Beaver

• 1. Introduction

  ›

  The beaver, a large semiaquatic rodent of the Northern Hemisphere, starts building a dam the moment it hears running water. The sound of a leak sends it straight back to patch the damage. That reflex ties together the two living species, the North American beaver and the Eurasian beaver, and helps explain why streams on two continents look the way they do. It also raises odd questions. Why would an animal that moves clumsily on land spend its life flooding the ground around it? What lets a rodent remake a whole valley? How close did hunters come to erasing it, and what brought it back? And why did people believe impossible things about its body for centuries?

  8788891820

• 2. Built for Water

  ›

  Capybaras are the only living rodents that outweigh the beaver. Most adult beavers weigh between 11 and 30 kilograms, though some reach 50. The head and body measure 80 to 120 centimeters, and the tail adds another 25 to 50. Males and females look almost identical from the outside. The body is streamlined much like a marine mammal's, and its sturdy build lets the animal haul heavy loads.

  Every square centimeter of a beaver's coat carries between 12,000 and 23,000 hairs. That dense pelt keeps the animal warm, helps it float, and protects it from predators. The long guard hairs are usually reddish brown, though they can be yellowish brown or nearly black. The shorter underfur is dark gray, and the whole coat is molted each summer.

  Four chisel-shaped incisors do the cutting, and they keep growing for the animal's entire life. Iron compounds tint the thick enamel orange or reddish brown. The roots of the lower pair run almost the length of the lower jaw. A beaver has 20 teeth in all, and its molars have winding ridges for grinding wood.

  The eyes, ears, and nostrils of a beaver sit high enough to stay above the surface while the rest of the body is submerged. Valves seal the nostrils and ears underwater, and membranes slide across the eyes. The epiglottis lies inside the nasal cavity rather than the throat, which shields the windpipe from water. The back of the tongue can also rise to form a waterproof seal. Lips close behind the incisors, so a beaver can gnaw underwater without swallowing any.

  A beaver can swim at 8 kilometers per hour using only its large webbed hind feet. The front feet fold under the chest while it swims. At the surface the hind limbs push one after the other, but underwater they thrust together. The dexterous front feet grasp food and objects and also dig. The second innermost toe on each hind foot carries a "double nail" used for grooming. On land a beaver is awkward, yet it can move quickly when threatened and can walk on its hind legs while carrying things.

  The tail has a muscular, hairy, conical base, and the remaining two-thirds is flat and scaly. It props the beaver upright when it chews a tree, steers like a rudder in the water, and stores fat for winter. A countercurrent arrangement of blood vessels lets the tail shed heat in warm weather and hold it in during cold.

  Fifteen minutes is the longest a beaver can hold its breath, though five or six minutes is the usual limit. Most dives last under 30 seconds and go no deeper than 1 meter. During a dive the heart rate falls to 60 beats per minute, half the normal pace, and more blood is sent to the brain. The body tolerates high levels of carbon dioxide, and one breath at the surface can replace 75 percent of the air in the lungs. A human replaces only 15 percent.

  The beaver's large cerebellum suits an animal that moves through three-dimensional space underwater, much as tree-climbing squirrels do. Touch and hearing dominate its neocortex, and the lips and hands are more sensitive than the whiskers and tail. Eyesight is poor, and a beaver sees less well underwater than an otter does. Smell is its sharpest tool for spotting land predators and reading scent marks. The intestine is six times the length of the body, and microorganisms in the caecum help it process around 30 percent of the cellulose it eats.

  A beaver has just one body opening, a cloaca, which handles reproduction, scent-marking, defecation, and urination. Most mammals lost this feature long ago, so it evolved again in beavers, and it may reduce the area exposed to infection in dirty water.

  1819202123

• 3. Dams, Lodges, and Larders

  ›

  A beaver can cut down a tree 15 centimeters wide in under 50 minutes. Trunks of 25 centimeters or more may stand for hours before falling. While chewing, the animal alternates between the left and right sides of its mouth. Strong jaw and neck muscles carry the cut branches to the building site, while the forelimbs tuck mud and rocks between chin and chest.

