Beaver

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Beavers are large, semiaquatic rodents in the genus Castor native to the temperate Northern Hemisphere. There are two extant species: the North American beaver (Castor canadensis) and the Eurasian beaver (C. fiber). Beavers are the second-largest living rodents after the capybaras. They have stout bodies with large heads, long chisel-like incisors, brown or gray fur, hand-like front feet, webbed back feet and flat, scaly tails. The Eurasian beaver has a more elongated skull with a more triangular nasal bone opening, lighter fur color and a narrower tail. The animals can be found in a number of freshwater habitats, such as rivers, streams, lakes and ponds. They are herbivorous, consuming tree bark, aquatic plants, grasses and sedges.

Beaver
Temporal range: Late Miocene – Recent
American Beaver.jpg
North American beaver (Castor canadensis)
Scientific classification e
Kingdom: Animalia
Phylum: Chordata
Class: Mammalia
Order: Rodentia
Family: Castoridae
Subfamily: Castorinae
Genus: Castor
Linnaeus, 1758
Species

C. canadensis – North American beaver
C. fiber – Eurasian beaver
C. californicus
C. praefiber
C. neglectus

Castor range.png
Range of the living beavers as of 2016 (including introduced C. canadensis populations in Europe and Patagonia, but missing C. fiber populations in Mongolia and northwestern China, as well as reintroduced populations in the United Kingdom)

Beavers build dams and lodges using tree branches, vegetation, rocks and mud; they chew down trees for building material. Dams impound water and lodges serve as shelters. Their infrastructure creates wetlands used by many other species, and because of their effect on other organisms in the ecosystem, they are considered a keystone species. Adult males and females live in monogamous pairs with their offspring. When they are old enough, the young will help their parents repair dams and lodges and may also help raise newly born offspring. Beavers hold territories and mark them using scent mounts made of mud, debris and castoreum, a urine-based substance excreted through the beaver's castor sacs. Beavers can also recognize their kin by their anal gland secretions and are more likely to tolerate them as neighbors.

Historically, beavers have been hunted for their fur, meat and castoreum. Castoreum has been used in medicine, perfume and food flavoring, while beaver pelts have been a major driver of the fur trade. Before protections began in the 19th and early 20th centuries, overhunting had nearly exterminated both species. Their populations have rebounded, and they are both listed as least concern by the IUCN Red List of mammals. In human culture, the beaver symbolizes industriousness and is the national animal of Canada.

Etymology

The English word "beaver" comes from the Old English word beofor or befor and is connected to the German word Biber and the Dutch word bever. The ultimate origin of the word is from an Indo-European root for "brown".[1] The genus name Castor has its origin in the Greek kastor and translates as "beaver".[2] The name "beaver" is the source for several names of places in Europe including Beverly, Bièvres, Biberbach, Biebrich, Bibra, Bibern, Bibrka, Bobr, Bjurbäcker, Bjurfors, Bober, Bóbrka and Bjurlund.[3]

Taxonomy

North American beaver (Castor canadensis)
Eurasian beaver (Castor fiber)

There are two extant species: the North American beaver (Castor canadensis) and the Eurasian beaver (C. fiber). The Eurasian beaver has a more elongated skull with a more triangular nasal bone opening, lighter fur color and a narrower tail. In addition, the North American beaver is slightly longer.[4]

Carl Linnaeus coined the genus Castor in 1758;[5] he also coined the specific (species) epithet fiber.[6] German zoologist Heinrich Kuhl coined C. canadensis in 1820.[7] However, they were not shown conclusively to be separate species until chromosomal evidence became available in the 1970s. (The Eurasian has 48 chromosomes, versus the North American's 40.) Prior to that, many considered them the same species.[8][9] The difference in chromosome numbers prevents them from interbreeding.[10] 25 subspecies have been classified for C. canadensis and nine for C. fiber.[6][7]

Evolution

Castorimorpha
Castoroidea
Castoridae

Agnotocastor coloradensis

Agnotocastor praetereadens

Anchitheriomys sp.  

Castorinae (modern beavers)  

Castoroidinae  

Agnotocastor sp.

