Owls Aren't Wise and Bats Aren't Blind (3 page)

The whole affair is really a beaver version of concrete. Just as we use cement, weak by itself, to bind gravel and iron rods into extremely strong concrete, beavers use mud as their cement, with brush, sticks, and other materials as the reinforcing medium. The end result is a structure that’s strong and nearly impervious to water, except for the overflow through the brush and sticks at the very top of the dam.

A beaver dam is such a fascinating edifice that it’s worth inspecting at close range, both for its ingenious construction and for the variety of materials often incorporated into it. Given the near ubiquity of beavers today, most people shouldn’t find it difficult to locate a beaver dam that they can scrutinize at leisure and thereby gain a much greater appreciation of this clever rodent’s abilities.

In addition to the physical construction of the dam itself, beavers have some related talents. For one thing, they seem to have an uncanny eye for selecting dam sites that offer a high ratio of pond acreage to size of dam. From the beaver’s point of view, the perfect dam site combines a short span between high banks with a wide, flat area just upstream. A minimum amount of construction in this sort of location will yield a large, deep pond.

Perhaps an even more remarkable instinct causes beavers to vary a dam’s conformation according to conditions. If the current is slow or the span short, the dam is constructed more or less in a straight line. However, longer dams where the current is strong are curved, with the convex side pointing upstream. Of course, this just happens to be the way in which humans engineer big dams!

After building their dam to a satisfactory height, the beaver pair next fabricate a dome-shaped house called a lodge. Like the dam, the lodge is made of a mixture of sticks and mud, although it’s constructed with far less care. The beavers simply heap everything up, starting from the bottom of the pond and extending the mass for several feet above water level. Then, beginning at a safe depth underwater, they gnaw and dig their way upward until they’re above water level. There they scoop and gnaw out a roomy chamber.

The size of the lodge depends on the number of beavers in the colony. (More about colony size and composition later.) Lodges six feet or more above water level aren’t uncommon, and in exceptionally old colonies—forty to fifty years old—lodges have been found as high as twelve to fifteen feet above water level.

The interior of the lodge may consist of a single, all-purpose chamber, or it may have a feeding chamber and a connected resting chamber. A good deal of room is required to house a number of beavers, and the interior of an extremely large lodge may be capable of seating several people at once!

Once the lodge is finished, it provides the beavers with great security. Just as the steel rods in reinforced concrete add a great deal of strength, the high ratio of sticks to mud in a beaver lodge makes it highly resistant to attack. Even in summer, with relatively thin walls surrounding the above-water chamber, it’s very difficult for a predator to break into the lodge, and in that event the beavers can simply exit into the safety of their pond. In winter, when the mud freezes, the lodge becomes as hard as iron, impregnable even to the largest, fiercest predators.

Although a beaver lodge is admirably suited for ensuring the survival of its occupants, we shouldn’t regard it as the equivalent of a human dwelling. Whenever he gives a lecture about beavers, Thomas Decker, a biologist with the Vermont Fish and Wildlife Department, likes to point out that a beaver lodge doesn’t fit our notion of a nice, cozy home. The interior is hot and humid in the summer, smelly at all seasons of the year, and infested with a variety of undesirable trespassers. Hordes of insects often abound inside a lodge, water snakes like to crawl into it, and otters may enter from time to time; the latter, incidentally, will try to kill young beavers up to the age of a month or two. Clearly, this is no palace by human standards!

It’s widely and erroneously believed that beavers include fish in their diet. However, beavers are entirely vegetarian. In summer, they mainly consume a variety of aquatic plants, algae, and some grasses and ferns. Their principal winter food supply is the inner bark of various species of trees and shrubs. Aspen, willow, alder, and maple are favorites, although they will eat a variety of other hardwood species, including ash and birch. Beavers normally eschew the bark of evergreens as food, but will turn to it if forced to by a shortage of better foods.

When winter draws near, the beavers cut quantities of tree branches and shrubs and bury the butt ends in the mud next to their lodge. As soon as the pond becomes icebound, the beavers retire to the safety of their lodge; when they want food, they swim out, cut sections of branches, bring them back to the lodge, and munch off the bark. The peeled sticks are carried outside to float up under the ice; otherwise their debris would soon fill the lodge. Although bark represents their primary winter food supply, beavers also supplement their diet with the thick, tuberous rootstocks of pond lilies, at least in those ponds where these plants grow.

Like a number of other wild creatures, beavers mate for life, but that doesn’t mean what most people seem to think it does. The common perception of lifetime mating in the wild is that the death of one partner relegates the survivor to a single existence for the remainder of its life. That’s not the case at all. Like many humans, a wild animal that normally mates for life will, after the death of its mate, seek another.

As previously noted, beavers live in a colony that consists exclusively of a single beaver family. Each colony begins with a mated pair of young beavers. After the pair have constructed dam and lodge, and laid down a winter’s food supply, they breed during the winter. After a gestation of about ninety days, the young, called kits, are born in the spring. There are usually three to five kits in a litter, and occasionally more.

The young remain with the parents through the winter, and the following spring the colony is enlarged by a second litter of kits. The colony now contains the parents plus the offspring from two successive years. All of the beavers in a colony share the work of maintaining the dam and lodge, and cutting and storing the winter food supply.

