European and American Beavers

There are two species of beavers, Castor fiber in Europe and Castor canadensis in North America. These two may look similar but are actually more different than you might realize. The starkest difference is in their chromosomes. European beavers have 48 pairs and North American beavers have 40. The two species have never been known to produce offspring and in all likelihood, they are unable to do so.

Light Fur Beaver
North American Beaver

There are many subspecies of Castor fiber but not of Castor canadensis. Across North America, beavers have interbred and now look very similar. While beavers further South tend to have lighter fur than those in the North, the difference is not so significant as to determine new subspecies. Both European and North American beavers have faced endangerment, though they are now protected and are making a comeback.



European Beaver
European Beaver

However, introducing beavers into new habitats can be detrimental. For example, beavers introduced into Tierra del Fuego have been the cause of some deforestation, as the trees there don’t regenerate as the ones in North America do. Beavers have also been blamed for building dams in rivers that are critical for salmon migration, thereby blocking the fish’s journey. Their dam-building behavior has been used by humans purposefully, too, in order to reduce the erosion of farmland.


The common ancestor of both extant beaver species diverged from giant beavers around 20 million years ago. At this point, all beavers were living aquatic lifestyles. An important feature that allows beavers to live in the water is their “combing claw”. This is a second claw on their second toe which has a serrated edge and allows them to groom debris from their fur and to waterproof their coat. This feature first has fossil evidence about 23 million years ago.


Baby BeaversAt the time that aquatic beavers first appear in the fossil record, all terrestrial beavers were disappearing. It is possible that the glaciation during this time led beavers into the water, which was more stable than the land.

The two extant species branched from a common ancestor about 7.5 million years ago, when some beavers migrated into North America from Asia. It was at this point that they began to exhibit woodcutting behaviors. Giant beavers had a diet of soft aquatic plants, not wood. While this doesn’t exclude the possibility of them cutting wood for dams, there isn’t enough evidence yet to say that they did.


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