Migmacastor procumbodens

Palaeocastorinae  

Geomyoidea

Heteromyidae (kangaroo rats and allies)  

Geomyidae (gophers)  

Phylogeny of extant and extinct relatives of modern beavers based on genetics and morphology.[11][12]

Beavers belong to the rodent suborder Castorimorpha, along with Heteromyidae (kangaroo rats, kangaroo mice, pocket mice and spiny pocket mice), and the gophers. Modern beavers are the only extant members of the family Castoridae. They originated in North America in the late Eocene and dispersed into Eurasia via the Bering Land Bridge in the early Oligocene, coinciding with the Grande Coupure, a time of great faunal turnover around 33 million years ago (mya).[13][14]

The more basal castorids had features 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), the stapedius muscle, a smooth palatine bone with the palatine foramen (opening) closer to the rear end of the bone, and an elongated snout. More derived castorids have less complex occlusion, upper tooth rows that diverge posteriorly, larger second premolars compared to molars, loss of P3 and stapedius, and more grooved palatine 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 having relatively large forelimbs, a low, broad skull and a short tail.[14]

In the early Miocene (about 24 mya), castorids evolved a semiaquatic lifestyle. Members of the subfamily Castoroidinae appeared around this time and included giants like Castoroides of North America and Trogontherium of Eurasia. Members of this group appear to have been less specialized for aquatic life than modern beavers.[12][14] Castoroides is estimated to have had a length of 1.9–2.2 m (6.2–7.2 ft) and a weight of 90–125 kg (198–276 lb).[15] Fossils of one genus in Castoroidinae, Dipoides, have been found near assemblages of chewed wood, though it appears to have been a rather poor woodcutter compared to Castor. Researchers suggest that modern beavers and Castoroidinae shared a bark-eating common ancestor.[16] Dam and lodge-building likely developed from bark eating and allowed beavers to survive in the harsh winters of Arctic latitudes.[12] There is no conclusive evidence for this behavior occurring in non-Castor species.[16]

The genus Castor likely originated in Eurasia;[17] the oldest fossil remains appear to be C. neglectus of Germany and dated 12–10 mya.[18] The ancestors of the North American beaver would have entered North America across the Bering Land Bridge in the late Miocene. Mitochondrial DNA studies place the common ancestor of the two living species at around 8 mya.[17] Castor may have competed with members of Castoroidinae, which led to niche differentiation.[19] The Eurasian beaver may have descended from C. praefiber.[20] C. californicus from the Early Pleistocene of North America was similar to but larger than the extant North American beaver.[21]

Characteristics and adaptations

 
Mounted North American beaver skeleton

Beavers are the second-largest living rodents, after the capybaras. 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).[9] Males and females are almost identical externally.[22] Their bodies are streamlined like marine mammals and their robust build allows them to pull heavy loads.[23][24] A beaver coat has 12,000–23,000 hairs/cm2 (77,000–148,000 per in2) and functions to keep the animal warm, to help it float in water, and to protect it against the teeth and claws of predators. Guard hairs are 5–6 cm (2.0–2.4 in) long and typically reddish brown, but can range from yellowish brown to nearly black; while the underfur is 2–3 cm (0.79–1.18 in) long and dark gray. Beavers molt during the summer.[9][25]

Beavers have massive skulls adapted for withstanding the forces generated by their powerful chewing muscles. Their four chisel-shaped incisors grow continuously. The incisors' outer enamel is very thick and colored orange due to the presence of iron compounds.[26][27] The roots of the lower incisors extend throughout the length of the lower jaw. Beavers have one premolar and three molars on each side of the upper and lower jaws, 20 teeth in total. The molars have meandering ridges for grinding woody food.[25] 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 that close underwater while nictitating membranes cover the eyes. Unusual among mammals, the epiglottis is contained in the nasal cavity rather than the throat, preventing water from flowing into the larynx and trachea. In addition, the back of the tongue can rise and create a waterproof seal. A beaver's lips can close behind the incisors, allowing for chewing in water.[28][29]

