Mammals
Beaver
Castor · also called beavers
A beaver is a rodent that rebuilds rivers. With self-sharpening orange teeth it fells trees, dams streams, and floods land to make the deep pond it needs to stay safe — and in doing so it stores water, traps silt, and creates wetland that hundreds of other species move into. The fur trade in beaver hats drove the European exploration of North America and nearly wiped the animal out; it has since come roaring back.
Overview
Beavers are the genus Castor, two species — the North American and the Eurasian beaver — and the second-largest living rodents after the capybara, at 16–30 kg. They live wherever there is year-round water and woody plants, across North America, Europe, and temperate Asia. They are herbivores, eating bark cambium, twigs, and buds in winter and aquatic plants in summer, and caching branches underwater to last the frozen months.
The engineering
A beaver’s four incisors grow continuously, with hard orange iron-rich enamel in front and softer dentine behind, so they wear to a permanent chisel edge — enough to drop a 15 cm tree in a night. The dam raises the water so the lodge entrance stays underwater and ice-free; the largest known, in Alberta, is over 850 m long. The broad scaly tail is a stool, a rudder, a fat store, and an alarm — slapped hard on the water, it sends every beaver in the pond diving.
Status
Both species are Least Concern and increasing — a major success. The North American beaver recovered to 10–15 million after the fur trade; the Eurasian beaver was rebuilt from about 1,200 survivors around 1900 to over a million today through protection and hundreds of reintroductions, including recent projects in Britain. Beavers are a keystone species: their ponds cut downstream flooding, sustain streams in drought, and support far more life than the bare stream did.
Beaver: common questions
Why do beavers build dams?
A beaver builds a dam to create a pond, and it needs that pond for safety. Beavers are slow and vulnerable on land but fast and agile in water, so their whole strategy is to keep deep water around them at all times. Damming a stream raises the water level until it is deep enough that the underwater entrances to their lodge cannot be reached by predators or blocked by ice, and until the pond reaches the trees and bankside plants they eat, letting them float food back rather than carry it overland. The sound of running water triggers the building and repair instinct, and beavers will pile material on a loudspeaker playing a recording of a trickle. The dam is infrastructure for the pond, and the pond is the point.
How do beavers cut down trees without hurting their teeth?
Their teeth are built for it. The four front incisors grow throughout life, so they cannot be worn away, and they are made of two materials, a very hard orange enamel layer on the front face and softer dentine behind. As the beaver gnaws, the dentine wears faster than the enamel, which keeps a sharp chisel edge on every tooth automatically. The orange colour comes from iron compounds in the enamel that also make it harder and more acid-resistant than human enamel. A beaver braces with its tail, grips the trunk with its forepaws, and bites out chips in a ring until the tree falls, usually toward the water because the beaver works the water side hardest.
Are beavers good or bad for the environment?
On balance, strongly positive, which is why they are described as a keystone species and an ecosystem engineer. Beaver ponds and the wetlands around them slow and store water, which reduces downstream flooding, keeps streams flowing in droughts, and recharges groundwater. They trap sediment and pollutants, improving water quality. The mosaic of pond, marsh, dead standing timber, and regrowing willow supports far more plants, insects, fish, amphibians, birds, and mammals than the plain stream did. The costs are local and real, including flooded fields, roads, and timber, and blocked culverts, but these can usually be managed with flow devices and protective fencing rather than by removing the beavers.
What is the difference between a beaver and a muskrat or a nutria?
All three are semi-aquatic rodents often seen swimming, but they differ in size and tail. A beaver is large, 16 to 30 kg, with a distinctive broad, flat, scaly, paddle-shaped tail. A muskrat is much smaller, around 1 to 2 kg, with a thin, black, vertically flattened tail that it sculls from side to side, and it builds domed houses of vegetation rather than dams. Nutria, or coypu, are an invasive South American species in between at 4 to 9 kg, with a round, thin, rat-like tail and white whiskers and front teeth. Only the beaver fells trees and builds true dams, and only the beaver has the flat tail.