|North American beaver (Castor canadensis)|
The beaver (genus Castor) is a large, primarily nocturnal, semiaquatic rodent. Castor includes two extant species, the North American beaver (Castor canadensis) (native to North America) and Eurasian beaver (Castor fiber) (Eurasia). Beavers are known for building dams, canals, and lodges (homes). They are the second-largest rodent in the world (after the capybara). Their colonies create one or more dams to provide still, deep water to protect against predators, and to float food and building material. The North American beaver population was once more than 60 million, but as of 1988 was 6-12 million. A similar trend occurred in Eurasia, with only around 1200 individuals remaining at the turn of the 20th Century, but by 2003, the population had resurged to about 639,000 individuals. This population decline is the result of extensive hunting for fur, for glands used as medicine and perfume, and because the beavers' harvesting of trees and flooding of waterways may interfere with other land uses.
The English word "beaver" comes from the Old English word beofor or befer (recorded earlier as bebr), which in turn sprang from the Proto-Germanic root *bebruz. Cognates in other Germanic languages include the Old Saxon bibar, the Old Norse bjorr, the Middle Dutch and Dutch bever, the Low German bever, the Old High German bibar and the Modern German Biber. The Proto-Germanic word in turn came from the Proto-Indo-European (PIE) word *bhebhrus, a reduplication of the PIE root *bher-, meaning "brown" or "bright", whose own descendants now include the Lithuanian bebras and the Czech bobr, as well as the Germanic forms.
There are two extant species of beavers; the North American beaver (Castor canadensis) and the Eurasian beaver (C. fiber). The genus Castor was coined by Carl Linnaeus in 1758 who also classified the species name fiber.C. canadensis was classified in 1820 by German zoologist Heinrich Kuhl. However, the two beavers were not conclusively shown to be separate species until the 1970s with chromosomal evidence. Prior to that, many still considered them the same species. As many as 24 subspecies have bee classified for C. canadensis while 9 have been for C. fiber. The North American beaver has 40 chromosomes while the Eurasian beaver has 48.
Beavers belong to the rodent suborder Castorimorpha along with kangaroo rats and pocket gophers. Modern beavers are the only extant members of the family Castoridae. The closet known relatives of the Castoridae are members of the extinct family Eutypomyidae. Castoridae originated in North America in the late Eocene and dispersed into Eurasia in the early Oligocene coinciding with the Grande Coupure, a time of great faunal turnover around 33 million years ago (mya).
The most basal group of castorids were the Agnotocastorinae, which had several "primitive" features shared with the Eutypomyidae; such as more complex occlusion between the cheek teeth, parallel upper tooth rows, premolars close to the molars in size, the presence of a third set of premolars (P3) and the stapedius muscle, smooth palatine bone with a posteriorly located palatine foramen and an elongated rostrum. Subsequent castorids would have less complex occlusion, upper tooth rows which diverge posteriorly, larger second premolars in comparsion to molars, loss of P3 and stapedius and more grooved plaatine with a palatine foramen shifted towards the front. Members of the subfamily Palaeocastorinae appeared in late Oligocene North America. This group was small-bodied and adapted to a fossorial or burrowing lifestyle, with relatively large forelimbs, a low, broad skull and short tail.
In the early Miocene (about 24 mya), castorids evolved a semiaquatic lifestyle and developed the ability to cut down trees and built infrastructure, an ability that allowed them to survive in the harsh winters of Arctic latitudes. It appears that these behaviors started as a side effect to the consumption of wood and bark. Members of the subfamily Castoroidinae appeared around this time and includes giants like Castoroides of North America and Trogontherium of Eurasia. Members of this group appear to have been less specialized for aquatic life.
There is evidence that least one genus in Castoroidinae, Dipoides, ate bark and built dams and lodges. Researchers suggest that eating bark and building are unlikely to have evolved twice, so modern beavers and Castoroidinae shared a wood eating common ancestor.Dipoides appears to have been a rather poor builder than modern beavers. The subfamily Castor likely originated in Eurasia and the ancestors of the North American beaver would have entered North America across the Bering Land Bridge in the late Miocene (8-7.6 mya). The linages of the two beaver species are estimated to have split around 7.5 mya.
