NATIONAL PARK SERVICE Wildlife Portfolio of the Western National Parks

THE BEAVER is the largest rodent native to North America. Formerly, it was found near lakes and streams throughout most of the wooded sections extending from Alaska to the Rio Grande. Today the boundaries of the range are almost as extensive, but large areas are unoccupied.

It is an aquatic mammal with a robust body, a broad, flat, scaly tail, and short ears. The hind feet are strongly webbed between the toes and there is a double claw on the second toe. The pelage is composed of long, coarse over-hairs and a short, soft underfur. An average adult has a total length of about 42 inches; tail, 16 inches long and 4 inches in width; weight about 45 pounds. The males and females are so similar in size and color that it is difficult to distinguish between them in the field.

These rodents spend most of their lives in or near the water and their home frequently consists of a burrow dug in some bank on the margin of a lake or stream. Beavers with this type of home are known as "bank" beavers. Other individuals, who build their "houses" out of sticks, stones, and pieces of saplings which they cut, drag, and pile up on the bank of a river or lake to form a large haycock, are known as "lodge" beavers. However, the same individual may build a "house" or dig a burrow according to the building site and conditions. The entrance to this lodge or house is through a tunnel that leads from an under-water opening to the center of the house. Thus, whenever danger threatens, the beaver is able to enter and leave its home without exposing itself above ground or water and thereby escapes the attacks of many of its enemies.

Beavers construct dams of sticks, branches, rocks, and earth across quiet streams and along the margins of lakes. Usually, they do this to impound additional water or to maintain the water at a constant level, in order to protect the entrance to their homes. Sometimes it is done to maintain a certain depth of water in the canals which they occasionally dig as a means of transporting limbs or branches of trees which they have cut for food.

The beaver shows considerable foresight in storing its winter food supply, particularly in regions where the ponds and lakes freeze over. Aspen and willow bark are favorite foods of this animal. To secure a year-round supply the beaver, in the fall, cuts many willow and aspen saplings and drags them through the water to his lodge, which usually is situated adjacent to deep water. Here the animal piles or anchors the green saplings and limbs at the bottom of the stream or lake adjacent to his home. Then, in midwinter, when the pond, lake, or stream is frozen over, the beaver is able to go from his house down under the ice and drag back into his den a branch or limb which provides a meal.

PHOTOGRAPH, SNELLING, MERCED COUNTY, CALIF., FEBRUARY 26, 1930. COURTESY OF MUSEUM OF VERTEBRATE ZOOLOGY.