Our Window on Nature

. . . exploring the world around us

Leave It To Beavers

Filed under: Mammals — Lowell Christie -- May 28, 2016 @ 8:24 pm

BeaverNature lovers can’t easily watch beaver behavior, because the animals are, for the most part, creatures of the night. Yet we needn’t crouch on a stream bank swatting mosquitoes to know beavers are around. A dam reveals their presence.

No matter how large this dam might be, a typical beaver pond supports just one family grouping: the parents (monogamous while both parties live), one or two yearlings, and the current year’s offspring. In addition to this core family group, older siblings occasionally return to their natal surroundings if they are unable to secure, or are evicted from, a homesite. Apparently, the group identity persists even after a long absence, because there have been cases when a familiar individual is welcomed back after several years.

Because young beavers have much learning to do, they don’t leave home nearly as quickly as do other members of the rodent clan. As the largest and most intellectually developed member of the rodent family, a beaver is far removed from its more diminutive cousins. A mouse, for example, is ready to make its way in the world within 20 days of its birth. But most of its essential behaviors are inbred. Beaver brains show the kind of well-developed cerebral cortex that an animal needs to be able to master complex operations such as dam building and communal living.

French biologist P. Richard devised a way of comparing the problem solving behaviors of a muskrat, a rat, and a beaver (all rodents) by suspending a piece of bread on a string. The muskrat and the rat simply jumped for the bait, trying to bite it to pieces. The beaver, on the other hand, studied the situation for a while, and then cut the string.

Richard further tested a beaver by presenting it with a willow tree that had been protected at the base by wire netting. The resourceful beaver stacked up a pile of refuse just high enough to allow it to reach the bare bark of the tree.

With those two experiments in mind, it isn’t surprising that young beavers stay with their parents longer than other rodents. They leave when they are nearly two years old, when they have the maturity and the education necessary to survive.

When the time does arrive for the youngsters to establish their own territories, they face several possibilities. Should a beaver find an active den that lacks an adult of its own sex, it might be able to move into an established residence. (Lodges are often used for decades by many generations of beavers.) When one member of the partnership dies, the survivor selects a replacement; but, since beavers don’t reach sexual maturity until they are approximately 3 1/2 years old, they are more likely to need bachelor quarters first.

The newly launched two-year-old needn’t worry about finding a readymade pond, however. It is the nature of this beast to build; so, if given access to a stream not already claimed by one of its kind, the beaver simply creates its own habitat from scratch.

Man is the only animal to surpass the beaver in engineering feats, and a human has the advantage of books and a formal education.  Human engineers place dams across rivers and streams to preserve water for various uses. Beaver dams raise the water level, too, but not for use in cities or for irrigation. Ponds are necessary to create a moat around the beaver lodge, thus making the castle less vulnerable to predators.

Humans use mathematical formulas to calculate how high and how bulky a dam must be in order to withstand the pressure of water. Beavers are less precise. Instinct guides them, not science, but they have enough intellect to solve some problems. They have the manual skills they learned while first watching and then helping their parents maintain the family homestead. The rest comes from practice.

When building a dam, a beaver typically fells trees that are approximately eight inches in diameter. Think about how long it would take you to ax down a tree of that size. A beaver can do it in five minutes. Those huge, chisel-like incisors certainly make the wood chips fly.

It’s an old grandfather’s tale that beavers always cut a tree so that it falls toward the water. However, the beaver is usually smart enough to get out of the way.

Cutting down trees isn’t the first step in dam building, however. The animal lays a foundation of stones, mud, sticks, and plant roots before it begins to weave brush and saplings into a web that can be packed solid with more mud, rocks, and twigs. Larger trees are included to add strength to the dam.

As the water level rises, so does the beaver dam. Considering the effort involved, it’s mind-boggling to think that it takes a family of beavers (with four to six builders) only a week to build a 40-foot-long dam. In 1927, naturalist William Carr wrote, “In the space of four months, they (two adults and two youngsters) had built a pond, a home, and five dams with a total length of about 380 feet. In addition to this, they had collected almost enough food to sustain them during the winter.” Carr also observed a 2,000-foot dam in Montana, another in New Hampshire that was nearly a mile long, and a dam in Wisconsin that backed up 12 feet of water.

The transformation of a meadow into a beaver pond may be irritating to ranchers, but it is a miracle to the rest of us. Gradually, as the water level rises, new plants establish themselves along the banks. Before long, ducks come to feed and, perhaps, to raise their families among the reeds growing in the shallow water. Herons feed upon fish, snails, and frogs. Smaller birds fill the air with song. By providing for themselves and their families, a family of beavers creates a miniature Eden, one that not only meets their needs, but those of many other creatures – even man’s need for peaceful reflection.

FMC792

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