Nature News from Greater Yellowstone #14 9/29/20

My first memory of being aware of a live beaver goes back to my early teens.  On a wilderness canoe trip with a couple of siblings, cousins and a brave aunt and uncle who exposed us to 10 nights in the Maine wilderness, we heard trees crashing down one dark night.  My petrified self was relieved to hear the explanation that beavers worked hard at night and they are expert tree fellers!   The next morning I wandered away from camp looking for them, but, of course, the only evidence was some gnawed stumps and down trees that had branches nibbled off and hauled away.

Cottonwood tree felled by beaver

Castor canadensis, the Beaver, is North America’s largest rodent. Adult beavers typically weigh 45 to 60 pounds, but have been known to grow to 100 pounds.

Getty images

Native Americans greatly respected beavers, calling them “Little People”. Beavers and humans are alike in their ability to modify their habitats to suit their own needs. They’ve been called “nature’s architects” . For this reason, they are considered a Keystone Species.
(Oxford dictionary: a species on which other species in an ecosystem largely depend, such that if it were removed the ecosystem would change drastically.)

A recent article was published talking about the fire break affect beaver ponds create in a wildfire event.  For more interesting information about that, see the link below. Be sure to watch the video!

Beavers find food and building materials by toppling large trees using nothing but their specially adapted incisor teeth and powerful lower jaw muscles. Beaver teeth never stop growing, so they do not become too worn despite years of chewing hardwoods. Beavers eat only plants including woody and aquatic vegetation. They eat fresh leaves, twigs, stems, and bark. Beavers will chew on any species of tree, but preferred species include alder, aspen, birch, cottonwood, maple, poplar and willow. Beaver also feed on cattails, water lilies, sedges and rushes. Cattail and water lily tubers are favorites. Using their nimble front paws, beavers will roll lily pads like cigars to eat them. Beavers do not eat fish or other animals.

Beaver Nibbling on vegetation, Getty Images

If one has the good fortune of seeing a beaver, they will first notice a large flat tail, which serves as a rudder when swimming, a prop when sitting or standing upright, and a storehouse of fat for the winter. Beavers slap their tail on the surface of the water as a danger warning to other beavers. Their front “paws” are like our hands which give them tremendous dexterity.  The back feet are webbed making it like they are wearing flippers for swimming!  Like many water creatures, their eyes are protected with a nictitating membrane, a clear membrane covering  the eyes as they dive underwater  and a flap in the back of the nasal cavity (similar to moose) preventing them from breathing in water.

Beavers live in “colonies”, typically family groups.   As I mentioned early, beavers are great builders.  They alter the landscape to form ponds and canals in slower moving stream environments by building dams. The average beaver colony will dam a half-mile length of a small stream. Lodges are often built in the center of a pond, or occasionally on the bank. Once the water is deep enough, the lodge is protected. Each lodge contains at least two water-filled tunnels leading from the main chamber to the pond so the beavers can enter and exit the lodge underwater without being spotted by predators. The walls of the conical lodge are very strong due to layers of mud and sticks ( which are replenished each year) , and are extremely insulated.

Beaver dam in Yellowstone
Beaver dam and ponds

What you’d see if you crawled inside a beaver lodge!

Diagram from pin-interest

Even with subzero outside temperatures it will not drop below freezing inside the lodge.  Their fur has both fine hairs for insulation and longer hairs for waterproofing.  Under their bellies is a castor gland which excretes and oily substance used in grooming and marking territory.

On larger rivers or fast moving streams, the beaver burrow in to a bank to form their homes. The entrance to the burrow is concealed as they pile sticks and secure the structure with mud, similar to building a free standing lodge in a pond they have constructed. Beaver do not hibernate so in the fall months they fill up their “pantry”. They will collect and store vegetation underwater They stay inside their lodge all winter except when they swim under the ice to their food cache for a stick to nibble on.

Beavers are monogamous and pair bond for life. They do not breed until they are two to three years old. Breeding occurs in the den, typically in January and the young are born, 1-6 kits, 107-110 days later. These young will nurse for 2-3 months. The availability of food appears to affect the size of the litter. Each established beaver “colony” consists of adult parents, and two years of offspring. The 2 years olds will typically leave the colony to find one of their own.

Beaver have lived in the greater Yellowstone area for millennia.  Beaver pelts were a valuable trade item in the recently formed US and Canada.  They were frequently acquired through trade with Native Indian people in the plains. 

This image has an empty alt attribute; its file name is beaver-hat.jpg
Beaver hat

Late 1700s the course fur trappers began building trading posts and continued a legacy of trading and trapping that came to a close when beavers became scarce by the 1840s.  By that time, a demand for buffalo (bison) hides replaced the beaver skin demand.  Much of the GYE was remote enough, the population of beavers remained intact. 

From the NPS/Yellowstone website below:

The first survey of beavers in the park, conducted in 1921, reported 25 colonies, most of them cutting aspen trees. Although it was limited to parts of the northern range, comparing the locations of those beaver colonies with subsequent survey results demonstrates how beavers respond and contribute to changes in their habitat. A 1953 survey found eight colonies on the northern range, but none at the sites reported in 1921 and a lack of regrowth in cut aspen. Willow were also in decline during this period.

To help restore the population of beavers on Gallatin National Forest, 129 beavers were released into drainages north of the park from 1986 to 1999. Park-wide aerial surveys began in 1996 with a count of 49 colonies and increased to 127 by 2007; dropping to 118 in 2009 and 112 in 2011. While the long-term increase is partly attributable to the improving ability of aerial observers to locate colonies, the park’s population of beavers probably has grown in the last 15 years. Some of the increase likely came from beavers dispersing from the national forest, but they would not have survived without suitable habitat. The increase has occurred throughout the park and is likely related to the resurgence in willow since the late 1990s, at least on the northern range, and possibly in the park interior. Nearly all of the colonies documented in recent years were located in or near willow stands, none near aspen.

Robust shrubby willow

Willow, which is more common in the park than aspen, is a hardier shrub that quickly regenerates after being clipped by beavers. The reason for the prolonged decline and relatively sudden release of willow on the northern range, and whether aspen have begun a sustained surge in recruitment, are topics of intense debate. Possible factors include the relationship of these plant species to changes in the abundance of beavers and elk, fire suppression, the reintroduction of wolves, and climate change.

The next time you hear the term “busy as a beaver” you’ll really have an idea how these mighty landscape changers have to keep moving all the time. They are hardworking and industrious!

Wishing you all some more beautiful autumn days and Happy trails from Greater Yellowstone. If you would like to contact me send a note to

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