.

In North America, beavers have made a dramatic comeback in numbers after being hunted almost to extinction in the late nineteenth and early twentieth centuries. In Europe, beavers have fared less well and in some areas are listed as threatened or endangered, due mostly to destruction of suitable habitat. North American beavers are also an invasive species in parts of South America. Beavers may face predation from wolves, coyotes, human hunters, wolverines, bears, foxes, and lynx.

Although the number of beavers in North America has been restored, many advocates are calling for an expansion in the protection of this species. Many detractors point to flooding caused by beavers and the destruction of woodlands; however, advocates point out that beavers assist in the issue of climate change. Beaver activity helps create and maintain wetlands. These environments can store carbon and inhibit its release into the atmosphere, where it will cause global temperature increases. According to scientists, wetlands can absorb more than ten times the amount of carbon as soils. Wetlands also serve as natural firebreaks, which hinder the spread of wildfires that are an annual hazard in the United States and Canada. The magnitude of Canadian wildfires in 2023 and their impact on the air quality in the northern portion of the United States attests to the scale of this environmental issue.

Principal Terms

castoreum: a secretion from the castor glands, used in scent marking

Castoridae: rodent family to which beavers belong

sexual dimorphism: a marked difference between males and females in some aspect of animal morphology, such as body size

Bibliography

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