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This article is about all beaver species worldwide. For the North American Beaver, see North American Beaver. For the Eurasian Beaver, see Eurasian Beaver. For other uses, see Beaver (disambiguation).

Beaver
Temporal range: Late Miocene – Recent
North American Beaver (Castor canadensis)
Scientific classification
Kingdom:
Animalia
Phylum:
Chordata
Class:
Mammalia
Order:
Rodentia
Family:
Castoridae
Genus:
Castor

Linnaeus, 1758
Species

C. canadensis – North American beaver
C. fiber – Eurasian beaver
†C. californicus

Distribution of both species of beaver. Red spots in Europe denote released or feral populations of the American beaver.

The beaver (genus Castor) is a primarily nocturnal, large, semi-aquatic rodent. Castor includes two extant species, 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. This population decline is due to extensive hunting for fur, for glands used as medicine and perfume, and because their harvesting of trees and flooding of waterways may interfere with other land uses.[1]

General

Beavers are known for their natural trait of building dams on rivers and streams, and building their homes (known as "lodges") in the resulting pond. Beavers also build canals to float build materials that are difficult to haul over land.[citation needed] They use powerful front teeth to cut trees and other plants that they use both for building and for food. In the absence of existing ponds, beavers must construct dams before building their lodges. First they place vertical poles, then fill between the poles with a crisscross of horizontally placed branches. They fill in the gaps between the branches with a combination of weeds and mud until the dam impounds sufficient water to surround the lodge.

A beaver skeleton
A beaver skeleton on display at The Museum of Osteology, Oklahoma City, Oklahoma.

They are known for their alarm signal: when startled or frightened, a swimming beaver will rapidly dive while forcefully slapping the water with its broad tail, audible over great distances above and below water. This serves as a warning to beavers in the area. Once a beaver has sounded the alarm, nearby beavers will dive and may not reemerge for some time. Beavers are slow on land, but are good swimmers, and can stay under water for as long as 15 minutes.

Beavers are herbivores, and prefer the wood of quaking aspen, cottonwood, willow, alder, birch, maple and cherry trees. They also eat sedges, pondweed, and water lilies.[2]

Beavers do not hibernate, but store sticks and logs in a pile in their ponds, eating the underbark. Some of the pile is generally above water and accumulates snow in the winter. This insulation of snow often keeps the water from freezing in and around the food pile, providing a location where beavers can breathe when outside their lodge.

Fossil remains of beavers are found in the peat and other superficial deposits of Britain and the continent of Europe; while in the Pleistocene formations of Britain and Siberia, occur remains of a giant extinct beaver, Trogontherium cuvieri, representing a genus by itself.

Beavers have webbed hind-feet, and a broad, scaly tail. They have poor eyesight, but keen senses of hearing, smell, and touch. A beaver's teeth grow continuously so that they will not be worn down by chewing on wood.[3] Their four incisors are composed of hard orange enamel on the front and a softer dentin on the back. The chisel-like ends of incisors are maintained by their self-sharpening wear pattern.

Beavers continue to grow throughout life. Adult specimens weighing over 25 kg (55 lb) are not uncommon. Females are as large asor larger than males of the same age, which is uncommon among mammals. Beavers live up to 24 years of age in the wild.

Etymology

The word is descended from the Proto-Indo-European name of the animal, cf. Sanskrit babhru's, brown, Lat. fiber, Ger. Biber, Russ. bobr; the root bhru has given "brown", and, through Romanic, "bronze" and "burnish".[4]

Species

They are the only extant members of the family Castoridae, which contains a single genus, Castor. Genetic research has shown the European and North American beaver populations to be distinct species and that hybridization is unlikely. Beavers are closely related to squirrels (Sciuridae), agreeing in certain structural peculiarities of the lower jaw and skull. In the Sciuridae the two main bones (tibia and fibula) of the lower half of the leg are quite separate, the tail is round and hairy, and the habitats are arboreal and terrestrial. In the beavers or Castoridae these bones are in close contact at their lower ends, the tail is depressed, expanded and scaly, and their habitats are aquatic.[4]

