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“A beaver colony in the wilds gives a touch of romance and a rare charm to the outdoors. The works of the beaver have ever intensely interested the human mind. Beaver works may do for children what schools, sermons, companions, and even home sometimes fail to do— develop the power to think. No boy or girl can become intimately acquainted with the ways of works of these primitive folks without having the eyes of observation opened, and acquiring a permanent interest in the wide world in which we live.”

Enos A. Mills, 1913, known as Father of Rocky Mountain National Park

On a recent March visit to Heron Pond, a part of the Cache River State Natural Area, it was a duck-kind of day, gray and raining. There were no wildflowers blooming, and birds were hidden or silent. Yet there was a lot to see and ponder. I noticed an increase in beaver activity. Quite a few small cypress saplings had been cut; along the boardwalks, I noticed many debarked twigs and branches, several trees bore scars from gnawing, and some new raised mounds had appeared. Back in Champaign, I visited our library, where I discovered the book The Beaver, Nat-ural History of a Wetlands Engineer by Dietland Muller–Schwarze and Lixing Sun; and I talked with Illinois Natural History Survey mammalogist Joe Merrit. My observations took on new meaning; they were more than just visions of gnawed wood!

The North American beaver, Castor canadensis, is one of the largest rodents in the world. Its scientific name Castor comes from the Greek for beaver; and canadensis refers to Canada, where the type specimen was collected. Common names include flat-tail, bank beaver, castor, and castor cat.

In North America, beaver occur from coast to coast and range from Alaska, Hudson Bay, and northern Labrador in the north to the United States–Mexico border, Gulf Coast, and Florida state line in the south. Their habitat must include a year-round supply of water for swimming and diving, floating logs, safety from predators, burrow entrances, and storage of winter’s food supply.  Beaver always escape into the water. They reside along rivers and streams or in small lakes bordered by stands of trees. Beaver are rare in sandy areas because mud is not available for their lodge. They normally avoid areas that flood often, but they are able to adapt! Once they have found a site, beaver rarely move and often stay in the same location for several years.

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In Illinois, they can be found throughout the state, but this was not always the case. An estimated 400 million beaver lived in precolonial North America. Beaver played a major role in the settling of North America, with a prime pelt netting about $65 during colonial times. Beaver were the backbone of the North American fur trade. During the Lewis and Clark Expedition, the number of beaver sighted and caught were noted, as well as beaver lodges, dams, beaver-cut trees, and trees suited for beaver. The country had “beaver fever.” Due to unregulated trade, beaver began to disappear. By 1850, they had all but disappeared from Illinois. In 1861, in an account of the mammals of Illinois, Cyrus Thomas wrote, “That any individuals of this species now exist in this State is doubtful, yet possible ….”

Illinois began restocking beaver during 1929 when a pair was released at the Savanna Army Depot in Carroll County. Several more pairs were released there; and eventually, offspring from these moved to other areas or they were live trapped and dispersed to other locations in the state. Beaver were also released at several sites in the southern half of the state, and emigration has occurred from neighboring states. Today, whether good or bad, every county in Illinois supports beaver.

A Compromise of Life on Land and Water

Adult beaver range in length from 36 to 53 inches and weigh 30 to 50 pounds. They have rich “beaver brown” fur—which is a range of brown, black, or yellow hairs that is slightly paler on the underside. Their fur is dense and heavy. It has a waterproof outer layer of guard hairs, which are underlain by a dense layer of insulative fur. Beaver maintain the waterproof quality of their fur by using specialized claws on their hind-feet to apply oil that is taken from two abdominal glands. These double claws located on the two inner toes serve as a handy comblike feature and are unique to beaver.

Their oval, dorsoventrally flat tails are covered with black scales and are used for temperature regulation, fat storage, communication, a rudder during diving and underwater swimming, and a prop for balance when they are cutting trees. Webbed hind-feet serve in the water while non-webbed forepaws are used for digging, grooming, grasping, and manipulating.

While underwater, a nictitating membrane covers their eyes. This membrane acts as a transparent inner eyelid covering the eye and permits a clear field of vision. Their ears, nose, and mouth can be closed voluntarily, with their lips closed behind their incisors. Adapted for gnawing, beaver teeth are very hard. Their four incisors have a bright orange enamel layer and are formidable chisels that continue to grow. Living a crepuscular and nocturnal life, a beaver’s sense of smell is acute, while their eyesight is poor. They have small eyes and ears but an extremely sensitive nose, allowing them to smell their food before they choose it.

Beaver form permanent breeding pairs, with mating taking place in winter. Females give birth to one or more kits during May or June. The kits begin to eat tender tree twigs after 2 weeks; and at a month, they begin to explore outside the lodge. The young beaver leave the family group after 2 years, usually right before the next litter is born. Dispersal is usually around the time of the spring thaw. While they do not like to travel any farther than necessary, they can disperse more than 30 miles. In the first 2 to 3 years of life, mortality is highest. They can perish from predators, disease, starvation, or trapping. Although now they have few predators, historically, wolves were important predators. Humans have now taken on that role. A beaver in the wild can live up to 10 years.

Adults are always busy as they repair dams, fortify the lodge, clean out dens, dig canals, cut down trees, scent-mark, patrol their pond, feed and preen themselves, and bring leafy twigs to the kits in the lodge.