 
The fore foot, hind foot, and tail of a beaver

The beaver's front feet are dexterous, allowing them to grasp and manipulate objects and food, as well as dig. The hind feet are larger and have webbing between the toes, and the second innermost toe of the hind foot has double nails used for grooming.[29][30] Beavers can swim at 8 km/h (5.0 mph);[24] only their webbed hind feet are used while the front feet are tucked under the chest.[29] On the surface, the hind limbs thrust alternately while underwater they move simultaneously.[31] Beavers are awkward on land but can move quickly when frightened. They can carry objects while walking on their hind legs.[23][29] The beaver's distinctive tail consists of a conical, muscular, hairy base; and a flat, scaly end that makes up two-thirds of the appendage. The tail has multiple functions; it provides support for the animal when it is upright (such as when chewing down a tree), acts as a rudder when it is swimming and stores fat. It also has a countercurrent blood vessel system which allows the animal to lose heat in warm temperatures and retain heat in cold temperatures.[32]

The beaver's sex organs are inside the body, and the male's penis has a cartilaginous baculum. They have a single opening, a cloaca, that contains the genital, digestive and excretory openings. The cloaca evolved secondarily, as mammals have lost this feature, and may reduce the area vulnerable to infection when swimming in dirty water. The beaver's intestine is six times longer than its body, and the caecum is twice the volume of its stomach.[33] Microorganisms in the caecum allow them to digest around 30 percent of the cellulose they consume.[23] The beaver's feces take the form of balls of sawdust, which it deposits into the water. Female beavers have four mammary glands; these produce milk with 19 percent fat, higher than in other rodents. Beavers have a pair of castor sacs between the kidneys and urinary bladder that empty into the urethra and anal glands. The castor sacs secrete castoreum, a urine-based substance used mainly for marking territory. Anal glands produce an oily substance which beavers rub on their fur to make it waterproof. This plays a role in individual and family recognition. Anal secretions are darker in females than males among Eurasian beavers, while the reverse is true for the North American species.[34]

 
Eurasian beaver swimming

Compared to many other rodents, a beaver's brain has a smaller hypothalamus in relation to the cerebrum; 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 dedicated mainly to touch and hearing. Touch is more advanced in the lips and hands than the whiskers and tail. Vision in the beaver is comparably poor, and the beaver eye is not as well adapted to seeing underwater as that of an otter. Beavers have an acute sense of smell, particularly important for sniffing out scent marks and detecting land predators.[35]

Beavers can hold their breath for as long as 15 minutes. However, they typically remain underwater for no more than five or six minutes.[36] Dives generally last less than 30 seconds and are usually shallow, at less than 1 m (3 ft 3 in).[37] Beaver tissue contains less myoglobin than fully aquatic mammals.[38] When diving, their heart rate decreases to 60 beats per minute, around half its normal function, while blood flow to the brain increases. Beavers also have a high tolerance for carbon dioxide in their body. When surfacing, the animal can replace 75 percent of the air in its lungs in one breath, compared to 15 percent for a human.[29][36]

Distribution and status

 
North American beaver in Yellowstone National Park

The IUCN Red List of mammals lists both beaver species as least concern.[39][40] The North American beaver is widespread throughout most of the United States and Canada and can be found in northern Mexico. The species was introduced to Finland in 1937 (and then spread to northwestern Russia) and to Tierra del Fuego, Patagonia, in 1946.[39] The introduced population in Finland has been moving closer to the range of the native Eurasian beaver as of 2019.[41] Historically, the North American beaver was trapped and almost extirpated because its fur was highly sought after. Protections have allowed the beaver population on the continent to rebound to an estimated 6–12 million by the late 20th century; this is a fraction of the originally estimated 60–400 million North American beavers before the days of the fur trade.[42] The introduced population in Tierra del Fuego is estimated at 35,000–50,000 individuals as of 2016.[39]

The Eurasian beaver's range is not contiguous but fragmented. It was historically widespread throughout Eurasia, but overhunting had greatly reduced its range by the early 20th century. In Europe, beavers 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 with combined numbers estimated at 1,200 individuals. The beaver has since returned to parts of its former range because of management measures and reintroductions. Beaver populations now range from Spain and France, through central and eastern Europe and into Scandinavia and Russia.[40] Beginning in 2009, beavers have been reintroduced successfully to parts of Great Britain.[43] In 2020, the total beaver population in Europe was estimated at over one million.[44] Small native populations are also present in Mongolia and northwestern China; their numbers were estimated at 150 and 700 respectively as of 2016.[40] Under New Zealand's Hazardous Substances and New Organisms Act 1996, beavers are classed as a "prohibited new organism" preventing them from being introduced into the country.[45]