North American beaver tracks

Although superficially similar to each other, there are several important differences between the two species. Eurasian beavers tend to be bigger, with larger, less rounded heads, longer, narrower muzzles, thinner, shorter and lighter underfur, narrower, less oval-shaped tails and shorter shin bones, making them less capable of bipedal locomotion than the North American species. Eurasian beavers have longer nasal bones than their North American cousins, with the widest point being at the end of the snout for the fomer, and in the middle for the latter. The nasal opening for the Eurasian species is triangular, unlike that of the North American race which is square. The foramen magnum is rounded in the Eurasian beaver, and triangular in the North American. The anal glands of the Eurasian beaver are larger and thin-walled with a large internal volume compared to that of the North American breed. Finally, the guard hairs of the Eurasian beaver have a longer hollow medulla at their tips. Fur colour is also different. Overall, 66% of Eurasian beavers have pale brown or beige fur, 20% have reddish brown, nearly 8% are brown and only 4% have blackish coats. In North American beavers,50% have pale brown fur, 25% are reddish brown, 20% are brown and 6% are blackish.[5]

The two species are not genetically compatible. North American beavers have 40 chromosomes, while Eurasian beavers have 48. Also, more than 27 attempts were made in Russia to hybridize the two species, with one breeding between a male North American beaver and a female European resulting in one stillborn kit. These factors make interspecific breeding unlikely in areas where the two species' ranges overlap.[5]

Eurasian Beaver

Main article: Eurasian Beaver
A European Beaver

The Eurasian Beaver (Castor fiber) was nearly hunted to extinction in Europe, both for fur and for castoreum, a secretion of its scent gland believed to have medicinal properties. However, the beaver is now being re-introduced throughout Europe. Several thousand live on the Elbe, the Rhone and in parts of Scandinavia. A thriving community lives in northeast Poland, and the Eurasian Beaver also returned to the Morava River banks in Slovakia and the Czech Republic. They have been reintroduced in Scotland (Knapdale),[6] Bavaria, Austria, Netherlands, Serbia (Zasavica bog), Denmark (West Jutland) and Bulgaria and are spreading to new locations. The beaver became extinct in Great Britain in the sixteenth century: Giraldus Cambrensis reported in 1188 (Itinerarium ii.iii) that it was to be found only in the Teifi in Wales and in one river in Scotland, though his observations are clearly second hand. In October 2005, six Eurasian beavers were reintroduced to Britain in Lower Mill Estate in Gloucestershire; in July 2007 a colony of four Eurasian beavers was established at Martin Mere in Lancashire,[7] and a trial re-introduction occurred in Scotland in May 2009. Feasibility studies for a reintroduction to Wales are at an advanced stage and a preliminary study for a reintroduction of beavers to the wild in England has recently been published.[8][9]

North American Beaver

Main article: North American Beaver
A North American Beaver

The North American Beaver (Castor canadensis), also called the Canadian Beaver (which is also the name of a subspecies), American Beaver, or simply Beaver in North America, is native to Canada, much of the United States and parts of northern Mexico. The chief feature distinguishing C. canadensis from C. fiber is the form of the nasal bones of the skull.[4] This species was introduced to the Argentine and Chilean Tierra del Fuego, as well as Finland, France, Poland and Russia.

The North American beaver's preferred food is the water-lily (Nuphar luteum), which bears a resemblance to a cabbage-stalk, and grows at the bottom of lakes and rivers.[10] Beavers also gnaw the bark of birch, poplar, and willow trees; but during the summer a more varied herbage, with the addition of berries, is consumed. These animals are often trapped for their fur. During the early 19th century, trapping eliminated this animal from large portions of its original range. However, through trap and transfer and habitat conservation it made a nearly complete recovery by the 1940s. Beaver reintroduction in British Columbia was facilitated by Eric Collier as recounted in his book Three Against the Wilderness. Beaver furs were used to make clothing and top-hats. Much of the early exploration of North America was driven by the quest for this animal's fur.[11][12] [13] Native peoples and early settlers also ate this animal's meat. The current beaver population has been estimated to be 10 to 15 million; one estimate claims that there may at one time have been as many as 90 million.[14]

Habitat

Beaver swimming

The habitat of the beaver is the riparian zone, inclusive of stream bed. The actions of beavers for hundreds of thousands of years in the Northern Hemisphere have kept these watery systems healthy and in good repair, although a human observing all the downed trees might think that the beavers were doing just the opposite.