  Dam construction begins in late summer or early fall, and repairs continue whenever they are needed. Beavers stack long, thick logs between the banks, wedge heavy rocks in for stability, and pack grass between the logs. They keep adding material until the dam slopes toward the upstream side. Heights run from 20 centimeters to 3 meters, and lengths from 0.3 meters to several hundred. These dams trap water, and leak it slowly, more effectively than man-made concrete dams. Beavers living on lakes need no dam at all, and where trees are missing they dig burrows into a bank instead.

  Beavers build two kinds of lodge. Bank lodges are burrows dug along the shore and covered with sticks. Open-water lodges are freestanding structures raised over a platform of piled sticks. Mud seals most of the surface, leaving a hole at the top as an air vent. Entrances are underwater, and the dry space inside is called the "living chamber," sometimes with a "dining area" near the water entrance. Newcomers build small, sloppy lodges. Experienced families can raise one 2 meters high and 6 meters across, and a lodge sturdy enough for the coming winter can be finished in just two nights.

  Seasons shape where a family sleeps. Bank lodges stay cooler in summer, while open-water lodges serve in winter. The vent clears newly added carbon dioxide within an hour, and oxygen and carbon dioxide levels stay steady from season to season. North American beavers build more open-water lodges than their Eurasian cousins do.

  Some beavers dig canals that fill with groundwater and link to their ponds. These channels can reach 1 meter wide, 0.5 meters deep, and more than 0.5 kilometers long. They make hauling easier and offer an escape route from predators. One hypothesis treats them as an extension of the "central place" around the lodge or food store. On dry ground, dragged timber leaves behind trails called "slides," which the beavers reuse.

  In fall and winter a beaver eats mostly bark and cambium from woody plants such as aspen, birch, oak, dogwood, willow, and alder. Spring and summer meals lean toward leaves, roots, ferns, grasses, sedges, water lilies, and cattails. Researchers disagree about why beavers pick particular trees. Some find they favor species that are easier to digest, while others say stem size matters most.

  Many beaver families stockpile winter food by piling wood in the deepest part of the pond, out of reach of other browsers. This pile is called a "raft," and when its top freezes it becomes a "cap." The beaver reaches the food by swimming beneath the ice. Many Eurasian populations skip rafts and forage on land through winter. Beavers do not hibernate; they are mainly active at night and at twilight, and in northern winters an activity cycle can stretch to as long as 29 hours, mostly spent inside the lodge.

  3435323187

• 4. Landscape Shapers

  ›

  In one study, 160 percent more open water was available during droughts after beavers returned to an area than in the years when they were absent. Apart from humans, few living animals do more to reshape their surroundings. By redirecting streams and rivers, beaver dams create extensive wetlands. They also raise water tables in mineral soils and in peatlands, where steadier water levels lead to greater carbon storage.

  Beaver ponds, and the wetlands that follow them, filter sediments and pollutants out of waterways and can stop the loss of valuable soil. Nitrogen builds up in the pond sediments, which can raise the productivity of freshwater ecosystems. In northern latitudes, ice thaws earlier on the warmer dammed water. In Arctic areas, however, the floods can thaw permafrost and release methane into the atmosphere.

  In the Adirondacks, one study found that beaver engineering increased herbaceous plant diversity along the water's edge by more than 33 percent. In semiarid eastern Oregon, the band of streamside vegetation grew several-fold as dams watered dry terraces beside the stream. Ponds also shelter riverbank plants during wildfires, supplying enough moisture to help them resist the flames.

  Dragonflies, oligochaetes, snails, and mussels thrive in the still water behind a dam. Black flies, stoneflies, and net-spinning caddisflies, which prefer rapid water, lose out. Drowned trees give Drosophila flies and bark beetles dead wood to live and breed in. Wild salmon and trout become more numerous and larger where beavers work, using the ponds for spawning, overwintering, feeding, and refuge from changing flows. The benefits to fish appear to outweigh the drawback of blocked migration. Frogs raise their larvae in protected, warm shallows, and freshwater turtles find stable water.

