Beaver - Natural History


Text by Liz McKenzie and Photos by Richard Nelson

The North's Busiest Animal

The North American beaver is a crafty, intelligent, and industrious animal best known for ingeniously altering its environment to create the watery world it needs. Beavers are famous for their dam building skills and their elaborate lodges. But much of their lives go on hidden inside their houses and under thick winter ice—and what we can't easily see of the beaver's life is just as impressive as what we can see.

Beavers (Castor canadensis) are the largest members of the North American rodent family, but they look very different from familiar rodents like mice and rats. Adult beavers have thick bodies that can weigh 50 pounds or more and reach 3 to 4 feet long. They have rich brown fur, webbed hind feet and a large, flat, paddle-shaped tail.

As impressively large as beavers are, they'd be dwarfed beside their distant ancestor, the giant beaver. This mega rodent lived during the last Ice Age, tipping the scales at about 200 pounds and measuring 8 feet long.

The modern beaver's distribution is the best demonstration of this amazing animal's success. Beavers live from western Alaska across all of Canada, and south through nearly all of the lower 48 states. Mexico was also once part of this animal's range, but it has mostly been exterminated there.

Of course, beavers are famous for their rich, soft, luxurious fur. They also have glands that produce an oil, called castoreum, used by beavers to mark their territory, but also prized as a trapping lure, a perfume ingredient, and an additive to many sweets and desserts.

The Beaver's World

Beavers are water animals, spending much of their time plying the liquid world around their dens, carrying sticks to feed piles, building and repairing damns. Although awkward and vulnerable on land, they must make forays for food and building materials. Numerous beaver trails lead to and from the water.

On land, beavers are prey to wolves, wolverines and bears. In the winter, they are less vulnerable but not entirely safe. Wolverines sometimes dig into a beaver den and eat the inhabitants. Otters can swim into the lodge at any time of year and this is bad news for beavers. Sometimes otters will even occupy a lodge, after killing the beavers that lived there.

Beaver are strict vegetarians: their food includes bark, cambium, twigs, buds, and leaves from trees like willow, aspen, poplar, alder, and birch. They also eat other terrestrial plants and roots, as well as aquatic plants like cattails and water lilies. Contrary to a common belief, beavers do not eat fish.

The beaver's most important tool is a pair of large, wide incisor teeth that make excellent chisels. They'll gnaw around the base of a tree until it falls, then cut it into smaller pieces that they can drag to the water, and then swim with sticks and branches trailing behind—all of this is made possible by those strong, sharp teeth.

Beavers also use their dexterous front paws to work with mud, stones, and sticks–the raw materials for their lodges, dens, and dams.

Sometimes beavers live in a den tunneled into the bank of a river or lake.  These are called "bank dens" and they're pretty common, especially in rivers where damming isn't possible and periodic flooding would wash away a lodge. 

Beavers are incredibly hard workers, and with good reason. During the short, ice-free time they must do everything possible to ensure winter survival—the lodge or den must be in good repair, the dams secure, and the food they need for 8 months of winter must be cut, moved, and stored. And this is where the genius of beavers is most evident.

The family will rest inside their house during the summer days, then head out to work and gather food through the long twilight hours. What do beavers do with all that food? Near the lodges and dens, they maintain a huge feed pile. It's mostly underwater, so beavers can reach it even after thick ice forms over the rivers and ponds, but usually some sticks poke out above the surface not far from the lodge—a telltale sign of the feed pile.

Beavers work so hard that they become lean by the end of the summer. Luckily, if all goes well, they'll have the winter to loaf around and grow fat again.

Winter Life

During the long winter months, beavers stay relatively warm in their lodges or dens. These can be big, elaborate, multi-roomed structures up to 12 feet wide and 6-8 feet high—built from sticks, stones, and mud. Usually 6 or 7 beavers live in the house, but there can be as many as 12. There's even an air intake on top of the lodge.

There are two underwater entrances to the lodge, giving beavers a chance for escape if one tunnel gets blocked or an otter shows up. After freeze up, beavers exit the lodge and swim under the ice to their food stockpile, never having to surface in the frigid outside air.

Beaver lodges may house three generations—an older pair mated for life, a few adolescent offspring, and the kits born the previous spring.

Snug in their lodges and well fed, beavers regain the fat lost during the busy summer months. They mate in January or February. Before she gives birth, the mother beaver sends her mate away to a nearby den.

The kits are born in May or June—usually 3or 4 in the litter. They're already prepared for the world--fully furred, sharp teeth, good eyesight, and the ability to walk and swim–but the kits spend at least a month in the den before starting to explore their surroundings.

If all goes well, beavers can live to be 12 years old.

Humming Along

Anyone who has walked beside a beaver pond or paddled a lake where beavers live knows the sound of the beaver's tail slapping on the water.

This noise is a warning that you've come to close…or a warning to other beavers that there is danger nearby.

Beavers can be noisy chewers too. Listen to this one whittling the bark from a summer-green branch at the edge of an Alaskan pond.

If a person is really lucky, they might hear the sound of a beaver humming. A Koyukon elder explained this in a beautiful way:

"You know, beavers work hard all fall and winter, so every spring they take a vacation. They drift down the river someplace or they wander around through the lakes. Then, later on in summer they start heading back home, and it really makes them feel happy. So that's when they like to sing the most, while they swim along towards home."

- Steven Attla as told to Richard Nelson, Make Prayers to the Raven

The Dam Builders

Beavers build and maintain dams—sometimes just a few feet long to block a tributary creek but, sometimes big ones are several hundred yards long and more than six feet high. Perhaps the biggest known beaver dam is in Wood Buffalo National Park, Alberta, measuring over a half-mile long….2,790 feet to be precise. This is more than twice as long as the famous Hoover Dam in Arizona and Nevada.

Why do beavers work so hard to dam the waterways around their homes?

First, when a beaver moves into an area, building a dam across a creek or small river will create a pond. Then the beaver can build a lodge. But to maintain the pond, the dam must be kept in good repair.

A beaver's security lies in its ability to live inside lodges, which are only accessible via an underwater entrance. So in the summers, the water helps to protect the beaver from some of its predators.

In the frigid northern winters, beavers could not survive for long out in the open air—they need access to their home and feed pile via ice-free water. If the water is not deep enough, the pond could freeze right to the bottom and the beavers would be cut off from their home and food cache.

Beavers are meticulous builders and diligent repairers of their handiwork. It's not uncommon for a dam that's broken during the day to be repaired during the night, so that water levels can rise again by morning.

They also build canals—watery pathways to their food sources, which help them to transport the fruits of their labor back to the main pond, lake or river.

Beaver dams are incredibly important for habitat enrichment—they create rich wetlands that support wildlife like ducks, geese, frogs, and fish; and they nurture a complex plant community that's ideal for big animals like moose and small ones like songbirds.

So while the energetic, industrious beaver is busy creating and maintaining a good living situation for itself, it is shaping a world that is good for others too.

Works Cited:

Nelson, Richard K. Make Prayers to the Raven: A Koyukon View of the Northern
. Chicago: University of Chicago Press, 1983. Print.

Mark Bethka