Ecology

Eurasian beavers swimming and foraging

Beavers live in freshwater ecosystems such as rivers, streams, lakes and ponds. Water is the most important part of the beaver habitat. They require a year-round supply for swimming, diving, floating logs, protection of lodge entrances and safety from land-dwelling predators. Beavers prefer to use slower moving streams, typically with a gradient or steepness of one percent, though they have been recorded using streams with gradients as high as 15 percent. Beavers also prefer wider streams over narrower ones. They typically avoid areas with regular flooding and may abandon a location for years after a significant flood.[46]

Beavers prefer areas with flatter terrain and diverse vegetation close to the water. North American beavers colonize an area where trees are around 60 m (200 ft) from the water but can harvest trees several hundred meters away. Beavers have also been recorded in mountainous areas. Dispersing beavers will use certain habitats temporarily before arriving at their final destinations. These include small streams, temporary swamps, ditches and even backyards. These sites lack important resources, so the animals do not remain there for long. Beavers have settled increasingly at or near human-made environments, including agricultural areas, suburbs, golf courses and even shopping malls.[47]

 
North American beaver eating lily pads

Beavers have an herbivorous and a generalist diet. During the spring and summer, they mainly feed on herbaceous plant material such as leaves, roots, herbs, ferns, grasses, sedges, water lilies, water shields, rushes and cattails. During the fall and winter, they eat more bark and cambium of woody plants; tree and shrub species used include aspen, birch, oak, dogwood, willow and alder.[9][48][49] Beavers may cache their food for the winter, piling up wood in the deepest part of their pond where it cannot be reached by other browsers. This is known as a "raft" the top of which becomes frozen creating a "cap".[23] The animal accesses the raft by swimming under the ice. Many populations of Eurasian beaver do not make rafts but forage on land during winter.[9]

Beavers usually live up to 10 years. Felids, canids and bears may prey upon them. Beavers are cautious on land and escape into the water when they sense a threat, their shelters providing them with protection. Their parasites include the bacteria Francisella tularensis, which causes tularemia; the protozoan Giardia duodenalis, which causes giardiasis (beaver fever); and the beaver beetle and mites of the genus Schizocarpus.[50][51] They have also been recorded with the rabies virus.[52]

Infrastructure

 
North American beaver chewing down a tree

Beavers need trees and shrubs as building material for dams, which impound flowing water to create a pond for them to live in, and lodges, which provide shelter and protection. Without such material, beavers dig burrows into a bank to live. Construction begins in late summer or early fall, and they repair them whenever needed. Beavers can fell trees 15 cm (5.9 in) wide or less in under 50 minutes; trees as large as 25 cm (9.8 in) can require over four hours.[53] When chewing down a tree, beavers bite the trunk at a 45-degree angle and chew with the side of the mouth; alternating between the left and right sides. Tree branches are cut and carried through land and water using the powerful jaw and neck muscles. Other building materials, like mud and rocks, are carried under the chin with the forelimbs.[54]

The sound of running water appears to stimulate dam-building, and the sound of a leak in a dam triggers them to repair it.[55] To build a dam, beavers use log poles, around 2 m (6 ft 7 in) long and 5 cm (2.0 in) in diameter, to brace against the banks. They align these in the direction of the water's flow at an angle of around 30 degrees. Heavy rocks weigh down the poles, and grass is stuffed between them. Beavers continue to pile on more material until the dam settles into a compact slope on the enclosed side. Dams can be as low as 20 cm (8 in) to as high as 3 m (10 ft) tall and can stretch from 0.3 m (1 ft 0 in) to several hundred meters long. Beaver dams appear to be more effective than human-made concrete dams in trapping water and slowly releasing it. Lake-dwelling beavers do not need to build dams.[56]