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Beaver need a mixed diet. They consume bark, leaves, twigs, and roots of woody plants growing near the water. Grasses, sedges, and rushes are usually consumed during the summer months. Food habits vary seasonally with plant forage availability. Cottonwood, aspen, poplar, birch, alder, willow, and maple are some of the preferred tree species; and these are also used as building materials. Many of these species grow fast and have soft wood that is easy to chop down and peel.

Beaver normally forage on small trees no more than 300 feet from the water, and only one beaver gnaws on a tree at a time. Small trees are gnawed from one side but larger ones are gnawed from all sides resulting in the hourglass shape of the cut. They do not know the direction the tree will fall. While holding the stick or branch in their front paws they chew the bark. A feeding bed (many debarked sticks floating on top) results where beavers continue to consume the bark of twigs and logs in shallow water

How do they choose a tree? Observations near a wetland often have trees that have been nibbled but then left alone. It appears the beaver are assessing nutritional value. Distance from the water plays a role; the greater the distance from the water, the more selective the beaver are in what they choose and the smaller the diameter trees they cut. Beaver cut only small specimens of less preferred trees, while size doesn’t seem to matter for their favorites. During late winter and spring, conifers are accepted due to renewed sugar flow. Tree cutting intensifies in the fall because beaver prepare for winter by building an underwater food cache. These food caches do not meet all their energy requirements; so during the winter, the adults reduce their body temperature and metabolism and lose weight.

Beaver store energy for lean times in two forms—a food cache near the lodge and as body fat. Fat accumulates not only in “normal” areas, but also in the tail. From spring to fall, the volume of the tail doubles: As the fat is used, the dimension of the tail decreases.


The tail slap is the most familiar alarm signal. It is given in response to any disturbance near the beaver pond, with tail slaps by older beaver eliciting more escape responses than those by younger beaver. Inside the lodge, a variety of noises are made—burps, whines, gnawing, chewing, gargling, and bubbling. (Over indulgence at Thanksgiving comes to mind!) On the water, hissing is common. And at night, a beaver’s gnawing can be heard. Young beaver whine, a sound that has been compared to a baby crying.

Beaver also communicate by chemical signals—a scent mound serves as a warning that this area is occupied. The beaver deposits dredged mud on land close to the bank. It then arches its back, straddles the pile, and applies castoreum or anal-gland secretions. A mound elevates the point of odor release, while the moist mud helps to intensify the odor. The raised mound also offers high ground when water levels fluctuate. The majority of scent mounds are constructed during the spring.

Beaver have several unique structures, including their pouchlike castor sacs. These sacs contain a yellowish fluid called castoreum, which is made up of many different compounds, one of which is castoramine. Castoramine is also found in pond lilies. Many of the compounds in the castoreum are also compounds plants use to defend themselves.

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Nature’s Engineer

Called one of the animal kingdom’s architectural masterpieces, the beaver dam is built to impound water along streams. These impoundments keep the lodge entrance under water, allow the floating of logs and branches, provide winter food storage, and implement travel to feeding areas. The larger the pond, the more potential feeding range is available without increasing risky land travel. Dams are built in late summer or fall.

Building materials are varying lengths of logs, rocks, wads of grass, and mud: All are interwoven, with the shape depending upon topography and water flow. The optimal site would be slowly moving water with a stream gradient of 1% or lower, but dams may be built in a variety of aquatic locations.

The lodge is the principal shelter for the beaver colony; and they may share their lodge with muskrats, mice, voles, and moths. Lodges range from a burrow in the side of a steep slope to the more familiar free-standing mound surrounded by water. Using logs and mud, a winter lodge can be built in two nights. The entrance is under water, and the beaver rest on a platform above the water. In the summer, beaver prefer bank lodges because they are cooler.

What Good Are Beaver?

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The work of beaver helps maintain a healthy hydrologic balance. Their dams help replenish the water table and store water. Stored water and a raised water table are helpful to plants and animals during a drought. The water-flow pattern is altered, reducing erosion. When they open up forests along the streams, they create new habitats—ponds, swamps, and meadows. New habitat attracts other organisms, and soon a complex community develops.

As stated by Joel Greenburg in his book, A Natural History of the Chicago Region, “Even in an area teeming with millions of people, the marvels wrought by this amazing animal can still unfold. On a forest preserve land in northern Cook County, beavers transformed what had been an ordinary field into a beautiful open water cattail marsh. In early April, hundreds of newly arrived tree swallows skimmed the smooth surface for insects. Painted turtles soaked in the spring sun, and chorus frogs filled the air with music. Yellow-headed blackbirds, pied-billed grebes, great blue herons, and other water-dependant birds nested or tarried during migration …. Then, after a few years, the beaver disappeared. There hasn’t been as much water or wildlife since, but some day a young beaver looking for a home of its own will come upon the site, find it hospitable and stay.”

Susan L. Post is the staff writer for The Illinois Steward magazine and a scientist with the Illinois Natural History Survey, Institute of Natural Resource Sustainability at the University of Illinois at Urbana–Champaign. This article was supported with Fish and Wildlife Restoration Funds, U.S. Fish and Wildlife Service through the Illinois Department of Natural Resources.