Window on Nature
By Lowell & Kaye Christie, F47246
Imagine that you’re hiking along a stream and run across a beaver, North America’s largest rodent, busy at work. An adult beaver can grow to be four feet long (including its 12- to 16-inch tail) and weigh anywhere from 30 to 68 pounds. The beaver’s dark brown fur, which consists of tough “guard hair” on top of soft, dense fur, makes for a striking contrast to its flat, scaly tail. Its front paws are equipped with claws and its hind feet are webbed for swimming. Given the species’ lifestyle, this arrangement serves it well.
Beavers range widely in North America, except for the extreme northern regions of Canada, the desert Southwest of the United States, and Florida. Given that the entire beaver population was nearly eradicated more than a century ago, the species has recovered remarkably well. Beavers are picky about where they live, though, choosing sites in streams, marshes, and ponds.
The beaver lifestyle is almost entirely aquatic, but when the need arises, the critters are quite capable of traveling on land. They are, however, slow-moving and a little awkward, especially when compared to their agility in the water.
Over the years we’ve seen a lot more beaver handiwork than we have beavers. They are primarily nocturnal creatures, so early morning and early evening are the best times for a sighting. When out of the water, their efforts are focused on cutting trees for food or to use when building or repairing their lodges and dams.
It’s claimed that a beaver needs only five minutes to drop a small tree (one whose trunk is 4 inches in diameter). Standing erect and using its flat tail for balance, the beaver first uses its large, very sharp upper incisors to cut a ring entirely around the tree trunk. Next it will sink its equally well-equipped lower incisors into the space below the ring and start levering out the wood, chip by chip. Once the tree falls, the beaver gets busy trimming off branches and bark, cutting the wood into logs, and dragging it to the building site.
A beaver lodge must be in water deep enough to provide storage room for food and to keep the entrances submerged. To ensure proper water depth, the beaver may first need to build a dam. It’s quite a laborious project, but here’s a quick explanation of the process. A beaver will cut down a series of trees and drag the logs and branches to a narrow part of the stream to create the bottom layer of the dam. Next comes a layer of mud and stones, much of which seeps into the gaps between the trees. By alternating layers of trees and branches with more mud and rock, the dam level gradually rises to the appropriate height. The upstream face of the dam is then waterproofed with more mud. As for size, dams a quarter-mile long and 12 feet high aren’t unheard of, but most are smaller.
Beaver dam designs vary widely, but one certainty is that, no matter how carefully a beaver does its job, the dam will need repairs. The slightest sight or sound of running water stimulates the beaver to get back into carpenter mode and repair the dam.
Beaver lodges are built using the same process “” first, a large dome is built with layers of logs and branches sealed with rocks and mud. The work doesn’t stop until the dome top reaches three to six feet above the water level. Once the structure is completed, the beaver submerges and gnaws its way into the mass, making access tunnels and chambers with raised sleeping platforms. Finally, the outside of the lodge is plastered with mud, except for the hole at the top that’s essential for ventilation.
Not all beavers build lodges; those living along a river generally dig long, branching burrows into the riverbank, but with the entrances underwater. These are known as bank beavers. Those living in quiet streams, lakes, and ponds usually build the lodges, as well as dams to control the water flow. The lodges provide a safe place for the beavers to sleep and raise their young, avoid predators, and store food. A working beaver can toil underwater for up to 15 minutes before surfacing for air.
On land, the beaver is far less at ease than when in the water, and frequently it will interrupt its activity to sniff the air and look for signs of danger. It also listens for warnings from family members. To sound the alarm, a beaver slaps its tail on the water loud enough to be heard by other beavers (and everyone else) at a considerable distance.
A single pond usually “belongs” to one family group, whether the beavers live in burrows or in lodges. Access to burrows and lodges is always underwater to discourage predators. And even if predators know there’s a meal inside, they usually don’t spend the necessary time and energy digging through the walls to get to the beavers within.
Because the purpose of a beaver dam is to ensure that the water will be deep enough to cover the lodge, the busy rodents keep adding branches, logs, mud, and stones to raise and to reinforce the dam. The lodge also will be enlarged in succeeding years of use, and older lodges can reach double their original size.
Beavers are monogamous, but should one mate die, the other finds a new partner. The family consists of two adults, the current year’s offspring, plus several adolescents from the previous year. Beavers reproduce in the winter and the female has its litter of two to six kits in April or May. Once the new litter arrives, the two-year-olds are “encouraged” to strike out on their own. For the remaining family “” parents, one-year-olds, and the new litter, family life is cooperative. Everyone helps with gathering food and building and repairing the lodge and dam.
Speaking of food, beavers are herbivores. Their favorite foods include water lily tubers; clover; and the leaves, twigs, and green bark from fast-growing trees such as willows, cottonwoods, and aspens. As autumn approaches, beavers will begin to harvest more trees and shrubs to fill their food cache. The quantity and availability of their food supply will determine the condition and survival of the colony.
A beaver pond benefits just about all types of wildlife, large and small. But after a period of years a beaver’s food supply may begin to run out. At that point it must find (or construct) a new pond; build a new lodge or burrow; and settle into a new home.
After the beavers desert their previous home, the results of their labor continue to benefit wildlife. The once carefully tended dams begin to erode and develop leaks, and the pond drains, leaving only the original small stream. Plant succession begins on the newly exposed banks, first with grasses and wildflowers, and later with small woody plants. The result is a wet “beaver meadow” that serves other small mammals, deer, birds, and butterflies and myriad other insects.
Even beaver meadows don’t last forever, and decades of plant succession eventually return the area to forest, much like the beavers originally found it.