North America’s largest rodent, the Canadian Beaver (Castor canadensis), better known as the North American beaver, exhibits a wide range of physical adaptations to its largely aquatic lifestyle. The heavily muscled body is shaped more like a marine mammal than like other terrestrial mammals, while the hind-feet are webbed for swimming. Furthermore, the characteristically flattened, scaly tail provides steering and propulsion, particularly when swimming fast or diving. While the colouration of the coarse outer-fur varies from chestnut to almost black, the dense underfur is typically dark grey and maintains body warmth even in freezing waters. The ears and nose are equipped with valve-like flaps that can be closed underwater, while the small eyes have a protective transparent eyelid (nictitating membrane). Owing to the need for a strong foundation for the prominent tree-felling incisors, the beaver has an exceptionally thick and heavy skull and jaw. Large claws on the short forefeet provide dexterity with handling food and also facilitate digging. As many as 24 subspecies of the American beaver are recognised, but reintroductions have blurred their geographic boundaries and resulted in genetic mixing.