Beaver continue to grow in size throughout life, and weights in excess of 60 or 70 pounds do occur when foods are abundant and accessible during the entire year. Unlike many other species, females are as large as males of the same age, and they sometimes are larger. A paddle shaped, leathery tail, positively identifies the species. An adult's tail is usually about 10 inches long, and 5 or 6 inches wide, with a thickness of 1/2 inch in the middle.
The hind feet of beaver are fully webbed, and large. These feet often measure 6 inches in length, and the spread of the toes is equal to or greater than the length as the beaver swims. Five toes with strong nails are found on the hind feet, including unique split toenail on one toe which serves the beaver as a comb for grooming. The front feet seem small in contrast to the hind feet. These feet measure 2 1/2 to 3 inches in length and are not webbed at all. Beaver normally swim with their front feet held against their chest, and the large webbed hind feet provide the propulsion with the tail acting as a rudder.
Guard hairs in beaver fur are 2 inches in length, overlaying a soft and dense underfur about an inch deep. Colors vary from section to section, and from blonde colors to nearly black. Both male and female beaver have large glands, called castors, beneath the skin on the lower bellies. These glands produce an oil which the beaver combs into its fur to waterproof it. This oil is also deposited by the beaver at selected locations as territorial markers or mating attractants in the spring of the year.
Beaver have transparent eyelids which cover the eyes as the beaver submerges, enabling the beaver to see well when submerged as the eyeball is protected from abrasive particles suspended in the water. The ears and nose of a beaver have valves that close as a beaver submerges, preventing the entry of water. Two upper and two lower incisor teeth dominate the front of a beaver's mouth. The upper incisors overlap the lower incisors, and friction from chewing causes the teeth to self-sharpen to chisel sharpness.
Similar to birds and reptiles, beaver have a single lower body opening, known as a cloaca. This single opening serves the urinary and bowel tracts, the secreted oil from the castor glands, and covers the reproductive organs of both males and females.