 
Open-water beaver lodge in Canada

Beavers make two types of lodges: bank lodges and open-water lodges. Bank lodges consist of tunnels and holes in steep-sloped banks with sticks piled over them. The more complex freestanding, open-water lodges are built over a platform of piled-up sticks. The roof is sealed up with mud apart from an air vent at the top. Both types are accessed by underwater entrances.[23][57] The space inside the lodge is known as the living chamber which is above the water line. A dining area may exist near the water.[9] North American beavers build more open-water lodges than Eurasian beavers. Beaver lodges built by first-time settlers are typically small and sloppy. More experienced families can build structures that are 6 m (20 ft) in diameter (above the water line) and 2 m (6 ft 7 in) high. One sturdy enough to withstand the coming winter can be built in just two nights.[58]

Both lodge types can be present at a beaver site. During the summer, beavers tend to use bank lodges that are cooler than the surrounding air. They use open-water lodges during the winter, whose temperature is similar to that of the surrounding water. The air vent provides ventilation, and carbon dioxide can clear out in 60 minutes. The amount of oxygen and carbon dioxide in a lodge changes little with the seasons. During the winter, warm air coming out of the vent helps to melt the snow and ice on the lodge.[59]

Beavers in some areas will dig canals connected to their ponds. The canals fill up with groundwater and increase accessibility of river resources, facilitate transport of acquired resources, and lessen the risk of predation. These canals can stretch up to 1 m (3 ft 3 in) wide, 0.5 m (1 ft 8 in) deep and over 0.5 km (0.31 mi) long. It has been hypothesized that beavers' canals are not only transportation routes but an extension of their "central place" around the lodge and/or food cache.[49][60] Beavers create trails or "slides" as they drag wood, which then make it easier for the animals to transport new material.[23]

Environmental effects

Beaver dam enlargement
September 2009
December 2009
Images of a beaver dam over a four-month period. Dams block rivers and create ponds.

The beaver works as an ecosystem engineer and keystone species as its activities can have a great impact on the landscape and biodiversity of an area.[61] Aside from humans, no other extant animal appears to do more to shape its environment.[62] When building dams, beavers alter the paths of streams and rivers allowing for the creation of extensive wetland habitats.[63] In one study, beavers were associated with large increases in open-water areas. When beavers returned to an area, 160% more open-water was available during droughts than in previous years when they were absent.[64] Beaver dams have a tendency to raise the water table, both in mineral soil environments and in wetlands such as peatlands. In peatlands particularly, their dams can stabilize the often fluctuating water table, which controls the levels of both carbon and water.[65]

Beaver ponds, and the wetlands that succeed them, remove sediments and pollutants from waterways, and can stop the loss of important soils.[66][67] These ponds can increase the productivity of freshwater ecosystems by accumulating nitrogen in sediments.[61] Beaver activity can affect the temperature of the water. In northern latitudes, ice thaws earlier in the warmer beaver-dammed waters.[68] Beavers may contribute to climate change. In Arctic areas, the floods they create can cause permafrost to thaw, releasing methane into the atmosphere.[69][70]

As wetlands are formed and riparian habitats enlarged, aquatic plants colonize the newly available watery habitat.[61] One study in the Adirondacks found that beaver engineering leads to a 33 percent increase in the number of herbaceous plant species on river banks.[71] Another study in semiarid eastern Oregon found that the width of riparian vegetation on stream banks increased several-fold as beaver dams watered previously dry terraces adjacent to the stream.[72] Riparian ecosystems in arid areas appear to maintain more vegetation productivity when beaver dams are present.[73] Beaver ponds act as a refuge for river bank plants during wildfires and provide them with enough moisture to resist such fires.[74] Introduced beavers at Tierra del Fuego have been responsible for destroying the indigenous forest.[75][76] Unlike many trees in North America, trees in South America rarely regenerate when cut down.[77]

