The Carlisle Mosquito Online

Friday, February 8, 2002


Biodiversity Corner

Just when you thought the big story on unapproved tree-felling was over, I bring you the American Beaver or Castor canadensis or tree-feller-extraordinaire. A Kwakiutl Indian name for the beaver is Throw-Down-in-One-Day.

Evidence: photographed by Midge Eliassen, January 2002, in the Great Brook State Farm State Park. Midge did not see the beavers themselves since they are nocturnal, but her evidence of their presence is damning.

Distinguishing characteristics: The beaver is easily distinguished by its size. It is the largest rodent in North America and the second largest in the world ­ the capybara of South America being larger. Typical weight is in the range of 35 to 70 pounds, with a record size of 115 pounds recorded in 1981. Perhaps its most defining feature is the flat, broad, scaly tail about ten inches long and six inches wide. The tail is used to store fat, to warn other beavers of danger by slapping it on the surface of the water; and as a rudder. Like other rodents, beavers have large front teeth that continue to grow throughout their lives. To prevent becoming hopelessly long in the tooth, they wear these front teeth down by grinding them on the lower teeth. The front surface of the teeth is made of a very hard enamel. The back surface is not as hard and the grinding results in a chisel-like shape perfect for felling trees.
Incredible beaver activity shows up at Great Brook Farm State Park in the pond area near the dam by North Road. (Photo by Midge Eliassen)

Tree-felling: Beavers fell trees for food or for building dams and lodges. A beaver can drop a tree of five-inch diameter in less than 30 minutes. The partially chopped tree in the photo above was untouched the day before. Only one beaver at a time works to fell a tree, but once the tree is down all members of the colony chip in and cut the branches into small lengths. Tree-felling by beavers is a haphazard affair. Trees are chopped on whatever side is most convenient without consideration for where they may fall.

Adaptations for life in the water: Beavers have special muscles to close their ear canals and nasal openings to keep water out; they have an inner transparent eyelid to allow underwater vision; they are extremely efficient at extracting oxygen from the air in their lungs (they can use 75 percent of the available oxygen compared to our rather meager 15 percent) and can therefore stay underwater for as long as 10 to 15 minutes; they waterproof their fur by applying oil from the castor glands at the base of the tail; their underfur is very dense and traps air next to the skin for insulation; and they have large hind feet with webbed toes.

Diet: Beavers are herbivores. They eat leaves, young twigs, and the bark of trees and shrubs as well as roots and stems of aquatic plants. They do not eat the hardwood. Their favorite trees include willow, maple, poplar, beech, birch, alder, and aspen. Samuel Hearne, the 18

Housing: Beavers build lodges of piled branches and logs covered with smaller branches and plastered with mud, next to the shore, on islands, or in the still water behind a dam. The branches on a beaver lodge have no bark. The lodge has two or more entrances tunneled from below and a central chamber which is the living space. A lodge may be used for several years with successive generations of beavers adding to it. While they can become as large as eight feet tall and forty feet across, typical lodges are half that size. The lodge in the photo at top right is at the edge of an island at Great Brook Farm State Park. It is hard to be certain that it is in use since the entrances are below. Beavers will also build nesting burrows in river banks. This seems to be the strategy when the water is too deep or too swift to dam and so is unlikely at Great Brook.
An abandoned beaver lodge is shown on an island in "Beaver Pond." (Photo by Midge Eliassen)

Other Construction: Beavers are well known for building sturdy dams. The sound of running water seems to be the stimulus for dam building. The purpose of the dam is to create a pond and flood the vegetation which can then be eaten without leaving the safety of the water.

Other Notes: Beavers produce an orange-brown oily substance called castoreum from castor glands at the base of the tail. Castoreum is used in France and the Orient for making perfumes, and used by the beaver to mark mounds they build of mud and vegetation. These scent mounds can be up to two feet tall and may have a territorial function. Beavers don't hibernate (it would tarnish the brand image of industriousness). They are monogamous and live in family groups of six to eight members made up of the parents, yearlings and kits. A family group or colony in a prime habitat could be as large as 12.

Environmental Impact: Beavers modify their environment to suit their purposes more than other animals except humans. While their dams can interfere with our plans for land use, this is not a problem in areas set aside for wildlife habitat and for our appreciation of wildlife.

References: Donald & Lillian Stokes, Guide to Animal Tracking and Behavior; University of Michigan Museum of Zoology (at enter 'beaver' in the search window).

Ideas for species for the Biodiversity Corner are encouraged from anyone in town. The only requirements are that the species exists in the wild and was seen in Carlisle. Tell me what you are finding. Send a note to Kay Fairweather at 392 School St, Carlisle MA 01741 or to

2002 The Carlisle Mosquito