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.[15]

Beavers fell trees for several reasons. They fell large mature trees, usually in strategic locations, to form the basis of a dam, but European beavers tend to use small diameter (<10 cm) trees for this purpose. Beavers fell small trees, especially young second-growth trees, for food. Broadleaved trees re-grow as a coppice, providing easy-to-reach stems and leaves for food in subsequent years. Ponds created by beavers can also kill some tree species by drowning but this creates standing dead wood, which is very important for a wide range of animals and plants.[citation needed]

Dams

Main article: Beaver dam
These trees, up to 250 mm (9.8 in) in diameter, were felled by beavers in one night.

Beaver dams are created as a protection against predators, such as coyotes, wolves and bears, and to provide easy access to food during winter. Beavers always work at night and are prolific builders, carrying mud and stones with their fore-paws and timber between their teeth. Because of this, destroying a beaver dam without removing the beavers is difficult, especially if the dam is downstream of an active lodge. Beavers can rebuild such primary dams overnight, though they may not defend secondary dams as vigorously. (Beavers may create a series of dams along a river.)

Lodges

Beaver lodge, approx. 20-foot (6.1 m) diameter. Ontario, Canada

The ponds created by well-maintained dams help isolate the beavers' homes, their lodges, which are created from severed branches and mud. The beavers cover their lodges late every autumn with fresh mud, which freezes when the frost sets in. The mud becomes almost as hard as stone, and neither wolves nor wolverines can penetrate it.

The lodge has underwater entrances to make entry nearly impossible for any other animal (however, muskrats have been seen living inside beaver lodges with the beavers who made them).[16] A very small amount of the lodge is actually used as a living area. Contrary to popular belief, beavers actually dig out their dens with underwater entrances after they finish building the dams and lodge structures. There are typically two dens within the lodge, one for drying off after exiting the water, and another, drier one where the family actually lives.

Illustration of lodge

Beaver houses are formed of the same materials as the dams, with little order or regularity of structure, and seldom contain more than four adult and six or eight young beavers. Some of the larger houses have one or more partitions, but these are only posts of the main building left by the builders to support the roof, for the apartments usually have no communication with each other except by water.

When the ice breaks up in spring beavers always leave their embankments and rove about until just before fall, when they return to their old habitations and lay in their winter stock of wood. They seldom begin to repair the houses until the frost sets in, and never finish the outer coating until the cold becomes severe. When they erect a new habitation they fell the wood early in summer, but seldom begin building until nearly the end of August.

Water Quality and Beaver

Beaver ponds, and the wetlands that succeed them, remove sediments and pollutants from waterways, including total suspended solids, total nitrogen, phosphates, carbon and silicates.[17][18]

The term ‘beaver fever’ is a misnomer coined by the American press in the 1970s, following findings that the parasite Giardia lamblia, which causes Giardiasis, is carried by beavers. However, further research has shown that the many animals and birds carry this parasite, and the major source of water contamination is by other humans.[19][20][21] Recent concerns point to domestic animals as a significant vector of giardia with young calves in dairy herds testing as high as 100% positive for giardia.[22] In addition, fecal coliform and streptococci bacteria excreted into streams by grazing cattle have been shown to be reduced by beaver ponds, where the bacteria are trapped in bottom sediments.[23]

Invasiveness

Beaver dam in Tierra del Fuego

Beavers often modify their natural environment by building dams, and are therefore considered as pests or invasive species in some places. For example, in the 1940s, beavers were brought to the island and province of Tierra Del Fuego in southern Argentina for commercial fur production. However, the project failed and a few pairs of beavers were released into the wild. They quickly spread throughout the island, reaching a number of 100,000 individuals within just 50 years. As the habitat is not adapted to withstand the impact of beavers (e.g., unlike in the Northern Hemisphere, trees native to Tierra del Fuego do not coppice), beavers have had a detrimental impact. They are now considered a serious invasive species on the island, due to their massive destruction of forest trees, and efforts are being made for their eradication.[24]