Beavers are the second largest rodents after the capybara. They have a head-body length of 80-120 cm (31-47 in), with a 25-50 cm (9.8-19.7 in) tail, a shoulder height of 30-60 cm (12-24 in) and a weight of 11-30 kg (24-66 lb). Their bodies are drop-shaped like other aquatic animals. The fur comes in two forms; the longer, coarser guard hairs and softer, woolier underfur. The former are 5-6 m (16-20 ft) long and typically reddish brown, but can range from yellowish brown to nearly black; while the latter are 2-3 cm (0.79-1.18 in) long and dark gray. A beaver coat has 12,000-23,000 hairs/cm³ and functions to keep the animal warm, help it float in the water and protect against the teeth and claws of predators. Beavers molt during the summer.
Beavers have massive skulls which are adapted for withstanding the forces generated by their powerful chewing muscles. Their four incisors are chisel-shaped with continuous growth. The outer enamel of the incisors is very thick and colored orange due to the presence of iron. The roots of the lower incisors extend throughout the length of the lower jaw. Beavers have two premolars and six molars for each jaw adding up to 20 teeth in total. The molars have meandering ridges on a flat surface for grinding woody food. The eyes, ears and nostrils are arranged so that they can remain above water when the rest of the body submerges. The nostrils and ears have valves which close underwater, and nictitating membranes cover the eyes. The lips can close behind the teeth, allowing for chewing while submerged, and the tongue and epiglottis prevent water from flowing into the larynx and trachea. Beavers typically spend 5-6 minutes underwater per dive but they can hold their breath for as long as 15 minutes.
The front feet of the beaver are dexterous, allowing them to grasp and manipulate food as well as dig and groom. The hind feet are larger and have webbing in-between the toes which they use for swimming. The second claw of the hindfoot is split and is used for combing the fur to keep it fluffy. Perhaps the most distinctive feature of the beaver is its flat, scaly tail. The tail has multiple functions, it props up the animal when it is cutting down a tree and acts as a rudder when it is swimming and maneuvering underwater. It also stores fat and has a countercurrent blood vessel system which allows the animal to reduce heat loss by 25% during the summer and 2% in the winter.
Beavers have one opening, a cloaca, that contains the genital, digestive and excretory openings. This may serve to reduce areas for infections when swimming in dirty waters. Beavers have two castor sacs, which are found between the kidneys and urinary bladder and open into the urethra, and anal glands. The castor sacs secrete castoreum, a urine based substance, which is mainly used for marking territory. Anal sacs produce an oily substance which beaver use to rub on their rub to make it waterproof. It also plays a role in individual and family recognition.
Compared to many other rodents, the brain of a beaver has a smaller hypothalamus in relation to the cerebrum with a ratio of hypothalamus to cerebrum length ranging from 0.20-0.24; this indicates a relatively advanced brain with higher intelligence. The cerebellum is well-developed, giving the beaver coordination in three-dimensional space (such as underwater). The neocortex is mainly dedicated to touch and hearing. The former is more advanced in the lips and hands than the whiskers and tail. Vison in the beaver is comparably poor and the beaver eye is not as well adapted to seeing underwater than that of an otter. Beavers have an acute sense of smell with is particularly important for sniffing out scent marks as well as detecting land predators.
The North American beaver has a larger skull with a broader tail. In addition, the nasal openings of the North American beaver are square shaped while those of the Eurasian species are triangular.
Both beaver species are list as least concern by the IUCN Red List of mammals. The North American beaver is widespread throughout the continent down to northern Mexico; being absent only in the arctic, the deserts of the southwestern US and in peninsular Florida. The species was also introduced to Finland, where it spread to other parts of Europe (and coexists with the Eurasian species), the Russian Far East, and Tierra del Fuego, Patagonia. Historically, the North American beaver was trapped out and almost extirpated because its fur and castoreum were highly sought after. With protection in the late 19th and early 20th centuries, the beaver population has rebounded to an estimated 10 to 15 million by the early 21 century; this is a fraction of the originally estimated 100 to 200 million North American beavers before the days of the fur trade.
