American Beaver

The North American beaver is the largest rodent in North America, native across the continental United States, Canada, southern Alaska and parts of northern Mexico. It is semiaquatic, with a large scaly paddle-shaped tail, webbed hind feet and dexterous front feet. Mainly active at night, it can stay submerged for up to 15 minutes. Beavers build lodges of sticks, twigs, rocks and mud, and dam streams to make deep ponds that give refuge from predators. They eat bark, buds, leaves, twigs and roots, and in cold regions store branches underwater as a winter food cache.

About the American beaver

The American beaver is one of two living beaver species, alongside the Eurasian beaver, and it's the largest rodent in North America. It's also the official state mammal of Oregon and New York and one of Canada's national symbols. The species is widespread across the continental United States, Canada, southern Alaska, and parts of northern Mexico, and has also been introduced to Patagonia and parts of Europe, including Finland and Karelia.

Physically, the beaver is built for a life split between land and water. It has a large, scaly, paddle-shaped tail and webbed hind feet, while its smaller front feet are unwebbed, clawed, and highly dexterous — useful for digging, feeding, and maneuvering sticks while gnawing bark. A nictitating membrane covers the eyes so the beaver can see underwater, and its nostrils and ears seal shut when it's submerged. Its lips even close behind its front teeth, letting it keep gnawing wood while its head is underwater. A thick layer of fat beneath the skin provides insulation, and the fur itself is a combination of long, coarse outer hairs and short, fine inner hairs, usually dark brown. Scent glands near the genitals produce castoreum, an oily substance the beaver spreads through its fur to waterproof it.

Adults usually weigh between 11 and 32 kg (24 to 71 lb), with 20 kg (44 lb) being typical, though the species follows Bergmann's rule — animals from northern populations tend to run larger. In New York, average adult male weight was recorded at 18.9 kg (42 lb); in Ohio, both sexes averaged 16.8 kg (37 lb). Very old individuals can exceptionally exceed 40 kg (88 lb), and some have been recorded as heavy as 50 kg (110 lb). Like all rodents, beavers have continuously growing incisors, and they're hindgut fermenters — their cecum houses symbiotic bacteria that help digest the tough, lignocellulose-heavy plant material that makes up their diet.

Behavior and diet

Beavers are active mainly at night and are strong swimmers, capable of staying submerged for up to 15 minutes. Because they're more vulnerable on land, they stay in the water as much as possible. Their flat tail does double duty: it slaps the water's surface to signal danger to other beavers, and it stores fat.

Beavers are best known for building dams, which create the deepwater ponds that let them escape predators and reach food safely. Dams are constructed from branches the beavers cut with their strong incisors, along with rocks, grass, and mud; where wood is scarce, dams may be built largely from rocks. The sound of running water is what triggers a beaver to start building or repairing — a phenomenon striking enough that in one experiment, a beaver buried a tape player playing running water in branches and mud, even though it was on dry land. The largest known beaver dam, discovered by satellite imagery in 2007 in Wood Buffalo National Park, Alberta, stretches 2,790 ft (850 m) — more than twice the width of the Hoover Dam. Beavers build their lodges from sticks, twigs, rocks, and mud, with underwater entrances and dry platforms inside, and they plaster the structure with mud that freezes into a concrete-like shell by winter. In colder climates, they also stash a winter food cache of branches underwater near the lodge, accessible by swimming beneath the ice once the pond freezes over.

Beavers are herbivorous generalists with a diet that varies by taste, texture, and smell as much as availability. They eat bark, cambium, branches, twigs, roots, buds, leaves, stems, sprouts, and occasionally sap, with aspen and other poplars as preferred foods, along with birch, maple, willow, alder, black cherry, red oak, and beech.

Socially, beavers usually mate for life and live in family colonies. Kits are born precocious, already covered in fur, and typically stay with their parents for up to two years, using that time to learn dam-building and other skills needed for independent life. Communication includes scent marking at "scent mounds" built from castoreum-marked debris near lodges and trails, along with tail slapping and a small set of vocal sounds — most zoologists recognize a whine, a hiss, and a growl, used to beg for food, warn of predators, or drive predators away.

Range and habitat

American beavers are found across nearly the entire continental United States, Canada, southern Alaska, and some parts of northern Mexico, where they've been documented along the Sonora, Colorado, Bavispe, and San Bernardino rivers. Historically, they ranged from just south of the Arctic tundra to the deserts of northern Mexico, and from the Atlantic to the Pacific. Heavy trapping for the fur trade drove regional extirpations across the continent, particularly on the Great Plains, and by the early 1900s beavers survived mostly in isolated pockets. Reintroduction efforts and natural spread have since restored the species across most of its original range, and populations are rebounding in boreal and temperate regions. Beavers have recently even been observed colonizing arctic tundra, likely aided by climate-driven increases in the shrubs that grow along riverbanks there.

Their preferred habitat centers on lakes, streams, and tidal river deltas, where they build lodges either as freestanding structures surrounded by water or as burrows dug into riverbanks. Beavers also dig canals — sometimes more than half a kilometer long — to float logs toward their pond and to extend their foraging range while reducing exposure to predators.

Conservation status

The American beaver's conservation status is Least Concern (LC), reflecting its wide distribution and largely recovered population after the fur-trade era nearly wiped it out across large parts of its range. The species had been trapped extensively for economic and political reasons, causing regional extirpation in places like the Great Plains, with mixed-success conservation efforts by the Hudson's Bay Company slowing some declines. Reintroduction programs and the species' own ability to spread naturally have since restored beavers to most of their historical range, and populations in boreal and temperate ecoregions continue to rebound from that historic over-exploitation.