Mammals
North American Beaver
Castor canadensis · also called American beaver, Canadian beaver
The North American beaver changes the landscape more than any animal except humans. By felling trees and damming streams it turns running water into a chain of ponds and wetlands, and in doing so it creates habitat for hundreds of other species.
Overview
Beavers live in fresh water across almost the whole of North America, from Alaska and northern Canada to the deserts of northern Mexico, anywhere there is a year-round stream or lake and woody plants to eat. A family — an adult pair, that year’s kits, and the previous year’s young — occupies a stretch of water and defends it against other beavers with scent mounds of mud and castoreum.
Built for the water
The beaver is a rodent rebuilt for an amphibious life:
- A broad, flat, scaly tail that works as a rudder, a prop when gnawing, a fat store, and an alarm signal — slapped hard on the water to warn the family of danger.
- Webbed hind feet for propulsion and dexterous front paws for handling sticks and food.
- Valved ears and nostrils that seal shut underwater, transparent third eyelids that act as goggles, and lips that close behind the incisors so the beaver can carry and cut wood while submerged.
- A dense, waterproof coat kept oiled and groomed, insulating the animal in near-freezing water.
The dam and the pond
Given a small stream, a beaver family builds a dam of branches, stones, and mud, sometimes hundreds of metres long and maintained for generations. Behind it forms a pond deep enough not to freeze to the bottom, in which the family builds a lodge — a mud-plastered mound of sticks with underwater doorways and a dry chamber inside.
The pond is the whole point. It lets the beavers move and feed in safety, protects the lodge, and holds a submerged pile of branches — the winter food cache — within reach beneath the ice.
Diet
Beavers are strict herbivores. In the warmer months they eat leaves, ferns, grasses, and aquatic plants; in autumn and winter they live on the bark and soft cambium layer of felled trees, favouring aspen, willow, birch, and cottonwood. They do not eat wood itself — the trunks and large branches go into the dam and lodge.
Reproduction and life cycle
A pair mates in winter and the female gives birth to two to four fully furred kits in spring. The kits can swim within days and stay with the family for about two years before dispersing to find their own territory and mate. Beavers are monogamous and both parents, plus the yearlings, help raise each litter.
From near-extinction to recovery
Beaver fur — especially the felted underfur used for hats — drove European exploration and trade across North America for two centuries. By 1900 the continent’s beavers had been trapped down from perhaps 100–200 million to a few hundred thousand, and they were gone from most of their range.
Protection, regulated trapping, and reintroduction have brought them back to an estimated 10–15 million, and beavers now reoccupy landscapes they had been absent from for a century. The species is listed as Least Concern and increasing, and beaver wetlands are increasingly valued — and deliberately restored — for the water storage, fire resistance, and biodiversity they provide.
North American Beaver: common questions
Why do beavers build dams?
A beaver builds a dam to raise the water level and create a deep, stable pond. The pond lets it swim to food and building materials without leaving the water, surrounds the lodge entrance with a protective moat, keeps the entrance from freezing shut in winter, and provides an underwater larder for stored branches. Beavers are triggered to build and repair dams by the sound of running water.
How does a beaver cut down a tree?
With four large, chisel-shaped incisors that never stop growing. The front face of each tooth is coated in an iron-rich enamel that makes it orange, hard, and wear-resistant, while the softer back wears away faster, keeping a permanently sharp bevelled edge. A beaver gnaws a wedge from each side of the trunk and can drop a small tree in minutes, though it cannot control which way the tree falls.
What is a beaver lodge like inside?
A dome of branches and mud with one or more underwater entrances leading up to a dry living chamber above the waterline. The mud freezes near-solid in winter, making the lodge almost impregnable to predators. The family shares body heat inside, and a small gap at the top lets air in.
Are beavers good or bad for the environment?
Overwhelmingly beneficial. Beaver ponds store water, recharge groundwater, trap sediment and pollutants, reduce downstream flooding and drought, and create wetland habitat that supports fish, amphibians, waterfowl, moose, and countless invertebrates. They can flood roads and timber, but "beaver-based restoration" is now used deliberately to repair damaged streams.