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 2003, the population comprised about 639,000 individuals. 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.