The Eurasian beaver's range is less continuous. It was historically widespread throughout Europe and Asia, but overhunting greatly reduced its range by the early 20th century. In Europe, beaver were reduced to isolated populations in the Rhône of France, the Elbe in Germany, southern Norway, the Neman river and Dnieper Basin in Belarus and the Voronezh river in Russia; their combined numbered estimated at 1,200 individuals. The beaver has since returned to parts of its former range due to management measures and reintroductions. Beaver populations currently range from Spain and France, though central and eastern Europe and into Scandinavia and western Russia and their population totaled at least 639,000 by 2006. Starting in 2009, beavers have also been reintroduced to parts of Great Britain. Small populations are also present in Mongolia and northwestern China, their numbers estimated at 300 and 700 respectively.
Beavers live in freshwater ecosystems like rivers, streams, lakes and ponds. Water is the most important part of beaver habitat and they require a yearly supply that is sufficient enough for swimming, diving, floating logs. protection of entrances to their lodges, and safety from land-dwelling predators such as canids, felids and bears. Beavers tend to be cautious when on land and escape in to the water when they sense a threat. With streams, beavers prefer to utilize slow moving water, typically with a gradient or steepness of 1%. In some areas, beavers have been recorded using streams with gradients as high as 15%. In those same areas, wider streams around 8 m (26 ft) are used more then narrower ones around 1.4 m (4 ft 7 in). Beavers typically avoid areas with regular flooding, and may abandon a place for years after a significant flood.
Beavers prefer areas with flatter terrain but can be found in mountainous areas. They have a herbivorous and generalist diet and can utilize numerous plant communities. Trees and shrubs are needed for building lodges and dams. They can fall trees at an average of 1.24 minutes per plant. This can depend on the circumference of the trunks; those 25 cm (9.8 in) and larger require over four hours. North American beavers prefer aspen trees while Eurasian beavers prefer willow. Both tree species grow quickly and have soft wood for chewing and peeling. North America beavers can colonize an area where aspen is around 60 m (200 ft) from the water, but can harvest trees several hundred meters away. Beavers can harvest other tree species such as maples, alders, cherry, beech and hornbeam. In areas with no trees, beavers may use brush.
Beavers eat the bark of trees. Non-woody plants consumed by the animals include aquatic plants like water-shields and lilies, as well as ferns, raspberrys, grasses and sedges. Beavers also take agricultural crops like corn and sugar beets. Dispersing beavers will use certain habitats temporarily before arriving at their final destinations, particularly in spring. These include small streams, temporary swamps, ditches and even backyards. These sites lack important resources and the animals do not stay there long. Beavers have increasingly settled at or near human-made environments, including agricultural areas, suburbs, golf courses and even shopping malls.
Beaver dams serve to impound flowing water and submerge the entrance to their homes. The enclosed water also allows the floating of building material, depths to dive for safety and a wider area for feeding. This is not needed for lake-dwelling beavers and thus they do not build dams. Dams consist of logs, rocks, wads of grass and mud. They can be as low as 20 cm (7.9 in) to as high 3 m (9.8 ft) tall and can stretch 0.3 m (1 ft 0 in) to several hundred meters long.
How a beaver builds a dam depends on water flow. With shallow stagnant water, the animals slowly build up mud, leaves and small twigs. With large streams, a more solid base is needed. Beaver use log poles, around 2 m (6 ft 7 in) long and 5 cm (2.0 in) around, to brace against the banks and align in the direction of the water flow and at an angle of 30 degrees. These poles are weighed down by heavy rocks with grass stuffed in-between them. Beavers continue to pile on more material, until the dam settles into a compact slope on the enclosed side.
Beavers use three types of shelters for protection from the weather and predators, as well as sites for eating, resting, sleeping, mating, birthing and rearing. They may dig tunnels, or bank holes, into steep sloped banks with entrances another water. In some cases, they may pile sticks over and around the entrance, creating bank lodges. The most complex are freestanding open-water lodges which are built over a platform in shallow water and consist of sturdy logs and mud. The entrances to it are submerged under the water and roof is sealed up with mud aside from an air vent at the top.
