Description The beaver Castor canadensis is the largest rodent in North America and the largest rodent in the world except for the capybara of South America. On average, an adult weighs from 16 to 32 kg and, including its 30-cm tail, a large beaver may measure 1.3 m long. Some may even weigh more than 45 kg! Its ancestors were even larger. In the Pleistocene ice age—the era of the mastodons and the mammoths—the giant beavers that inhabited the expanses of Eurasia and North America measured just under 3 m in length, including the tail, and probably weighed 360 kg. Being short and rotund, the beaver is ungainly and slow on land. Not so in the water. The beaver is a graceful, strong swimmer, both under water and on the surface, attaining speeds approaching 7 km per hour if it is alarmed. The beaver’s body is adapted in many ways to the animal’s watery habitat. The beady eyes see as well in the water as out of it thanks to a specialized transparent membrane that can be drawn over the eyes for protection while diving. The nostrils are small and can be closed for underwater swimming, as can the ears. The beaver’s tail has important uses both in the water and on land. The tail of a large beaver may be 30 cm long, up to 18 cm wide, and 4 cm thick. It is covered with leathery scales and sparse, coarse hairs. Although fat, the tail is flexible and muscular. In the water, the animal can use its tail as a four-way rudder. On land, the tail acts as a prop when the beaver is sitting or standing upright. It also serves as a counterbalance and support when the animal is walking on its hind legs while carrying building materials like mud, stones, or branches with its front paws. The beaver’s hind feet are very large, with five long blunt-clawed toes which are fully webbed, for swimming. In the water, the beaver holds its forepaws close to its body, using only its hind feet to propel itself, with occasional aid from its tail. Its forepaws are small, without webs, and the toes end in long sharp claws suited to digging. These delicate paws are very dexterous—almost like hands—and with them the beaver can hold and carry sticks, stones, and mud and perform a variety of complex construction tasks. The beaver also uses its paws to groom its coat. The second toe on each hind foot is double-clawed, the claws being hinged to come together like tiny pliers. These specialized claws, along with the front claws, are used for grooming the fur, which is brown in colour. The beaver has several reasons to groom itself. Its fur is very dense, consisting of a mat of fine underfur about 2 cm long and an outer layer of heavy guard hairs about 7 cm long. Through constant preening and oiling, this dense pelt is kept waterproof: even after swimming under water for six or seven minutes the beaver is not wet to the skin. Oil is obtained from two glands near the anus and, like preening, application of oil is done with both front and hind feet. Preening also removes dirt, straightens matted fur and removes mites and other insect parasites. Members of family groups spend considerable time preening each other. The beaver moults in spring and in late autumn. Figure 1: Beaver tracks Finally, the animal has exceptional teeth. Its long, sharp, strong incisors grow continuously and are hardened with a dark orange enamel on the forward face. Consequently, as the upper and lower incisors are ground against each other, the outer tips of these teeth are maintained chisel-sharp. With them, a beaver is able to fell very large trees. The lips can be closed behind the incisors, permitting the beaver to gnaw on twigs while under water. Beavers may live as long as 12 years. Signs and sounds Beavers have a system of communication that includes physical markers and noises. They often leave “mud pies” on their trails, on which they leave paw prints and the oily deposit of a musky oil that they secrete. (See Figure 1 for illustrations of their tracks.) They also communicate through low whines and bellows and the pistol-shot-like slap of the tail on water, which warns other beavers that danger is near.
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