Owls Aren't Wise and Bats Aren't Blind (2 page)

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Nature’s Engíneer: The Beaver

MYTHS

Beavers pack mud with their tails.

Beavers can fell trees in a desired direction.

Beavers always build dams.

“Bank beavers” are a different species.

Beavers eat fish.

If a beaver loses its mate, it won’t mate again.

“Nuisance” beavers should be moved to another location.

THE SINGLE MOST BASIC AND IMPORTANT FACT ABOUT THE BEAVER
(CASTOR
CANADENSIS)
IS THIS: NEXT TO HUMANS, THE BEAVER IS UNDOUBTEDLY THE MOST PROFICIENT OF NORTH AMERICAN INHABITANTS AT MANIPULATING THE ENVIRONMENT FOR ITS OWN BENEFIT. For several hundred thousand years we humans have been able to harness fire and manufacture a variety of tools, ranging from primitive stone and bone implements to modern steam shovels, to assist us in modifying our surroundings. Beavers, however, have become legendary dam builders by using only instinct and the tools with which nature has endowed them.

Beavers have the ability to modify their immediate environment quite radically, even when they have very little to work with. Give a pair of beavers a reasonably flat area, a scant trickle of water, a modest supply of trees or shrubs, and they can quickly create a dam—large or small, as the situation demands—a pond, and a dwelling secure from predators. In the process they quite unintentionally create a great deal of valuable marsh or wetland habitat for many other species.

Perhaps because beavers are such intriguing creatures, with astonishing engineering and construction ability, they’re one of our most misunderstood mammals. Through the years, they’ve become encrusted with a number of major misconceptions that often obscure their true nature. For example, one of the most common folk myths is that beavers pack mud, or carry mud, with their tails. They don’t. How this notion gained such currency is anyone’s guess, but it simply isn’t true: although a beaver’s tail has many uses, packing mud isn’t one of them. Whenever beavers need to carry or pack mud while constructing their dams or houses, they use their front paws.

That fact fails to diminish in the slightest the importance of the beaver’s tail. Broad, flat, hairless, and scaly, it’s shaped like the blade of an old-fashioned canoe paddle. In fact the familiar and still-popular canoe paddle design is referred to as a beavertail paddle.

The tail’s unique conformation makes it suitable for so many purposes that it serves its owner as a virtual tool kit. For openers, the tail is the beaver’s alarm signal. Anyone who’s spent much time around a beaver pond, especially at dusk, is familiar with the loud
ka-WHOP
that resounds across the still waters. When a beaver detects human presence—or any other form of potential danger—it warns the other members of its colony by slamming its tail forcefully onto the surface of the water. The result is a mighty splash similar to what one would expect if a fifty-pound rock were heaved into the water!

Of necessity, beavers spend much of their time swimming, and the wide tail acts as both rudder and diving plane while the beaver moves to and fro, up and down, within its watery domain. A beaver’s tail is also very fatty, and this fat can act as an emergency food supply in time of famine. And because it’s hairless, the tail functions as a heat exchange mechanism during periods when its owner is exceptionally active. It even serves as a brace or prop while the beaver sits up and gnaws during the process of felling trees. Clearly, the beaver’s tail is the very model of a utilitarian appendage—but the one thing it doesn’t do is pack mud!

Another common myth is that beavers, like human loggers, can fell trees in the desired direction. The truth is far less glamorous. The beaver is a rodent, with incisors that act as big, sharp chisels. When a beaver decides to cut down a tree, it usually gnaws a V-shaped notch completely round the tree, slanting up from the bottom and down from the top to meet at the apex of the cut.

The beaver continues to work around the tree, all the while deepening and enlarging the notch. Eventually there’s too little wood left in the middle to support the tree, and it falls, willy-nilly, in any old direction. Far from knowing or planning the direction of fall, beavers are occasionally killed when the trees they’re felling accidentally land on them. Sometimes a beaver will gnaw through a tree from only one side, but that choice assuredly isn’t dictated by any intention of felling the tree toward a particular spot.

It’s fascinating to speculate on the many ways in which misconceptions about wildlife begin. Some are fairly easy to fathom, others less so. We like to be charitable and think that people are honest, but some individuals make up tall tales and pass them off as the truth, or just plain lie for reasons best known only to themselves. A case in point is the conversation I had with one man about beavers.

He assured me with a perfectly straight face that he had personally watched beavers packing mud with their tails. Further, he said that he had seen a beaver fell a hardwood tree over a foot in diameter in less than five minutes. Beavers can remove sizable chips with those big front teeth, but they certainly aren’t that fast. Such a feat would tax most people with a sharp ax and saw! The fact is that a beaver requires at least two or three nights of hard work, and sometimes more, to take down a single large tree.

Beavers are indisputably dam builders
par excellence,
justly famous for this ability. They live exclusively in colonies, and construction of a dam is the first step in founding a colony for a newly mated pair of young beavers. There’s a good reason for this. A high percentage of our North American beavers live where winter is severe and the ice becomes very thick. Beavers store their winter food supply underwater, and they require water deep enough that neither the underwater entrances to their home nor their food supply will be encased by ice.

This is purely instinctive behavior that has nothing to do with reasoning. Beavers evolved under harsh conditions that necessitated water depth sufficient for winter survival. Now they’ll mindlessly build dams wherever they possibly can, even if there’s no apparent reason for it. Further, they’ll continue to raise the height of a dam year after year, oblivious to the fact that the depth of their pond was more than sufficient from the beginning. Even in large lakes and ponds, beavers will attempt to dam the outlet, despite water depth many times what they require for survival.

It’s incorrect to assume that beavers always build dams, however. Beavers often live in wide, deep streams and rivers that are too difficult a challenge even for their formidable dam-building skills. Under such conditions, beavers tunnel up into the riverbank to create a home. Beavers living in this fashion are often referred to as “bank beavers,” and old-timers frequently claimed that they were a different species. They aren’t, but they do seem to have learned that they can’t dam that particular stream, and have somehow managed to override their age-old compulsion to build a dam.

Dam construction itself is a fascinating process. The beavers begin by laying down brush, with the butts pointed downstream. Mud and rocks are brought to the site to anchor and coat the brush, and various materials, from pieces of driftwood to old bottles, are gradually incorporated into the dam as it grows higher and higher. Mud is constantly packed in among the sticks, brush, and other material, and the upstream surface of the dam is coated and recoated with mud.