Most animals aren't the marrying kind. Less than five percent are believed to pair together for life, and even if they do stay together they do plenty of cheating.
But not European beavers. Not only do they pair up for life, a new genetic analysis shows that they are faithful to each other.
A team led by Pavel Munclinger from Charles University in Prague, Czech Republic took samples from several European beaver colonies living in the Kirov region of Russia. They then analysed the genetic relationships among family groups.
In every colony, all the offspring belonged to both of the parents. None of them had been fathered by males from elsewhere.
The results are published in the journal Mammal Research.
The same cannot be said for their American counterparts. North American beavers are known to mate with beavers other than their bonded partners.
They cheat a lot. In 2008, researchers discovered that the "father" of a pair of young was unrelated to at least one of them about half of the time.
The difference came as a surprise to Munclinger, as the two species are closely related. "They have similar ecology and live similar lives, it's quite surprising that they differ so much."
However their social habits are different. American beavers are less aggressive than European beavers. Their populations are also larger and more tightly-packed, at least in some areas.
Cheating does have its advantages. If a mother mates with a healthier male than her main partner, she can pass better genes onto her young.
But there are also advantages to staying loyal. "Genetic monogamy lowers the risk of parasite transmission," says Munclinger.
"It also lowers the risk of partner desertion, which is very important in species with extensive parental care of both sexes."
Staying faithful seems to serve the European beavers well. Their populations have been climbing in areas of the UK where they have been reintroduced.
But most animals are less loyal. Only a few mammals pair up for life, including prairie voles, beavers and night monkeys.
Birds are more inclined to form permanent pairs. But even then, many of the offspring will be fathered by a male from another pair.