All three structures can be present at a beaver site. Beavers tend to use the bank lodges during the summer, which are around 2 °C (36 °F) cooler than the surrounding air. Open water lodges are used during the winter and their temperature is similar to that of the surrounding water, at around 0.5 °C (32.9 °F). The air vent provides ventilation, carbon dioxide can clear out in 60 minutes. The amount of oxygen and carbon dioxide in a lodge does not change much with the seasons. During the winter, warm air of coming out of the vent helps melt the snow and ice on the lodge. Beaver lodges are typically small and sloppy for first time settlers but more experienced families can build fortresses that are 6 m (20 ft) around and 2 m (6 ft 7 in) high. Lodge building takes place mainly at night and one sturdy enough to withstand the coming winter can be built in just two nights. Beaver will also cache their food for the winter, piling up branches and saplings next to their lodge.
Beavers create trails as they wonder onto land and wear down vegetation, particularly by dragging tree limbs back to the water. If the ground is level, these trails may fill with water, becoming canals. Beavers will make them deeper by digging up mud. These trails can stretch 15-18 m (49-59 ft), or as long as 129 m (423 ft) in tree depleted areas. These channels can make transporting logs to the pond easier. Beaver family may also dig channels at the bottom of their pond, which can retain water during a drought.
Dams and lodges build by North American beavers tend to be more advanced. In addition, Eurasian beavers tend to build more bank lodges while North American beavers build more freestanding ones.
The beaver works as a keystone species in an ecosystem by creating wetlands that are used by many other species. Next to humans, no other extant animal appears to do more to shape its landscape. When building their dams and lodges, beavers alter the paths of streams and rivers and allow for the creation of extensive wetland habitats. Beaver ponds, and the wetlands that succeed them, remove sediments and pollutants from waterways, including total suspended solids, total nitrogen, phosphates, carbon and silicates. One study found that engineering by beavers leads to a 33 percent increase in the number of herbaceous plant species in riparian areas. Another study found that beavers increase wild salmon populations. The presence of beaver dams has also been shown to have a positive effect on trout by increasing either the number of fish, their size, or both. Contrary to popular myth, most beaver dams do not pose barriers to trout and salmon migration, although they may be restricted seasonally during periods of low stream flows.
Beavers help waterfowl by creating increased areas of water, and in northerly latitudes, they thaw areas of open water, allowing an earlier nesting season. In a study of Wyoming streams and rivers, watercourses with beavers had 75-fold more ducks than those without. Widening of the riparian terrace alongside streams is associated with beaver dams and has been shown to increase riparian bird abundance and diversity, an impact that may be especially important in semiarid climates. As trees are drowned by rising beaver impoundments, they become ideal nesting sites for woodpeckers, which carve cavities that attract many other bird species. Fish-eating birds use beaver ponds for foraging and in some areas, certain species appeared more frequently where beaver were active than at sites with no beaver activity. Beaver modifications to streams in Poland have been associated with increased bat activity. While overall bat activity was increased, Myotis bat species, particularly Myotis daubentonii, activity may be hampered in locations where beaver ponds allow for increased presence of duckweed.
Beavers can have negative ecological effects as well. Intoduced beavers at Tierra del Fuego have destroyed around sixteen million hectares of indigenous forest. Unlike many trees in North America, trees in South America often do not regenerate when cut down. The ponds they have created certain have flooded other trees and vegetation. With no natural predtaors, the animals have spread beyond Tierra del Fuego itself into the Brunswick Peninsula of Chile, and the government fears further penetration into continental South America. However, areas with introduced beaver were associated with increased populations of native puye fish (Galaxias maculatus), whereas the exotic species had negative effects on native stream fishes in the Cape Horn Biosphere Reserve, Chile.
Beavers may also contribute to climate change. The stuctures they build flood valleys and form lakes which then cause permafrost to thaw and release methane in Arctic areas. As this gas is released to the atmosphere, climate change ensues. The beavers' recent migration and habitation in the Alaskan tundra is escalating this danger.
Beavers are mainly nocturnal, being active at night. They spend the daytime in their shelters and during the first half of the night they forage and during the second half they do construction. In some areas, beaver activity is decoupled from the 24-hour cycle, particularly during winter when confined to their lodge and may have an activity-cycle of as much as 28 hours.
