The North american Beaver, Castor canadensis (1820)
Phylum : Chordata
Class : Mammalia
Order : Rodentia
Family : Castoridae
Genus : Castor
Species : C. canadensis
Subspecies : C.c. acadicus, C.c. baileyi, C.c. belugae, C.c. caecator, C.c. canadensis, C.c. concisor, C.c. carolinensis, C.c. duchesnei, C.c. frondator, C.c. idoneus, C.c. labradorensis, C.c. leucodonta, C.c. mexicanus, C.c. michiganensis, C.c. missouriensis, C.c. pacificus, C.c. pallidus, C.c. phaeus, C.c. rostralis, C.c. repentinus, C.c. sagittatus, C.c. shastensis, C.c. subauratus, C.c. taylori, C.c. texensis
- Least concern
- 90 cm long and 30 kg (size)
- North America (map)
Beavers are mainly active at night. They are excellent swimmers and may remain submerged for up to 15 minutes. More vulnerable on land, they tend to remain in the water as much as possible. They use their flat, scaly tail both to signal danger by slapping the surface of the water and as a location for fat storage.
They construct their homes, or “lodges,” out of sticks, twigs, rocks and mud in lakes, streams, and tidal river deltas. These lodges may be surrounded by water, or touching land, including burrows dug into river banks. They are well known for building dams across streams and constructing their lodge in the artificial pond which forms. When building in a pond, the beavers first make a pile of sticks and then eat out one or more underwater entrances and two platforms above the water surface inside the pile. The first is used for drying off. Towards winter, the lodge is often plastered with mud which when it freezes has the consistency of concrete. A small air hole is left in the top of the lodge.
The dam is constructed using logs from trees the beavers cut down, as well as rocks, grass and mud. The inner bark, twigs, shoots and leaves of such trees are also an important part of the beaver’s diet. The trees are cut down using their strong incisor teeth. Their front paws are used for digging and carrying and placing materials. Some researchers have shown that the sound of running water dictates when and where a beaver builds its dam. Besides providing a safe home for the beaver, beaver ponds also provide habitat for waterfowl, fish, and other aquatic animals. Their dams help reduce soil erosion and can help reduce flooding. However, beaver dams are not permanent and depend on the beavers’ continued presence for their maintenance. Beavers generally concentrate on building and repairing dams in the fall in preparation for the coming winter. In northern areas they often don’t repair breaches in the dam made by otters, and sometimes breach the dam themselves and lower the water level in the pond in order to create more breathing space under the ice or get easier access to trees below the dam. In a 1988 study in Alberta, Canada, no beavers repaired “sites of water loss” during the winter. Of 178 sites of water loss, beavers repaired 78 when water was opened, and did not repair 68. The rest were partially repaired.
Beavers are best known for their dam-building. They maintain their pond-habitat by reacting quickly to the sound of running water, and damming it up with tree branches and mud. Early ecologists believed that this dam-building was an amazing feat of architectural planning, indicative of the beaver’s high intellect. This theory was questioned when a recording of running water was played in a field near a beaver pond. Despite the fact that it was on dry land, the beaver covered the tape player with branches and mud. The largest beaver dam is 850 m in length—more than half a mile long—and was discovered via satellite imagery in 2007. It is located on the southern edge of Wood Buffalo National Park in northern Alberta and is twice the width of the Hoover Dam which spans 379 m.
Normally, the purpose of the dam is to provide water around their lodges that is deep enough that it does not freeze solid in winter. The dams also flood areas of surrounding forest, giving the beaver safe access to an important food supply, which is the leaves, buds, and inner bark of growing trees. They prefer aspen and poplar, but will also take birch, maple, willow, alder, black cherry, red oak, beech, ash, hornbeam and occasionally pine and spruce. They will also eat cattails, water lilies and other aquatic vegetation, especially in the early spring (and contrary to widespread belief, they do not eat fish). In areas where their pond freezes over, beavers collect food in late fall in the form of tree branches, storing them underwater (usually by sticking the sharp chewed base of the branches into the mud on the pond bottom), where they can be accessed through the winter. Often the pile of food branches projects above the pond and collects snow. This insulates the water below it and keeps the pond open at that location.
Beavers usually mate for life. The young beaver “kits” typically remain with their parents for up to two years.