The beaver, Castor canadensis, is North America’s largest rodent. Before European colonists arrived, the species was plentiful from the Mexican border to the Arctic. Beaver fur is thick and considered valuable; untanned pelts brought four dollars each in the early 1800s, when the skins were used to make top hats and to trim clothes. Tremendous demand for beaver fur sent trapping expeditions throughout the unexplored West, stimulating expansion of the new American nation. By the end of the nineteenth century, uncontrolled trapping had eliminated beavers in Pennsylvania and other states, but today this aquatic furbearer is back. Aided by modern wildlife management and its own prolific breeding potential, the beaver has repopulated a great deal of its former range. Today, beavers are found throughout Pennsylvania. The highest concentrations are found in the northern counties, often in remote territory and always in areas with plentiful, constant water sources. Using branches, mud and rocks, beavers build dams and lodges on streams and creeks, and along the edges of lakes and rivers. Beavers are shy and mainly nocturnal, but people interested in catching a glimpse of a beaver may get lucky by staking out a beaver pond in the early morning and near sundown.
Adult beavers weigh 40-60 pounds and grow up to 40 inches in length. (An extinct giant beaver of the Pleistocene era was the size of a bear.) They have blunt heads, short necks and legs, and stocky bodies. The coat is glossy tan to dark brown above, paler below; it consists of dense underfur covered with longer guard hairs. The thick pelt and deposits of body fat insulate the animal and allow it to remain in the water many hours at a time. A beaver’s tail is trowel-shaped, 8-12 inches long and five or six inches wide. It has a scaly, leathery covering. When the animal swims, it uses its tail as a propeller and a rudder; the tail also supports a beaver when it sits erect or gnaws a tree on dry land. A sharp slap of the tail on water is a signal warning other beavers of danger. A beaver’s front feet are remarkably dexterous. They have long claws and are used for digging, handling food and working on dams. The thumb is small and weak, but the little finger is strong and has taken over the thumb’s role. The hind feet, broad and webbed between the toes, propel the animal through the water. The second claw from the outside on each hind foot is double (or split) and is used for grooming. A beaver’s vision is weak, but its hearing and sense of smell are acute. Most food is located by smell. Beavers are slow on dry land but quite mobile in the water. A beaver can stay submerged up to 15 minutes; membrane valves seal the ears and nostrils while it’s submerged. Both males and females possess musk sacs, or castors, which produce an oily, heavily-scented substance called “castoreum,” which the animals use to mark territories. Commercially, castoreum has been used as an ingredient for some medicines and perfumes, not to mention trapping lures. Beavers have two other sacs, one on each side of the urogenital opening, which secrete an oil. The animal rubs this oil into its fur to repel water. Because its front teeth never stop growing, a beaver must continually cut wood to offset incisor growth. The upper and lower incisors are the primary cutters. A beaver can close its lips behind its incisors to gnaw on and transport saplings while underwater.