 
Salmon (Oncorhynchus nerka) jumping a beaver dam

Beaver activity impacts communities of aquatic invertebrates. Damming typically leads to an increase of lentic (slow or motionless water)-dependent species, like dragonflies, oligochaetes, snails and mussels, at the expense of lotic (rapid water) species like black flies, stoneflies and net-spinning caddisflies.[61][78][79] Beaver floodings create an increase in dead trees which benefit terrestrial invertebrates like Drosophila flies and bark beetles, which live on dead wood.[61][80][81] The presence of beavers can increase wild salmon and trout populations, and the average size of these fish. These species use beaver habitats for spawning, overwintering, feeding and as refuges from increased water flow. The positive effects of beaver dams on fish appear to outweigh the negative effects, such as inhibition of migration.[82] Beaver ponds have been shown to be beneficial to frog populations by protecting areas for larvae to mature in warm water.[83] The slow-moving and standing waters of beaver ponds also provide ideal habitat for freshwater turtles.[84]

Beavers help waterfowl by creating increased areas of water. The widening of the riparian zone associated with beaver dams has been shown to increase the abundance and diversity of birds favoring river banks, an impact that may be especially important in semi-arid climates.[85] Fish-eating birds use beaver ponds for foraging, and in some areas certain species appear more frequently where beavers were active than at sites with no beaver activity.[61][86][87] In a study of Wyoming streams and rivers, watercourses with beavers had 75 times as many ducks as those without.[88] As trees are drowned by rising beaver impoundments, they become ideal nesting sites for woodpeckers, which carve cavities that attract many other bird species.[61][86] Beaver-caused ice thawing in northern latitudes allows Canada geese to nest earlier.[68]

Other semi-aquatic mammals, such as water voles, muskrats, minks and otters use beaver lodges.[61] Beaver modifications to streams in Poland have been associated with increased activity of bat species that hunt at the water surface and use moderate vegetation clutter.[89] Large herbivores such as deer benefit from beaver activity as they can access vegetation from fallen trees and ponds.[61]

Behavior

Beavers are mainly nocturnal and crepuscular and spend the daytime in their shelters. In northern latitudes, beaver activity is decoupled from the 24-hour cycle during the winter and may last as much as 29 hours. They do not hibernate during winter, and spend much of their time in their lodges.[9][23][90]

Family life

North American beaver family, with the center pair grooming one another
Eurasian beaver parent and kit

The basic unit of beaver social organization is the family, which is composed of an adult male and an adult female in a monogamous pair and their offspring, both from the current and previous years' litters.[9][29] Beaver families can have as many as ten members besides the monogamous pair. Groups about this size build multiple lodges, while smaller families usually need only one. However, large families have been recorded living in one lodge.[91] Mutual grooming and play fighting reinforces bonds between family members, and aggressive behavior amongst them is uncommon.[29]

Adult beavers mate with their partners, though partner replacement appears to be common. A beaver that loses its partner will wait for another one to come by. Females may have their first estrus cycle of the season in late December and peak in mid-January. They may enter estrus two to four times per season; 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 three minutes.[92] Gestation lasts 104–111 days, up to four young or kits being born.[29][93] Newborn beavers are precocial and fully furred and can open their eyes within a few days.[23][29] Their mother is the primary caretaker, while their father maintains the territory.[9] Older siblings from a previous litter also play a role.[94]

After they are born, the kits spend their first month or two in the lodge.[29] During this time, parents routinely clean out the young's "bedding", pushing out the decaying plants and bringing in new material.[95] Beavers nurse for the first two or three months but can eat solid food within their first week and rely on their parents and older siblings to bring it to them. Eventually, beaver kits explore outside the lodge and forage on their own but may follow an older relative and cling to their tails.[29] After their first year, young beavers participate in construction.[9] Beavers are sexually mature at 1.5–3 years of age.[23] They may leave their birth colonies at two years of age, but remain with their parents for an extra year or more during times of food shortage, high population density, or drought.[96][97]

Territories and spacing

 
Eurasian beaver near its dam

Beavers typically disperse from their parental colonies when the winter snow melts. They often travel less than 5 km (3.1 mi), but long-distance dispersals are not uncommon when previous colonizers have already exploited local resources. Beavers are able to travel greater distances when they have access to free-flowing water. Individuals meet their mates during the dispersal stage and the pair travel together. It may take them weeks or months to reach their final destination; longer distances may require several years.[98][99] Beavers establish and defend territories along the banks of their ponds, which may be 1–7 km (0.62–4.35 mi) in length.[100]

