Anyone who has canoed a still Canadian pond at dusk may have heard the sudden slap of a beaver's tail against the water. Two species of beaver exist in the world, and the North American beaver ranges across nearly every part of Canada, from the forests of British Columbia to the wetlands of Labrador. Trappers once pursued the animal so relentlessly for its dense fur that the fur trade shaped the country's early economy. Today the beaver is honoured as a national emblem and appears on the five-cent coin. Yet behind this familiar figure lies a rodent whose entire body is fitted for a life spent partly beneath the surface.

Few mammals are so thoroughly equipped for the water. A beaver's hind feet are broadly webbed for swimming, and a clear third eyelid slides across each eye like a pair of goggles, letting the animal see clearly while it is submerged. Its lips close behind the front teeth so it can gnaw wood underwater without swallowing any, and it can hold its breath for as long as fifteen minutes on a single dive. The animal's front teeth never stop growing throughout its life, which is why it must gnaw constantly to keep them worn down.