Eurasian beaver (Castor fiber)

The Eurasian beaver or European beaver (Castor fiber) is a species of beaver which was once widespread in Eurasia. It was hunted to near-extinction for both its fur and castoreum; and by 1900, only 1200 beavers survived in eight relict populations in Europe and Asia. Reintroduced through much of its former range, it now occurs from Great Britain to China and Mongolia, although it is absent from Portugal, the southern Balkans, and the Middle East.

Physiology 
Physical characteristics 
The fur colour of Eurasian beavers varies geographically. Light, chestnut-rust is the dominant colour in Belarus. In Russia, the beavers of the Sozh River basin are predominantly blackish brown, while beavers in the Voronezh Reserve are equally distributed between brown and blackish-brown. 

Eurasian beavers are one of the largest living species of rodents and are the largest rodent native to Eurasia. They weigh around 11–30 kg (24–66 lb). In Norway, adult males average 21.5 kg (47 lb) while females average 23.1 kg (51 lb), while another study found adults from the same country to average 18.4 kg (41 lb). By the average weights known, the Eurasian beaver appears to be the world's second heaviest rodent behind the capybara, but averaging slightly larger and heavier than the North American beaver.While the largest specimen confirmed on record weighed 31.7 kg (70 lb), the Smithsonian has reported that this species can exceptionally exceed 40 kg (88 lb). Typically, the head-and-body length is 80–100 cm (31–39 in) and the tail length is 25–50 cm (9.8–19.7 in). 

Differences from North American beaver 
Although the Eurasian beaver appears superficially similar to the North American beaver, there are several important differences, chief among these being that the North American beaver has 40 chromosomes, while the Eurasian beaver has 48. The two species are not genetically compatible: After more than 27 attempts in Russia to hybridize the two species, the result was one stillborn kit that was bred from the pairing of a male North American beaver and a female Eurasian beaver. The difference in chromosome count makes interspecific breeding unlikely in areas where the two species' ranges overlap. 

Body
The Eurasian beaver has a narrower, less oval-shaped tail; and shorter shin bones, making it less capable of bipedal locomotion than the North American species. The anal glands of the Eurasian beaver are larger, and thin-walled, with a large internal volume, relative to that of the North American beaver.

Fur
The guard hairs of the Eurasian beaver have longer hollow medullae at their tips. There is also a difference in the frequency of fur colours: 66% of Eurasian beavers overall have beige or pale brown fur, 20% have reddish brown, nearly 8% are brown, and only 4% have blackish coats; among North American beavers, 50% have pale brown fur, 25% are reddish brown, 20% are brown, and 6% are blackish.

Subspecies of Eurasian beaver 
Historically, eight subspecies of Castor fiber were described, one for each of the eight 19th– to 20th-century refugia where the species never became extinct. The basis of the differentiation was morphological, largely based on very small differences in cranial morphology, but has been recently refuted based on genetic studies.In 2005, Durka et al.showed that only two evolutionarily significant units exist based on mitochondrial DNA studies, a western phylogroup (C. f. gallicus, C. f. albicus, and C. f. fiber) and an eastern phylogroup (C. f. ssp., C. f. tuvinicus, C. f. pohlei, and C. f. birulai).In addition, Ducroz et al. found that even in the more genetically diverse eastern phylogroup, the degree of genetic divergence was below thresholds considered sufficient for subspecies differentiation. 

Reproduction 
Eurasian beavers have one litter per year, coming into estrus for only 12 to 24 hours, between late December and May, but peaking in January. Unlike most other rodents, beaver pairs are monogamous, staying together for multiple breeding seasons. Gestation averages 107 days and they average three kits per litter with a range of two to six kits. Most beavers do not reproduce until they are three years of age, but about 20% of two-year-old females reproduce. 

Range
The Eurasian beaver is recovering from near extinction, after depredation by humans for its fur and for castoreum, a secretion of its scent gland believed to have medicinal properties. The estimated population was only 1,200 by the early 20th century.  In many European nations, the beaver became extinct, but reintroduction and protection has led to gradual recovery to about 639,000 individuals by 2003.Milishnikov found in genetic studies that beaver likely survived east of the Urals from a 19th-century population as low as 300 animals, and that factors contributing to their survival include their ability to maintain sufficient genetic diversity to recover from a population as low as three individuals, and that beavers are monogamous and select mates that are genetically different from themselves.About 83% of Eurasian beavers live in the former Soviet Union due to reintroductions, but the result is that beavers in Mongolia or Siberia do not appear significantly genetically different from samples from the European part of Russia, despite the great geographical distance.

Ecology 
Beaver are a keystone species helping support the ecosystem of which they are a part. They create wetlands, which increase biodiversity and provide habitat for many rare species such as water voles, otters, and water shrews. They coppice waterside trees and shrubs so that they regrow as dense shrubs which provide cover for birds and other animals. Beaver dams trap sediment and improve water quality, and recharge groundwater tables and increase cover and forage for trout and salmon.A recent study in Poland found that beavers increased the abundance and diversity of bats, apparently because they create gaps in forest cover, making it easier for bats to navigate. 

Effects on fish
Beaver ponds have been shown to have a beneficial effect on trout and salmon populations; in fact, many authors believe that the decline of salmonid fishes is related to the decline in beaver populations. A study of small streams in Sweden found that brown trout in beaver ponds were larger than those in riffle sections, and that beaver ponds provide habitat for larger trout in small streams during periods of drought. These findings are similar to several studies of beaver effects on fish in North America. Brook trout, coho, and sockeye salmon were significantly larger in beaver ponds than those in unimpounded stream sections in Colorado and Alaska. In addition, research in the Stillaguamish River basin in Washington found that extensive loss of beaver ponds resulted in an 89% reduction in coho salmon smolt summer production and an almost equally detrimental 86% reduction in critical winter habitat carrying capacity. Migration of adult Atlantic salmon (Salmo salar) may be limited by beaver dams during periods of low stream flows, but the presence of juveniles upstream from the dams suggests that the dams are penetrated by parr. Downstream migration of Atlantic salmon smolts was similarly unaffected by beaver dams, even in periods of low flows. Two-year-old Atlantic salmon parr in beaver ponds in eastern Canada showed faster summer growth in length and mass and were in better condition than parr upstream or downstream from the pond. The importance of winter habitat to salmonids afforded by beaver ponds may be especially important (and underappreciated) in streams without deep pools or where ice cover makes contact with the bottom of shallow streams. A 2003 study showed that Atlantic salmon and sea trout (S. trutta morpha trutta) spawning in the Numedalslågen River and 51 of its tributaries in southeastern Norway were unhindered by beaver. In Norway, beaver dams are considered beneficial for brown and sea trout populations (these are potamodromous and anadromous forms of the same species). There, beaver ponds produce increased food for young fish and provide refuges for large adults heading upstream to spawn.

Effect on water quality
The misnomer ‘beaver fever’ was invented by the American press in the 1970s after an outbreak of Giardia lamblia, which causes giardiasis, was blamed on beavers. However, the outbreak area was also frequented by humans, who are generally the primary source of contamination of waters. In addition, many animals and birds carry this parasite. Giardiasis affects humans in southeastern Norway, but a recent study found no Giardia in the beavers there.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.New Zealand has Giardia but no beavers. In a 1995 paper recommending reintroduction of beaver to Great Britain, MacDonald stated that the only new diseases that beaver might convey to that country's birds and mammals, are rabies and tularemia - both diseases that should be preventable by statutory quarantine procedures and prophylactic treatment for tularemia. 

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.
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