Social behavior

Family life

A beaver pair

The basic social units of beaver social organization are families consisting of an adult male and adult female in a monogamous pair, their kit and yearlings.[25] 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. [25] However, large families in the northern hemisphere have been recorded live in one lodge. Beavers pairs mate for life, however if a beavers mate dies, it will partner with another one. Extra-pair copulation also occur.[25] In addition to being monogamous, both the male and female take part in raising offspring. They also both, mark and defend the territory and build and repair the dam and lodge. [25] When young are born, they spend their first month in the lodge and their mother is there 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.[25] 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.[25]

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.[25] While these helping two-year olds helps increase chance of survival for younger offspring, they are not essential for the family and two-year olds only stay and help their families if there is a shotage of resources like in times of food shortage, high population density, or drought.[25] When beavers leave their natal territories, they usually do not settle far.[26] Beavers can recognize their kin by detecting differences in anal gland secretion composition using their keen sense of smell.[27] Related beavers share more features in their anal gland secretion profile than unrelated beavers.[27] Being able to recognize kin is important for beaver social behavior and it causes more tolerant behavior among neighboring beavers.[26]

Territories and spacing

Beaver and its dam

Beavers maintain and defend territories, which are areas for feeding, nesting and mating.[25] They invest much energy in their territories, building their dams and becoming familiar with the area.[28] Beavers mark their territories by constructing scent mounts made of mud, debris and castoreum,[29] a urine based substance excreted though the beavers castor sacs between the pelvis and base of the tail.[28] These scent mounts are established on the border of the territory.[29] Once a beaver detects another scent in its territory, finding the intruder takes priority, even over food.[29] 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.[28] These encounters are often violent. In other to avoid such situations, a beaver marks its territory with as many scent mounds as possible, signaling to intruders that the territory holder has enough energy to maintain its territory and thus able to put up a good defense. As such, territories with more scent mounts are avoided more often than ones with fewer mounts.[28] Scent marking increases in August during the dispersal of yearlings, in an attempt to prevent them from intruding on territories.[28] Beaver also exhibit a behavior known as the "Dear Enemy Phenomenon". A territory holding beaver will investigate and become familiar with the scents of its neighbors.[26] As such they respond less aggressively to intrusions by their territorial neighbours than intrusions by nonterritorial floaters or "strangers".[26] This ability could be advantageous in facilitating differential treatment of wandering strangers versus established neighbours.[26]

Commercial uses

Oscar Wilde in beaver fur coat

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 (Eskimo) medicine used dried beaver testicles like willow bark to relieve pain. Beaver testicles were exported from Levant (a region centered on Lebanon and Israel) from the tenth to nineteenth century.[30] Claudius Aelianus comically described beavers chewing off their testicles to preserve themselves from hunters, which is not possible because the beaver's testicles are inside its body. European beavers (Castor fiber) 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.[31] Castoreum continues to be used in perfume production.

Much of the early exploration and trade of Canada was based on the quest for beaver. See Canadian canoe routes (early) and related articles. The most valuable part of the 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 an Indian had worn or slept on was more valuable than a fresh skin since this tended to wear off the outer guard hairs.

In culture

Beavers on a map of the Hudson River valley c. 1635

In wider culture, the beaver is famed for its industriousness. The English verb "to beaver" means to work hard and constantly.

As a national emblem

The importance of the Beaver in the development of Canada through the fur trade led to its designation as the national animal. It is depicted on the Canadian five-cent piece and was on the first pictorial postage stamp issued in the Canadian colonies in 1849 (the so-called "Three-Penny Beaver"). As a national symbol, the beaver was chosen to be the mascot of the 1976 Summer Olympics held in Montreal with the name "Amik" ("beaver" in Ojibwe). The beaver is also the symbol of many units and organizations within the Canadian Forces, such as on the cap badges of the Royal 22e Régiment and the Canadian Military Engineers. Toronto Police Services, London Police Service, Canadian Pacific Railway Police Service and Canadian Pacific Railway crest bears the beaver on their crest or coat of arms.

