Eurasian beaver

The Eurasian beaver (Castor fiber) or European beaver is a species of beaver widespread across Eurasia, with a rapidly increasing population of at least 1.5 million in 2020. The Eurasian beaver was hunted to near-extinction for both its fur and castoreum, with only about 1,200 beavers in eight relict populations from France to Mongolia in the early 20th century. It has since been reintroduced into much of its former range and now lives from Western, Southern, Central and Eastern Europe, Scandinavia, Russia through China and Mongolia, with about half the population in Russia. It is listed as least concern on the IUCN Red List.

Castor fiber was the scientific name used by Carl Linnaeus in 1758, who described the beaver in his work Systema Naturae. Between 1792 and 1997, several Eurasian beaver zoological specimens were described and proposed as subspecies, including:

These descriptions were largely based on very small differences in fur colour and cranial morphology, none of which warrant a subspecific distinction. In 2005, analysis of mitochondrial DNA of Eurasian beaver samples showed that only two evolutionarily significant units exist: a western phylogroup in Western and Central Europe, and an eastern phylogroup in the region east of the Oder and Vistula rivers. The eastern phylogroup is genetically more diverse, but still at a degree below thresholds considered sufficient for subspecific differentiation.

The Eurasian beaver's fur colour varies between regions. Light, chestnut-rust is the dominant colour in Belarus. In Russia's Sozh River basin, it is predominantly blackish brown, while in the Voronezh Nature Reserve beavers are both brown and blackish-brown.^{[page needed]}

The Eurasian beaver is one of the largest living rodent species and the largest rodent native to Eurasia. Its head-to-body length is 80–100 cm (31–39 in) with a 25–50 cm (9.8–19.7 in) long tail length. It weighs around 11–30 kg (24–66 lb). By the average weights known, it appears to be the world's second heaviest rodent after the capybara, and is slightly larger and heavier than the North American beaver. One exceptionally large recorded specimen weighed 31.7 kg (70 lb), but it is reportedly possible for the species to exceptionally exceed 40 kg (88 lb).

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: the result of over 27 attempts in Russia to hybridise the two species was just one stillborn kit, 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.

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.

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 Eurasian beaver became extinct, but reintroduction and protection programmes led to gradual recovery so that by 2020, the population was at least 1.5 million. It likely survived east of the Ural Mountains from a 19th-century population as low as 300 animals. 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.