Beavers mark their territories by constructing scent mounds made of mud and scented with castoreum.[101] Those with many territorial neighbors create more scent mounds. Scent marking increases in spring during the dispersal of yearlings to deter interlopers.[102] Beavers are generally intolerant of intruders and fights may result in deep bites to the flanks, tail and rump.[29] They exhibit a behavior known as the "dear enemy effect". A territory-holder will investigate and become familiar with the scents of its neighbors. They respond less aggressively to the scents of their territorial neighbors than those made by strangers.[103] Beavers are also more tolerant of individuals that are their kin. They recognize them by using their keen sense of smell to detect differences in the composition of anal gland secretions. Related beavers share more features in their anal gland secretion profile than unrelated beavers.[104][105]

Communication

Beavers within a family greet each other with whines. Kits will attract the attention of adults with mews, squeaks and cries. Defensive beavers produce a hissing growl and gnash their teeth.[29] Tail slaps, which involve an animal hitting the water surface with its tail, serve as alarm signals warning other beavers of a potential threat. An adult's tail slap is more successful in alerting others, which then escape into the lodge or deeper water. Adults normally ignore those of juveniles, who have not yet learned the proper use of a tail slap.[106][107] Eurasian beavers have been recorded using a "stick display" at the borders of their territories. This involves individuals rising out of the surface of shallow water while holding a stick in the mouth and front feet and bobbing up and down, sometimes creating splashes.[29][108]

Interactions with humans

 
Grey Owl feeding his beaver

Beavers sometimes come into conflict with humans over land use; the individual beavers being labeled a "nuisance beaver". Beavers can damage crops, timber stocks, roads, ditches, gardens and pastures via cutting, burrowing or flooding.[23] They occasionally attack humans and domestic pets, particularly when infected with rabies, in defense of their territory, or when they feel threatened. Some of these attacks have been fatal, including at least one human death.[109][110][111] Beavers can spread giardiasis by contaminating surface waters.[51]

Flow devices, like beaver pipes, are used to manage beaver flooding, while fencing and hardwire cloth protects trees and shrubs from beaver damage. If necessary, dams are removed by hand tools, heavy equipment or explosives.[112][113] Hunting, trapping and relocation may be permitted as forms of population control and for removal of individuals.[23] In both Argentina and Chile, governments have encouraged hunting and trapping of invasive beavers in hopes of their eradication.[114] Landscape designers, ecologists, and land managers have recognized the ecological importance of beavers, particularly in urban areas where they help maintain green spaces. Cities like Seattle have designed their parks to accommodate the animals.[115] The Martinez beavers became famous in the mid-2000s for their role in improving the ecosystem of Alhambra Creek in Martinez, California.[116]

Beavers have historically been kept in captivity for entertainment, fur farming and conservation breeding. Zoos have displayed them since at least the 19th century, though not commonly. Captive beavers require access to water, substrate for digging, and artificial shelters.[117] Archibald Stansfeld "Grey Owl" Belaney pioneered beaver conservation in the early 20th century. Belaney wrote several books and made the first professional film about beavers in their natural habitat. In 1931 he moved to a log cabin in Riding Mountain National Park, after he had been given the job of "caretaker of park animals", and raised four rescued beaver kits which were later released.[118]

Commercial use

Depiction of a beaver hunt from a medieval bestiary with the beaver depicted as biting off its testicles
Beaver pelts were the driving force of the North American fur trade.

