<\/a>A beaver hauls a load of ferns and lupine. (Courtesy Photo \/ Jos Bakker)<\/p><\/div>\n\t\t\t\t
Dams are built to create ponds deep enough that they don’t freeze to the bottom. They are built of sticks, mud and vegetation. All members of the family work on building and on repairing breaches in the dam. In flat country, dams can be many meters long; in steep canyons, they can be over five meters high. In addition, sometimes beavers dig canals from the home pond out into the surrounding wooded area; this facilitates dragging branches from the woods back to the pond.<\/p>\n\t\t\t\t
In regions where ponds are ice-covered in winter, beavers make caches of branches for winter food. Caches may poke up above the ice but the animals have access to the branches under the ice, and they seldom do much wood-cutting in winter. Although they can use any kind of tree or shrub, willows and aspens or cottonwoods are favorites. They eat the twigs and inner bark, with the aid of special bacteria that break down cellulose (in wood). In summer, however, they depend a lot on softer, herbaceous vegetation.<\/p>\n\t\t\t\t
Beavers are the largest rodent in North America (and second largest in the world). They commonly weigh up to 70 pounds but occasionally exceed a hundred pounds. They’re heaviest in fall, after putting on fat for the winter. The muscular body is capable of hauling large branches for many meters.<\/p>\n\t\t\t\t
Beavers have a very specialized way of life, maintained by behavioral adaptations (building dams, etc.) with a variety of physical adaptations to suit. They can stay underwater for fifteen minutes, if they have to, swimming more than half a kilometer. Their lungs are very efficient, capable of exchanging 75% of their capacity (three or four times more than humans) with each breath. Oxygen storage capacity is not outstanding but they have a high tolerance for carbon dioxide. The mammalian diving reflex works well; when submerged, the heart rate slows and most blood is shunted to brain and heart.<\/p>\n\t\t\t\t
The big incisor teeth grow constantly; chewing wears down the back side of the teeth but leaves a sharp edge where the iron-hardened (and orange colored) front side of the teeth endures. When biting into wood, they commonly anchor the top teeth and actively chew with the lower jaw, which is moved by a powerful muscle in the muzzle. They can close their lips behind the front teeth, keeping wood chips out of the throat. The jaw joint is high on the skull, allowing the hard-ridged molars to meet in parallel for grinding vegetation. Eyes and ears are also high on the skull, so a swimming beaver can sense the upper world; ears and nostrils can be closed while underwater, and a protective nictitating membrane slides over the eyes.<\/p>\n\t\t\t\t
Toes on the big hind feet are webbed, giving good thrust for swimming. The front feet are quite dexterous, the fingers adept at handling small items; they also carry wads of mud and vegetation while a beaver is swimming or walking. The wide tail is a multi-purpose appendage: a rudder when swimming, a brace when standing to chew a tree trunk, a stabilizer when walking on hind feet, sometimes a cushion for sitting on cold or lumpy ground, and the well-known tool for slapping the water to give the alarm signal.<\/p>\n\t\t\t\t