Summit Outside: Understanding beaver biology | SummitDaily.com
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Summit Outside: Understanding beaver biology

David Hannigan/Colo. Division of Wildlife
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Editor’s note: This is the first in a three-part series examining the beaver.

Beavers are an integral part of American history and commerce. They are considered a “keystone species” in riparian (the interface between land and a river or stream) communities.

They are unique among mammals in that they alter their habitat to meet their needs, primarily by damming up small rivers and streams to form ponds, and their skills as builders in the animal kingdom are unparalleled. This is the first of a three-part series on the beaver.



Beavers are found frequently in Dillon Reservoir and the rivers and wetlands of Summit County. They are semi-aquatic mammals spending approximately 80 percent of their time in water.

Adult beavers have few predators and may live up to 20 years or more. Beaver continue to grow in size throughout life, and reach 60 or 70 pounds where food is abundant and accessible during the entire year. Females are as large as males of the same age, and sometimes larger.



The beaver has a paddle shaped, leathery tail, about 10 inches long, 5 or 6 inches wide, and 1/2 inch in the middle. Their large flat tail, serves as a rudder when swimming, a prop when sitting or standing upright and a storehouse of fat for the winter.

When startled or frightened in the water, a beaver will rapidly dive while forcefully slapping the water with its broad tail. This is heard over great distances above and below the water and warns other beavers in the area. Once a beaver has sounded the alarm, nearby beavers will dive and may not re-emerge for some time.

Beavers webbed hind-feet are much larger than the forepaws, and have specialized claws for digging. The hind feet of beavers are fully webbed, with five toes and strong nails including a unique split toenail on one toe which serves as a comb for grooming. Hind feet often measure 6 inches in length, and the spread of the toes is equal to or greater than the length.

The front feet are small in contrast to the hind feet. Front feet measure 2 1/2 to 3 inches in length and are not webbed.

Beavers normally swim with their front feet held against their chest, with the large, webbed, hind feet providing the propulsion and the tail acting as a rudder. Their large lungs store lots of air, and they can submerge for about 10 minutes.

Beavers have special muscles that seal their ears while they are diving; and their eyes are protected with a pair of see-through eyelids. Although beavers are fantastic swimmers they move very slowly on land.

Guard hairs in beaver fur are 2 inches in length, and overlay a soft and dense under fur about an inch deep. Colors vary from blonde to nearly black.

Both male and female beavers have large glands, called castors, beneath the skin on the lower bellies. These glands produce oil which the beaver combs into its fur to waterproof it. This oil is also secreted at selected locations as territorial markers or mating attractants in the spring of the year.

Adult beavers mate for life and a beaver colony may include a pair of adults, yearlings and kits. They breed once a year during the winter (January to March) and gestation lasts four months.

During this period the female makes her den warm and comfortable by

splitting soft wood into thin chips.A litter of is born, with fur, front teeth and their eyes open. The kits spend most of their time in the lodge, where they can stay relatively safe from predators. As soon as one-half-hour after birth, they can swim, and if tired, may be carried on their mother’s back.

The young remain with their parents for two years before they leave to start their own families. A single family unit is called a colony, and is typically made up of six to eight individuals that consist of two adults, that year’s kits and the young from the previous year. Young beavers may travel more than 30 miles before finding a mate and a place to settle.

Beavers maintain and defend territories, which are areas for feeding, nesting and mating. They invest much energy in their territories, building their dams and lodges.

Beaver feed on woody plants, consuming leaves, bark, buds, sap, roots and fruits. In winter when available food is restricted to an underwater cache, the diet may be restricted to bark.

Beavers consume a large amount of cellulose. About 32 percent of the cellulose is digested by symbiotic bacteria which break down the indigestible material (cellulose) in the small intestine. The beaver’s small intestine is very long, and is the major site for nutrient absorption and digestion is also enhanced by special stomach glands.

The beaver is a very important animal in American culture. Beavers play a variety of roles in Native American folktales from different tribes. In some tribes, they are portrayed as hardworking and persevering, while in others, they represent selfishness and stubbornness.

In some stories, Beaver causes floods or droughts by building dams without considering how they will affect other animals. In some tribes of Alaska, beaver is the culture hero, a benevolent transformer who uses his wits to slay monsters and shape the world to the benefit of the people. “Beaver” is a very popular name for places, and businesses. There are at least a dozen in the Denver area. We have the ski area Beaver Creek. The mascot for MIT is the beaver. The “Adventures of Paddy the Beaver” is a popular children’s story.

Breckenridge resident Dr. Joanne Stolen is retired from teaching microbiology Rutgers University, and has taught classes at CMC. She is now pursuing a career in art, specializing in nature and many of the animals she writes about. Her work can be seen locally.


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