The basic units of beaver social organization are families consisting of an adult male and adult female in a monogamous pair and their kits and yearlings. Beaver families can have as many as ten members in addition to the monogamous pair. Groups this size or close to this size build more lodges to live in while smaller families usually need only one. However, large families have been recorded living in one lodge. Beaver pairs mate for life; however, if a beaver's mate dies, it will partner with another one. Extra-pair copulations also occur. Females may have their first estrus cycle of the season in late December and peak in mid-January. Female may enter estrus two to four times per season and each cycle lasts 12-24 hours. Mating typically takes place in the water but may also occur in the lodge, and lasts 30 seconds to 3 minutes. Gestation may last 104-111 days, with three or four kits being born.
Both the male and female take part in raising offspring. When young are born, they spend their first month in the lodge and their mother is the primary caretaker while their father maintains the territory. In the time after they leave the lodge for the first time, yearlings will help their parents build food caches in the fall and repair dams and lodges. Still, adults do the majority of the work and young beavers help their parents for reasons based on natural selection rather than kin selection. They are dependent on them for food and for learning life skills. Young beavers spend most of their time playing but also copy their parents' behavior. However, while copying behavior helps imprint life skills in young beavers, it is not necessarily immediately beneficial for parents as the young beaver do not perform the tasks as well as the parents. Beaver kits are weaned at ten weeks.
Within the lodge, beavers communicate with burgs and whines. They produce gargles and bubbles when entering and exiting. When outside, tail-slaps, which involve an animal hitting the water surface with its tail, serve as alarm signals for a potential threat. The tail slap of an adult is more successful in alerting the entire family, which escape into the lodge. Adults normally ignore those of juveniles who have not yet learn the proper use of a tail-slap.
Older offspring, which are around two years old, may also live in families and help their parents. In addition to helping build food caches and repairing the dam, two-year-olds will also help in feeding, grooming and guarding younger offspring. Beavers also practice alloparental care, in which an older sibling may take over the parenting duties if the original parents die or are otherwise separated from them. This behavior is common and is seen in many other animal species, such as the elephant and fathead minnow. While these helping two-year-olds help increase the chance of survival for younger offspring, they are not essential for the family, and two-year-olds stay and help their families only if there is a shortage of resources in times of food shortage, high population density, or drought.
Beavers typically leave their paternal colony at 23 months old, and usually do not settle far. Beavers can recognize their kin by detecting differences in anal gland secretion composition using their keen sense of smell. Related beavers share more features in their anal gland secretion profile than unrelated beavers. Being able to recognize kin is important for beaver social behavior, and it causes more tolerant behavior among neighboring beavers.
Beavers maintain and defend territories which contain the dams, canals, trails and food caches while the outside feeding areas are encompassed within a home range. They invest much energy in their territories, building their dams and becoming familiar with the area. Beavers mark their territories by constructing scent mounts made of mud, debris and castoreum. These scent mounts are established on the border of the territory. Once a beaver detects another scent in its territory, finding the intruder takes priority, even over food.
Because they invest so much energy in their territories, beavers are intolerant of intruders and the holder of the territory is more likely to escalate an aggressive encounter. These encounters are often violent. To avoid such situations, a beaver marks its territory with as many scent mounds as possible, signalling to intruders that the territory holder has enough energy to maintain its territory and is thus able to put up a good defense. As such, territories with more scent mounts are avoided more often than ones with fewer mounts. Scent marking increases in August during the dispersal of yearlings, in an attempt to prevent them from intruding on territories. Beavers also exhibit a behavior known as the "dear enemy effect". A territory-holding beaver will investigate and become familiar with the scents of its neighbors. As such they respond less aggressively to intrusions by their territorial neighbors than those made by non-territorial floaters or "strangers".
Both beaver testicles and castoreum, a bitter-tasting secretion with a slightly fetid odor contained in the castor sacs of male or female beaver, have been articles of trade for use in traditional medicine. Yupik medicine used dried beaver testicles like willow bark to relieve pain. Dried beaver testicles were also used as contraception. Beaver testicles were used as medicine in Iraq and Iran during the tenth to nineteenth century.Aesop's Fables describes beavers chewing off their testicles to preserve themselves from hunters, which is not possible because the beaver's testicles are inside its body. This belief, also recorded by Pliny the Elder, persisted in medieval bestiaries.