Others who have used the beaver in their company or organizational symbol or as their mascot include:

In dietary law

In the 17th century, based on a question raised by the Bishop of Quebec, the Roman Catholic Church ruled that the beaver was a fish (beaver flesh was a part of the indigenous peoples' diet, prior to the Europeans' arrival) for purposes of dietary law. Therefore, the general prohibition on the consumption of meat on Fridays during Lent did not apply to beaver meat.[33][34][35] 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.[36] This is similar to the Church's classification of the capybara, another semi-aquatic rodent.[37]

In computer science

In computability theory, a busy beaver (from the colloquial expression for "industrious person") is a Turing machine that attains the maximum "operational busyness" (such as measured by the number of steps performed, or the number of nonblank symbols finally on the tape) among all the Turing machines in a certain class.

Trapping

Beavers have been trapped for millennia, and this continues to this day.[38] 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.[39]

References

  1. ^ Nowak, Ronald M. 1991. pp. 364–367. Walker's Mammals of the World Fifth Edition, vol. I. Johns Hopkins University Press, Baltimore.
  2. ^ http://www.nwrc.usgs.gov/wdb/pub/wmh/13_4_7.pdf
  3. ^ "Beaver Biology". Beaver Solutions. Retrieved 2010-03-15.
  4. ^ a b c Encyclopædia Britannica, 11th Ed.
  5. ^ a b Kitchener, Andrew (2001). Beavers. Stowmarket: Whittet. p. 144. ISBN 1-873580-55-X.
  6. ^ Carrell, Severin (2008-05-26). "Beavers returning to UK after 400 years". The Guardian. London. Retrieved 2010-03-26.
  7. ^ "Beavers are back after 500 years". BBC News. 2007-07-11.
  8. ^ Return of the Beavers at MSN.co.uk
  9. ^ "Beavers could be released in 2009". BBC. 2007-12-24. Archived from the original on 2008-02-06.
  10. ^ "The Beaver (Castor canadensis)". 2002. Archived from the original on 2008-04-30. Retrieved 2008-05-19.
  11. ^ Manifest Destiny: The Oregon Country and Westward Expansion
  12. ^ Innis, Harold A. (1999). The Fur Trade in Canada: An Introduction to Canadian Economic History. Revised and reprinted. Toronto: University of Toronto Press. ISBN 0-8020-8196-7.
  13. ^ Idaho Museum of Natural History
  14. ^ Seton-Thompson, cited in Sun, Lixing (2003). The Beaver: Natural History of a Wetlands Engineer. Cornell University Press. ISBN 0-8014-4098-X. {{cite book}}: Unknown parameter |coauthors= ignored (|author= suggested) (help) pp. 97–98; but note that to arrive at this figure he assumed a population density throughout the range equivalent to that in Algonquin Park
  15. ^ Beaver. In Animals. Retrieved June 15, 2009, from http://animals.nationalgeographic.com/animals/mammals/beaver.html (beavers “second only to humans in their ability to manipulate and change their environment”)
  16. ^ The Life of Mammals Episode 4, "Chisellers".
  17. ^ "Beaver pond biogeochemical effects in the Maryland Coastal Plain". Biogeochemistry: 217–239. 2000-06. Retrieved 2011-03-05. {{cite journal}}: Check date values in: |date= (help); Unknown parameter |authors= ignored (help)
  18. ^ Sarah Muskopf (October 2007). The Effect of Beaver (Castor canadensis) Dam Removal on Total Phosphorus Concentration in Taylor Creek and Wetland, South Lake Tahoe, California (Thesis). Humboldt State University, Natural Resources. Retrieved 2011-03-05.
