Beaver

A beaver is a large rodent that lives in rivers, streams and lakes and is famous for cutting down trees and building dams. There are only two living species. The North American beaver, Castor canadensis, lives across Canada, the United States and northern Mexico, and the European or Eurasian beaver, Castor fiber, lives in Europe and northern Asia. The two look almost the same, but they have a different number of chromosomes, 40 in the North American beaver and 48 in the European one, and they cannot breed with each other. Both belong to their own family, Castoridae, which during the last ice age also included the giant beaver of North America, an animal about the size of a black bear. Beavers are the second largest rodents in the world after the capybara, and adults usually weigh between 11 and 32 kilograms. In Canada the beaver is a national symbol: it has appeared on the five cent coin since 1937 and was made an official emblem of the country in 1975.

Beavers build dams to make deep, calm water. A pond gives them a safe place to swim away from wolves, coyotes and bears, keeps the entrances of their lodge under water, and in cold places stays deep enough not to freeze to the bottom, so they can reach the branches they store under the ice for winter. To build one, a family wedges sticks and logs across a stream, then packs the gaps with mud, stones and plants, carrying mud against their chest with their front paws. The sound of running water seems to be the main trigger: beavers rush to plug any spot where they hear water leaking through, and they repair their dams almost every night. A dam can be a few meters long or much more. The longest known, in Wood Buffalo National Park in northern Canada, measures about 775 to 850 meters and was found in 2007 by the ecologist Jean Thie on satellite images. Several generations of beavers have worked on it since the 1970s.

The beaver's tail is one of the most unusual in the animal world: broad, flat, about 25 to 50 centimeters long and covered in black scales instead of fur. It does several jobs. In the water it works as a rudder that helps the beaver steer, and on land it is a prop that lets the animal sit up while it gnaws a tree or walk on its back legs while carrying branches and mud. It is also a store of fat, and beavers build it up in autumn to help them get through the winter. When a beaver is frightened it dives and slaps the water hard with its tail, a loud crack that warns the rest of the family that danger is near. The tail also helps keep body temperature steady, losing heat in summer and saving it in cold water. One popular idea is not true: beavers do not use their tails as trowels to pat mud onto their dams and lodges, a job they do with their front paws.

The North American beaver ranges across most of Canada and Alaska and the contiguous United States, from the forests of the north to the rivers of Texas and northern Florida, and into northern Mexico, mainly along the Rio Grande and the Colorado River. Beavers thrive wherever slow or still fresh water meets woodland. Rivers, streams, lakes, ponds and the edges of marshes can all be home. Trees such as aspen, willow, birch, alder and cottonwood are essential, since they give both food and building material. In big rivers, where a dam is impossible, beavers dig dens into the banks instead of building lodges. They usually avoid fast mountain torrents and treeless areas. Within North America they recolonize streams quickly when they are protected, and many are now returning to cities and suburbs. People have also introduced them to southern South America and to parts of northern Europe and Russia.

Few mammals are as well equipped for life in the water as the beaver. Its dense fur has two layers that trap air close to the skin, keeping it warm in icy water while staying almost waterproof, and beavers spend hours grooming it with an oil from glands near the tail. Clear inner eyelids, called nictitating membranes, cover the eyes like goggles under water. The nostrils and ears close during dives, and the lips close behind the front teeth, so a beaver can gnaw wood under water without swallowing it. Beavers can stay submerged for up to 15 minutes. Large webbed hind feet push them through the water, and one claw on each back foot is split, which they use like a comb. Their famous orange incisors are covered on the front with enamel that contains iron, which makes it hard and orange, while the softer layer behind wears down faster and keeps a sharp chisel edge. Castor glands near the tail produce castoreum, a strong smelling substance used to mark territory.

Beavers are monogamous and live in close family groups, usually an adult pair, their kits of the year and the young from the year before, known as yearlings. A family typically has four to eight members. Kits are born in spring, usually two to four at a time, already furred and with their eyes open, and they can swim within days. Young beavers usually leave home at around two years old to find a territory of their own, and some travel many kilometers along rivers to do it. The whole family helps keep up the dams and lodges, hauling branches and packing mud through most of the year. Beavers are mostly active at night and around dawn and dusk. Before winter they gather a large pile of branches and logs under the water near the lodge entrance, a food store they can reach when ice covers the pond. They mark the edges of their territory with scent mounds made of mud and castoreum, and they warn one another of danger with the famous tail slap.

Where people moved them, North American beavers have often become a serious problem. In 1946, about 20 animals were brought from Canada to Tierra del Fuego, at the southern tip of South America, to start a fur industry. The fur trade never took off, but the beavers had no predators there and spread across the whole island and nearby islands. Estimates now range from about 100,000 to 200,000 animals, and their dams have killed large areas of lenga and ñire forests, which do not grow back from the stump. In 2008 Argentina and Chile agreed to try to eradicate them, and a pilot project on the Argentine side removed more than 1,000 beavers between 2015 and 2018. In Finland, seven North American beavers released in 1937 to help the native beaver recover were later found to be a different species, and their descendants now outnumber the European beaver in much of the country. From Finland they spread into Russian Karelia.

The North American beaver is listed as Least Concern on the IUCN Red List, and its comeback is one of the great wildlife recoveries of North America. Before Europeans arrived there may have been tens of millions of beavers. From the 1600s, the demand for beaver fur, which was pressed into felt for fashionable hats in Europe, drove a huge trade, and companies such as the Hudson's Bay Company, founded in 1670, were built on it. By the early 1900s, centuries of trapping had wiped beavers out of large parts of their range. Legal protection, controlled trapping seasons and reintroductions during the twentieth century let them recover, and today the population is often estimated at 10 to 15 million. Beavers are still trapped legally for fur, but pelts are worth far less than in the past. Many scientists now see beavers as a cheap way to restore streams, since their dams raise water tables, reduce erosion and help land hold water during droughts and wildfires.