  Watercourses with beavers in a Wyoming study held 75 times as many ducks as those without. Wider riparian zones draw more birds that favor the water's edge, and fish-eating birds hunt in the ponds. Woodpeckers carve cavities in trees drowned by rising water, and other birds later move in. Earlier ice melt allows Canada geese to nest sooner. Water voles, muskrats, minks, and otters shelter in beaver lodges, and some deer species reach vegetation on fallen trees. In Poland, beaver changes to streams create habitat for bats that forage at the water surface and "prefer moderate vegetation clutter."

  2737445847

• 5. Family and Territory

  ›

  Up to ten members can make up a beaver family, which is built around one adult male and one adult female in a monogamous pair, plus their offspring. A group that large needs several lodges. Mutual grooming and play fighting keep the bonds strong, and aggression inside the family is uncommon. Partners are often replaced; a beaver that loses its mate waits for another to wander by.

  Estrus cycles begin in late December and peak in mid-January. A female may have two to four per season, each lasting 12 to 24 hours. The pair usually mate in the water, and each encounter lasts from half a minute to three minutes. After a gestation of three or four months, up to four kits are born in spring and summer. Newborns arrive with a full fur coat and open their eyes within days. The mother's milk is 19 percent fat, richer than that of other rodents.

  The mother is the main caretaker, while the father maintains the territory, and older siblings from an earlier litter help too. Kits stay in the lodge for their first one to two months. They can eat solid food in their second week, though they suckle for as long as three months. Later they forage alone, sometimes riding on the back of an older relative. After their first year, young beavers join in repairing dams and lodges. They mature at 1.5 to 3 years and become independent at two, though food shortage, crowding, or drought can keep them home for another year or more. Beavers usually live up to 10 years, and felids, canids, and bears may prey on them.

  Spring, or the melt of winter snow, is when young beavers typically leave their parents' colony. They often travel less than 5 kilometers. Long journeys are not unusual where earlier settlers have used up local resources. Mates may meet during this dispersal and travel together, taking weeks or months to arrive. Longer distances can take years. Each pair then defends a stretch of bank between 1 and 7 kilometers long.

  Scent mounds of mud and vegetation, perfumed with castoreum, mark the boundary. Castoreum is a liquid made in the castor sacs, which are part of the urethra. Beavers with many neighbors build more mounds, and marking rises in spring when yearlings are dispersing. Intruders are not welcome, and fights can leave deep bites on the sides, rump, and tail. Beavers show a "dear enemy effect": a territory-holder learns the smell of its neighbors and reacts more aggressively to passing strangers. Kin get extra tolerance. Beavers recognize relatives by the makeup of their anal gland secretions, which are more alike among relatives than among strangers.

  Within a family, beavers greet one another with whines. Kits call for attention with mews, squeaks, and cries, while a defensive beaver gives a hissing growl and gnashes its teeth. A tail slap on the water surface works as an alarm, and other beavers flee into the lodge or deeper water. Adults' slaps succeed in alerting others, but juveniles have not yet learned to slap properly and are usually ignored. Eurasian beavers have also been recorded holding up a stick and bouncing in shallow water, a territorial "stick display."

  6466676870

• 6. Deep Time and Names

  ›

  Beavers began in North America during the late Eocene, then crossed the Bering Land Bridge into Eurasia in the early Oligocene. That crossing coincided with the Grande Coupure, a time of major change among animal species about 33 million years ago. Modern beavers are the only surviving members of the family Castoridae. Their closest living relatives among rodents include kangaroo rats, kangaroo mice, and gophers. Late in the Oligocene, a group called the Palaeocastorinae appeared in North America. These were mostly small animals with big front legs, flattened skulls, and reduced tails, all signs of a burrowing life. Around 24 million years ago, in the early Miocene, the family evolved a semiaquatic way of life.