  19. ^ "Reintroducing the European Beaver in Britain" (PDF). British Wildlife. 2008. Retrieved 2011-03-05. {{cite journal}}: Unknown parameter |authors= ignored (help)
  20. ^ Waterborne giardiasis: sources of Giardia cysts and evidence pertaining to their implication in human infection in P. M. Wallis and B. R. Hammond (ed.), Advances in Giardia research. Calgary, Alberta, Canada: University of Calgary Press. 1988. pp. 227–236. {{cite book}}: Unknown parameter |authors= ignored (help)
  21. ^ "Prevalence of Giardia spp. in Beaver and Muskrat Populations in Northeastern States and Minnesota: Detection of Intestinal Trophozoites at Necropsy Provides Greater Sensitivity than Detection of Cysts in Fecal Samples". Applied and Environmental Microbiology: 31–36. 1990-01. Retrieved 2011-03-05. {{cite journal}}: Check date values in: |date= (help); Unknown parameter |authors= ignored (help)
  22. ^ R. C. A. Thompson (2000-11). "Giardiasis as a re-emerging infectious disease and its zoonotic potential". International Journal of Parasitology: 1259–1267. Retrieved 2011-03-05. {{cite journal}}: Check date values in: |date= (help)
  23. ^ "Stream Water Quality as Influenced by Beaver within Grazing Systems in Wyoming". Journal of Range Management: 142–146. 1984-03. Retrieved 2011-03-05. {{cite journal}}: Check date values in: |date= (help); Unknown parameter |authors= ignored (help)
  24. ^ "Argentina eager to rid island of beavers". CNN. 1999-07-09. Retrieved 2010-03-15.
  25. ^ a b c d e f g h i Sun, Lixing, and Dietland Muller-Schwarze. The Beaver: Natural History of a Wetland Engineer. Ithaca: Cornell University Press/Comstock Publishing Associates, 2003.
  26. ^ a b c d e Bjorkoyli, Tore, and Frank Rosell. (2002) "A Test of the Dear Enemy Phenomenon in the Eurasian Beaver." Animal Behavior 63:1073-78.
  27. ^ a b Sun, Lixing, and Dietland Muller-Schwarze. (1998) "Anal Gland Secretion Codes for Relatedness in the Beaver, Castor Canadensis." Ethology 104: 917-27.
  28. ^ a b c d e Rosell, Frank, and Bart A. Nolet. (1997) "Factors Affecting Scent-Marking Behavior in Eurasian Beaver (Castor Fiber)." Journal of Chemical Ecology 23(3): 673-89.
  29. ^ a b c Rosell, Frank, and Andrezej Czech. (2000) "Response of Foraging Eurasian Beavers Castor Fiber to Predator Odours." Wildlife Biology 6(1): 13-21.
  30. ^ PMID 12576209
  31. ^ Stephen Pincock (2005-03-28). "The quest for pain relief: how much have we improved on the past?". Retrieved 2007-06-17.
  32. ^ "AAC template". Beginnings.ioe.ac.uk. Retrieved 2010-03-15.
  33. ^ [1][dead link]
  34. ^ "Lenten Reader Roundup". Jimmy Akin.Org. Retrieved 2010-03-15.
  35. ^ Template:Fr iconLacoursière, Jacques. Une histoire du Québec ISBN 2-89448-050-4 Explains that Bishop François de Laval in the 17th century posed the question to the theologians of the Sorbonne, who ruled in favour of this decision.
  36. ^ The Summa Theologica of St. Thomas Aquinas II. 147:8 provides legal foundation upon which theologians argued in favour of beaver being like fish.
  37. ^ "In Days Before Easter, Venezuelans Tuck Into Rodent-Related Delicacy". The New York Sun. 2005-03-24. Retrieved 2010-03-15.
  38. ^ "Park District Kills Beaver in Lincoln Park". MyFoxChicago.com. April, 2009. Retrieved Dec. 4, 2009. {{cite news}}: Check date values in: |accessdate= and |date= (help)
  39. ^ Nowak, Ronald M. 1991. pp. 638. Walker's Mammals of the World Fifth Edition, vol. I. Johns Hopkins University Press, Baltimore.

Further reading

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