Eurasian beavers were eventually hunted nearly to extinction in part for the production of castoreum, which was used as an analgesic, anti-inflammatory, and antipyretic. Castoreum was described in the 1911 British Pharmaceutical Codex for use in dysmenorrhea and hysterical conditions (i.e. pertaining to the womb), for raising blood pressure and increasing cardiac output. The activity of castoreum has been credited to the accumulation of salicin from willow trees in the beaver's diet, which is transformed to salicylic acid and has an action very similar to aspirin. Castoreum continues to be used in perfume production.
Castoreum can be used as an enhancer of vanilla, strawberry and raspberry flavorings. It is sometimes added to frozen dairy, gelatins, candy, and fruit beverages. Due to the difficulty and expense in obtaining castoreum, it is very rarely used in common food products.
Much of the early European exploration and trade of Canada was based on the quest for beaver. The most valuable part of a beaver is its inner fur whose many minute barbs make it excellent for felting, especially for hats. In Canada a "made beaver" or castor gras that a native had worn or slept on was more valuable than a fresh skin since this tended to wear off the outer guard hairs.
In the 17th century, based on a question raised by the Bishop of Quebec, the Roman Catholic Church ruled that beavers are fish (beaver flesh was a part of the Yuko indigenous peoples' diet, prior to the Europeans' arrival) for purposes of dietary law. Therefore, the general prohibition on the consumption of meat on Fridays did not apply to beaver meat. The legal basis for the decision probably rests with the Summa Theologica of Thomas Aquinas, which bases animal classification as much on habit as anatomy. This is similar to the Church's classification of other semi-aquatic rodents, such as the capybara and muskrat.
Beavers have been trapped for millennia, and this continues to this day. Beaver pelts were used for barter by Native Americans in the 17th century to gain European goods. They were then shipped back to Great Britain and France where they were made into clothing items. Widespread hunting and trapping of beavers led to their endangerment. Eventually, the fur trade declined due to decreasing demand in Europe and the takeover of trapping grounds to support the growing agriculture sector. A small resurgence in beaver trapping has occurred in some areas where there is an over-population of beaver; trapping is done when the fur is of value, and the remainder of the animal may be used as feed. In the 1976/1977 season, 500,000 beaver pelts were harvested in North America.
In wider culture, the beaver is famed for its industriousness and its building skills. They have also lent themselves to everyday language. The English verb "to beaver" means to work hard and constantly and a "beaver intellect" refers to a slow but honest mentality. The name "beaver" is also used as a slang term for beards and the female genitalia.
Beavers have been commonly featured in the mythologies of Native Americans. In a Haida tale, a raven tried to steal salmon from beavers by rolling up their stream like a carpet but had to keep stop to rest at treetops. The beaver followed as kept gnawing down the tree, causing the raven to keep losing fish and water. In a Cree story, the tricker Wisakedjak hacked at the lodge and dam of the Great Beaver. However, the beaver used its power to stop the water from rushing though and caused the water to build up and flood the world.
Beavers were often depicted fantastically in medieval European art, sometimes looking dog-like, given dagger-like tusks or even fish-tails. They were identified as beavers by their testicles. Beavers also appear in Dante Alighieri's Divine Comedy and in the writings of Athanasius Kircher, who wrote that when the beavers entered Noah's Ark they were given a stall near to a water-filled tub which they shared with mermaids and otters. The anthropomorphic Mr. and Mrs. Beaver have an important role in the plot of The Lion, the Witch and the Wardrobe, first part of C.S.Lewis' The Chronicles of Narnia. Beavers have also been featured in animated series such as the The Angry Beavers and Happy Tree Friends.
The importance of the beaver in the development of Canada through the fur trade led to its official designation as the national animal in 1975. The animal has long been associated with Canada, appearing on the coat of arms of the Hudson's Bay Company in 1678. It is depicted on the Canadian five-cent piece and was on the first pictorial postage stamp issued in the Canadian colonies in 1851 (the so-called "Three-Penny Beaver"). The city of Montreal and the Canadian Pacific Railway bear the beaver on their crest or coat of arms. The beavers has also been used in heraldry in Europe, including the coats of arms of the city of Biberach and the University of Oxford.