Beavers have been hunted, trapped and exploited for their fur, meat and castoreum. Since they typically stayed in one place, trappers could easily find the animals and would kill entire families in a lodge.[119] Pre-modern people appear to have believed that the castor sacs of the beaver were its testicles. Aesop's Fables describes beavers chewing off their testicles to preserve themselves from hunters, which is impossible because a beaver's testicles are inside its body; this myth has persisted for centuries.[120][121] Tools for hunting beavers included deadfalls, snares, nets, bows and arrows, spears, clubs, firearms and steel traps. Castoreum was used to bait the animals.[122][123]

Castoreum was used for a variety of medical purposes; Pliny the Elder suggested it could treat vertigo, seizures, flatulence, sciatica, stomach diseases and epilepsy. He stated it could cure hiccups when mixed with vinegar, toothaches if mixed with oil and injected into the ear opening on the same side as the tooth, and could be used as an antivenom. The substance has traditionally been prescribed to treat hysteria in women, which was believed to have been caused by a "toxic" womb.[124] Castoreum's properties have been credited to the accumulation of salicylic acid from willow and aspen trees in the beaver's diet, and has a physiological effect very similar to aspirin.[125] Today, the medical use of castoreum has declined and is limited mainly to homeopathy.[9] The musky odor of the substance has lent itself as an ingredient in perfumes and for food flavorings.[126]

Various Native American cultures have historically hunted beavers for food.[122] Beaver meat was advantageous since it had more calories and fat than other red meats, and the animals remained plump in winter. The bones were used to make tools.[127] In medieval Europe, the Catholic Church considered the beaver to be part beast and part fish and allowed followers to eat the scaly, fishlike tail on meatless Fridays during Lent.[128] Beaver tails were thus a delicacy in Europe and were described by French naturalist Pierre Belon as having the flavor of a nicely dressed eel.[129]

Beaver pelts were used to make hats; felters would remove the guard hairs. A good-quality top hat required two to three pelts, while Cavalier and Puritan hats used more.[130] In the late 16th century, Europeans began to deal in North American furs since there were no taxes or tariffs in the New World and fur-bearing animals in Europe declined. Beaver pelts were valuable enough to cause or contribute to the Beaver Wars, King William's War and the French and Indian War. Fur traders were the main driver of the westward expansion of Europeans into the continent and were the first to meet and negotiate with the native peoples, who traded with the Europeans.[131] Between 1860 and 1870, the peak of the fur trade, the Hudson's Bay Company and fur companies in the US bought over 150,000 beaver pelts per year.[132] Conservation, anti-fur and animal rights campaigns have led to lower demand for beaver pelts and the global fur trade is no longer profitable.[9][123]

In culture

 
Beaver sculpture over entrance to the Canadian Parliament Building

The beaver has been used to represent industry, tradition, masculinity and respectability. References to the beaver's skills are reflected in everyday language. The English verb "to beaver" means to work energetically or to be "as busy as a beaver", and a "beaver intellect" refers to a slow but honest mentality. The name "beaver" is also a sexual slang term for the human vulva.[133][134]

Native American myths emphasize the beaver's skill and industriousness. In the mythology of the Haida, beavers are descended from the Beaver-Woman, who built a dam on a stream next to their cabin while her husband was out hunting and gave birth to the first beavers. In a Cree story, the Great Beaver and its dam caused a world flood. Other tales involve beavers using their tree chewing skills against an enemy.[135] Beavers have been featured as companions in some stories, including a Lakota tale where a young woman escapes her evil husband with the help of her pet beaver.[136]

Europeans have traditionally thought of beavers as fantastical animals due to their amphibious nature. They depicted them as looking dog-like with dagger-like tusks, fish tails and visible testicles. French cartographer Nicolas de Fer illustrated beavers building a dam at Niagara Falls; fantastically depicting them like human builders. Beavers have also appeared in literature such as 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 a water-filled tub which they shared with mermaids and otters.[137]

The beaver has long been associated with Canada, appearing on the first pictorial postage stamp issued in the Canadian colonies in 1851—the so-called "Three-Penny Beaver". It was declared the national animal in 1975. The five-cent coin, the coat of arms of the Hudson's Bay Company and the logos for Parks Canada and Roots Canada use its image. Bell Canada used two cartoon beavers, Frank and Gordon, in their advertising campaign from 2005 to 2008. However, the beaver's status as a rodent has made it controversial, and it was not chosen to be on the Arms of Canada in 1921.[138][139] The beaver has commonly been used to represent Canada in political cartoons, typically to signify it as a benign nation, and as subject to both affection and ridicule.[134] In the United States, the beaver is the state animal of New York and Oregon.[140] It is also featured on the coat of arms of the London School of Economics.[141]

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Bibliography

External links