North American Beaver: Facts, Dams, Lodges and Diet

Appearance: teeth, tail and waterproof fur

A beaver is built for life in water. It has a stocky, rounded body, a broad head with small ears and eyes, and short legs. The thick fur is usually dark or reddish brown. It has two layers: long, glossy guard hairs on the outside and a dense, fine underfur that traps air and keeps the skin dry and warm.

The most recognizable feature is the tail. It is flat, paddle-shaped and covered in leathery, blackish scales. The beaver uses it as a rudder when swimming, as a prop when sitting upright to gnaw a tree, as a fat store for winter, and to sound the alarm with a loud slap on the water.

The hind feet are large and webbed, and they drive the beaver through the water. The front feet are small, clawed and remarkably dexterous. Beavers use them to dig, carry mud and stones, and hold sticks while they strip off the bark. A split claw on each hind foot works as a grooming comb.

At the front of the mouth are four large, chisel-like incisors with bright orange enamel. They never stop growing, as explained in why are beaver teeth orange? The beaver can close its lips behind these teeth, so it can gnaw underwater without swallowing water. Its ears and nostrils seal shut when it dives, and a clear third eyelid protects the eyes.

How big is a North American beaver?

Adult beavers are about the size of a medium dog, but much heavier for their height. Males and females look alike and are similar in size. Beavers in the north tend to be larger than those in the south.

The capybara of South America is the world’s largest rodent. The North American beaver and its close relative, the Eurasian beaver, are the next largest. The Eurasian species is slightly heavier on average, though the heaviest individuals recorded come from North America. See how big is a capybara? for the record holder.

Habitat and range

Beavers need two things: water that is deep enough to protect them, and trees or shrubs to eat and build with. They live along slow streams, rivers, ponds, lakes and marshes, from the edge of the Arctic tundra to the deserts of northern Mexico, and from the Atlantic to the Pacific coast.

Before European settlement, North America may have held 100 to 200 million beavers, by commonly cited estimates. Centuries of trapping wiped them out of large areas. Today, thanks to protection and reintroduction, they are back in most of their former range, and are even spreading onto Arctic tundra as shrubs grow there in a warming climate.

People also moved the species outside North America. In the 1940s, beavers were released in Tierra del Fuego at the southern tip of South America, where they spread widely and damaged native forests. In 1937, North American beavers were released in Finland, and their descendants also live in nearby parts of Russia (Karelia). The European Union now lists the species as an invasive alien species.

Dams, lodges and canals

Beavers are best known for their building. A family dams a stream with branches, logs, stones, grass and mud to create a pond. The deep water protects them from land predators, lets them float heavy branches, and in cold climates keeps water from freezing to the bottom. Learn more in why do beavers build dams?

Their home is a lodge: a dome of sticks plastered with mud, built in the pond or against the bank. Entrances open underwater, and inside is a dry chamber above the waterline. A small gap at the top lets air in. In winter the mud freezes almost as hard as concrete. An experienced family may build a lodge about 2 m (6.5 ft) high and up to 6 m (20 ft) across above the water. Where water is already deep, as in large rivers and lakes, beavers often live in bank burrows instead and may not build dams at all.

Beavers also dig canals from their pond to stands of trees. Canals can be over 0.5 km (0.3 mi) long and let them float branches home instead of dragging them over land, where they are more exposed to predators.

What do North American beavers eat?

Beavers are herbivores. They do not eat the wood itself but the soft inner bark (the cambium layer), along with twigs, buds and leaves. Favorite trees include aspen and other poplars, willow, birch, alder and maple. In spring and summer they eat many soft plants, such as water lilies, cattails, grasses, sedges and ferns.

In the fall, families in cold regions sink a large pile of branches into the pond near the lodge. When ice covers the water, they swim out under the ice to fetch food from this underwater pantry. More detail is in what do beavers eat?

Bark is hard to digest. Beavers have a long gut and a large cecum, a pouch in the intestine full of microbes that break down cellulose, the tough material in plant cell walls. They can digest roughly 30% of the cellulose they eat.

Behavior and family life

Beavers live in family groups called colonies. A colony is usually an adult pair and their young from the last one or two years, often about four to eight animals in all. Pairs often stay together for many years. Young beavers help repair dams and lodges and care for younger siblings before they leave home.

Beavers are mostly active at dusk and at night. They are excellent swimmers, reaching about 8 km/h (5 mph), and can stay underwater for up to about 15 minutes. On land they are slow and clumsy, so they rarely travel far from water.

Each family defends a territory. Beavers mark it with scent mounds: small piles of mud and plants on which they leave castoreum, an oily, musky substance from glands near the base of the tail. Neighboring families read these mounds as “keep out” signs. Beavers also communicate with whines, hisses and growls, and with the famous tail slap.

Reproduction and life cycle

Beavers breed once a year. Mating takes place in winter, mostly in January and February in northern areas and earlier in the south. After a pregnancy of about three and a half to four months, the female gives birth in spring, usually between April and June.

A litter has about three to six kits, most often four. Kits are born with fur and open eyes and can swim within a day or so. They stay in the lodge for about a month, start eating plants within a few weeks and are weaned by about three months. Most young leave the family at about two years old to find a mate and a territory of their own, sometimes traveling many kilometers along waterways. Most beavers first breed at about three years old.

Lifespan, predators and threats

Wild beavers usually live up to about 10 years, though some reach 15–20 years. Adults have few natural predators. Wolves, coyotes, cougars and black bears take beavers, especially when they are on land. Kits are also taken by lynx, bobcats, foxes, large birds of prey and, rarely, river otters.

People were once the beaver’s biggest threat. Today the main causes of death in many areas are road traffic, conflicts over flooding, disease and harsh winters.

Conservation status: a comeback story

The North American beaver is listed as Least Concern on the IUCN Red List, with a stable population. Recent estimates put the population at roughly 10 to 15 million, still only a fraction of the numbers before the fur trade.

From the 1600s to the mid-1800s, beaver pelts were among the most valuable goods in North America. Felt made from beaver underfur was used for fashionable hats in Europe, and the search for beaver drove much of the exploration of Canada and the American West. By around 1900, beavers had disappeared from much of their range. Hunting limits, protection and relocation programs, including famous airdrops of beavers by parachute in Idaho in the 1940s, brought them back.

Beavers and people

Beavers are a keystone species, meaning their effect on the ecosystem is far larger than their numbers suggest. Their ponds slow floods, store water during droughts, trap sediment, raise groundwater levels and can act as green firebreaks during wildfires. Many land managers now use beavers to restore streams.

The same activities can cause problems when beavers flood roads, farmland or culverts or cut down garden trees. Water-level devices and tree guards often let people and beavers live side by side.

The beaver is a national symbol of Canada and appears on the Canadian five-cent coin. It is also the state mammal of New York and Oregon, which is nicknamed “The Beaver State”. In Europe, its relative the Eurasian beaver is also making a comeback, including in Britain, as described in are there beavers in the UK?