  Giants once lived beside the ancestors of modern beavers. Castoroides of North America is estimated at 1.9 to 2.2 meters long and 90 to 125 kilograms. Trogontherium filled a similar role in Eurasia. Fossils of another relative, Dipoides, turn up near piles of chewed wood, but it seems to have been a poorer woodcutter than Castor. Researchers suggest that modern beavers and these giants shared a bark-eating ancestor. Dam and lodge building probably grew out of bark eating and let beavers survive harsh subarctic winters. No conclusive evidence shows the behavior in any beaver relative outside the genus Castor.

  Germany has yielded the earliest fossils of the genus Castor, the species C. neglectus, dated to 10-12 million years ago. That fits the idea that the genus began in Eurasia. DNA studies of mitochondria place the last common ancestor of the two living species around 8 million years ago. Ancestors of the North American beaver would have crossed the Bering Land Bridge about 7.5 million years ago. Competition with the giant relatives may have driven the beavers into different niches. The fossil species C. praefiber was probably an ancestor of the Eurasian beaver, and C. californicus, from the Early Pleistocene of North America, resembled today's American beaver but was larger.

  Carl Linnaeus coined the genus name Castor, along with the species name fiber for the Eurasian beaver. German zoologist Heinrich Kuhl named C. canadensis in 1820. For a long time, many scientists treated the two as one species. Chromosome evidence in the 1970s settled the matter: the Eurasian beaver has 48 chromosomes and the North American beaver has 40, so the two cannot interbreed. Scientists have classified 25 subspecies of the American beaver and nine of the Eurasian. Beyond chromosomes, the Eurasian animal is slightly longer, with a more elongated skull, triangular nasal cavities instead of square ones, lighter fur, and a narrower tail.

  The English word beaver descends from the Old English beofor or befor, and it is related to German biber and Dutch bever. Its ultimate source is an Indo-European root. The genus name Castor comes from the Greek word kastor. Relatives of the old word survive as the roots of European place names, including Beverley, Bièvres, Biberbach, Biebrich, Bobr, Bjurholm, and Bjurum.

  10121498

• 7. Hunted and Returned

  ›

  Late in the 16th century, Europeans began trading for North American furs, partly because the continent had no taxes or tariffs and partly because fur-bearing animals were declining at home. Felters made hats from beaver pelts after removing the guard hairs. Cavalier and Puritan hats took more fur than top hats did. Because beavers stayed in one place, trappers found them easily and could kill an entire family inside a lodge. Hunters used deadfalls, snares, nets, bows and arrows, spears, clubs, firearms, and leg-hold traps, and castoreum served as bait.

  The trade shaped history. Pelts contributed to the Beaver Wars, King William's War, and the French and Indian War, and they made John Jacob Astor and the owners of the North West Company very wealthy. For Europeans in North America, the fur trade drove exploration westward and brought contact with native peoples, who traded with them. Between 1860 and 1870, the Hudson's Bay Company and American fur companies bought more than 150,000 pelts a year. The modern fur trade is less profitable because of conservation efforts and anti-fur and animal rights campaigns.

  Before the fur trade, North America may have held between 60 million and 400 million beavers. Protection allowed numbers to recover to an estimated 6-12 million by the late 20th century. Across Eurasia, hunting had decimated the animal by the early 20th century. Surviving populations were fragmented, totaling about 1,200 animals, on the Rhône in France, the Elbe in Germany, southern Norway, the Neman and Dnieper basins in Belarus, and the Voronezh river in Russia. Conservation policy and reintroductions have since spread beavers across much of western, central, and eastern Europe, western Russia, and Scandinavia. Reintroductions in parts of Great Britain began in 2009. Europe now holds more than a million Eurasian beavers. Mongolia and northwestern China have small native groups, estimated in 2016 at 150 and 700. Both species are listed as least concern by the IUCN.

  Some beavers have gone where nature never sent them. North American beavers were introduced to Finland in 1937 and spread into northwestern Russia, moving closer to Eurasian beaver territory. In 1946 they were introduced to Tierra del Fuego, where 35,000-50,000 now live. There they have destroyed native forest, because many South American trees cannot regrow after being cut, unlike North American ones. Argentina and Chile have authorized trapping in hopes of eliminating them. New Zealand's Hazardous Substances and New Organisms Act 1996 lists beavers as a "prohibited new organism."

  Beavers that clash with people are sometimes labeled "nuisance beavers." Gnawing, eating, digging, and flooding can damage crops, timber, roads, ditches, gardens, and pastures. Flow devices such as beaver pipes control flooding, while fencing and hardware cloth shield trees. Seattle has designed parks and green spaces to make room for the animals. The Martinez beavers became famous in the mid-2000s for improving the ecosystem of Alhambra Creek in Martinez, California. Early in the 20th century, Archibald Stansfeld Belaney, known as "Grey Owl," pioneered beaver conservation. He wrote several books and was first to film beavers professionally in their environment. In 1931 he moved to a log cabin in Prince Albert National Park, where he served as "caretaker of park animals" and raised a beaver pair and their four offspring. Inspired by him, Dorothy Burney Richards lived with beavers in her home at Beaversprite.

  8386482829

• 8. Medicine, Myth, and Emblem

  ›

  Pliny the Elder promoted castoreum as a remedy for stomach problems, flatulence, seizures, sciatica, vertigo, and epilepsy. Mixed with vinegar, he said, it could stop hiccups. Mixed with oil and put in the ear on the same side as the aching tooth, it could cure toothache. He also claimed it worked as an antivenom. Its effects have been credited to salicylic acid that builds up from the willow and aspen in the beaver's diet, giving it an action comparable to aspirin. Medical use has since faded, remaining mainly in homeopathy, but castoreum still appears in perfumes, tinctures, and food and drink flavoring.

  Aesop's Fables describes beavers chewing off their own testicles to escape hunters. Many pre-modern people mistakenly believed castoreum came from the testicles. The story is impossible, because a beaver's testicles are internal. French physician Guillaume Rondelet corrected the myth in the 1500s, after it had lasted for centuries. In medieval Europe, the Catholic Church counted the beaver as part mammal and part fish, so the scaly tail was allowed on meatless Fridays during Lent. French naturalist Pierre Belon said it tasted like a "nicely dressed eel." Various Native American groups also hunted beavers for food, preferring the meat to other red meats for its higher calorie and fat content, and they made tools from the bones.

  In Haida mythology, the Beaver-Woman built a dam on a stream next to her cabin while her husband was hunting, then gave birth to the first beavers. A Cree story credits the Great Beaver and its dam with causing a world flood. A Lakota tale has a young woman escaping her evil husband with the help of her pet beaver. Europeans, meanwhile, treated the amphibious animal as a fantasy. Artists drew it with tusk-like teeth, a dog's or pig's body, a fish tail, and visible testicles. French cartographer Nicolas de Fer pictured beavers building a dam at Niagara Falls like human builders. Beavers also appear in Dante Alighieri's Divine Comedy. Athanasius Kircher wrote that on Noah's Ark the beavers lived near a water-filled tub that mermaids and otters also used.

  Everyday English remembers the beaver's labor. To "beaver" means to work with great effort, and people are "as busy as a beaver." A "beaver intellect" describes a slow and honest way of thinking.

  Canada claimed the beaver early. It appeared in 1851 on the first pictorial postage stamp issued in the Canadian colonies, known as the "Three-Penny Beaver." The animal was declared the national animal in 1975. Its image appears on the five-cent coin, the coat of arms of the Hudson's Bay Company, and the logos of Parks Canada and Roots Canada. Two fictional beavers, Frank and Gordon, starred in Bell Canada advertisements from 2005 to 2008. Yet the beaver's status as a rodent stirred controversy, and it was not chosen for the Arms of Canada in 1921. Political cartoonists still draw the beaver as Canada, usually as a friendly but relatively weak nation.

  8685888182