Although a number of myths have grown up concerning beavers, the phrase “busy as a beaver” most assuredly isn’t one of them. Beavers labor intensively during the warm months, preparing for the long winter to come. Constant diligence and great effort are required to ensure an adequate and secure habitat, as well as an ample winter food supply. Only when the pond finally becomes icebound can the beavers relax and enjoy the fruits of their labors.

Big changes occur during the colony’s third spring. As another litter of kits is born, the young of two years before either leave the colony voluntarily or are driven out by their parents. These “dispossessed” beavers must now seek new territory, find mates, and begin new colonies. Stark evidence of this springtime diaspora is often visible in the form of dead beavers killed by automobiles as they try to cross highways.

Thereafter, for the life of the colony, a new set of kits will be born each spring, and the two-year-olds will either set out on their own or be driven out. Although this system may seem harsh, it’s necessary for the colony’s survival. Without this annual dispersal, a beaver colony would soon become impossibly overpopulated, to the detriment of all.

The manner in which the dispersed two-year-olds find mates is an interesting one, worthy of a bit of exploration. The beaver’s genus name,
Castor,
comes to us through Latin from
Kastor,
the Greek name for the beaver. Eventually,
castor
even became a term for a beaver hat during the heyday of that article of apparel.

Of greater relevance in this instance is that beavers of both sexes secrete an oily, pungent substance that’s also known as castor. This rather malodorous liquid is produced by a pair of glands in the inguinal region, close to the anus. Beavers are highly territorial and have a well-developed sense of smell, so as soon as a young beaver finds a suitable site for a dam, it marks the boundaries of its territory with what are called
castor mounds.

Castor mounds are piles of mud and debris carried by the beaver with its front paws. These heaps are normally placed close to the water, and the beaver deposits castor on them. Such mounds can grow into sizable structures, especially where the territories of two or more colonies intersect. In that situation, a beaver from one colony will try to cover the scent of another colony by heaping more material on the mound and then depositing its own castor. As this “castor duel” continues, the mounds can sometimes grow to waist height on a human!

Because of its excellent sense of smell, a two-year-old beaver scouring the countryside for a home will soon locate and investigate castor mounds. If the mounds’ originator proves to be an unmated beaver of the opposite sex, a new colony has its start.

Beavers are obviously highly aquatic mammals, and they have a number of wonderful evolutionary adaptations that make it easy for them to function in and under the water. When it comes to moving about in the water, beavers are extremely well equipped. In addition to the nearly all-purpose tail, the beaver has large, fully webbed hind feet to provide efficient propulsion. These can thrust the big animal through the water with ease, even when the front feet are occupied with carrying mud or rocks.

Beavers can also stay underwater for an extraordinarily long time—at least fifteen minutes at a stretch—and they accomplish this feat with lungs of normal size. How is this possible? First, their heartbeat drops to half its usual rate when the beavers are underwater, thereby reducing their metabolic rate and their oxygen consumption. Second, they can also utilize most of the oxygen in their lungs—an astonishing 75 percent versus 15 percent in humans!

Nor does working underwater pose problems for the beaver. The instant a beaver submerges, certain reflex actions take place. A transparent “third eyelid,” called a nictitating membrane, covers each eye to allow good underwater vision. Simultaneously, special membranes close off the ears and nostrils. Meanwhile, a similar membrane blocks the throat and permits the beaver to gnaw wood or gather aquatic plants beneath the surface without swallowing water.

To top off the long list of adaptations, the beaver also has a double fur coat. The outer coat consists of long, glossy guard hairs, while the inner coat is so dense that water doesn’t even reach the beaver’s skin. With this sort of protection, the beaver has no difficulty swimming and diving in the most frigid water.

The beaver’s wonderful fur coat, so necessary for its survival, was once the cause of its near demise. When explorers and settlers came to North America, they found beavers in enormous quantities. As they sent beaver pelts, often obtained by trading with Native Americans, back to Europe, beaver hats and coats grew steadily in popularity. Soon beaver garments became all the rage in Europe, and the North American fur trade grew to enormous proportions. Indeed, it’s been said, without too much exaggeration, that the trade in beaver furs built North America.

Literature from the late sixteenth century until well into the nineteenth century reflects the immense and continuing popularity of beaver hats. Oliver Wendell Holmes, for example, describing the well-dressed man, adjured his readers to “Have a good hat; the secret of your looks / Lives with a beaver in Canadian brooks.”

An idea of the immense demand for beaver fur can be gleaned from the fact that the Hudson’s Bay Company alone sold over three
million
beaver pelts during a twenty-five-year period in the second half of the nineteenth century. As beavers were trapped relentlessly in the absence of any conservation laws, forests were simultaneously felled to make room for farmland. The combination of unregulated trapping and vanishing habitat proved deadly. By 1900, beavers had become scarce virtually everywhere and were extirpated throughout much of their range. Then the pendulum slowly began to swing.

With the advent of modern wildlife management techniques, laws to protect beavers were passed and strictly enforced. Beavers were also live-trapped in areas where they were still plentiful, and restocked in regions where they had been eliminated. For example, in 1921, six beavers were trapped in New York State and released in southern Vermont; eleven years later, beavers were reintroduced from Maine into northeastern Vermont.

No species, however rigorously protected, can survive without adequate habitat, and here the ways of humans, once so inimical to the beaver’s survival, began to favor it. Concurrent with protective laws and the reintroduction of beavers, there was a slow but steady abandonment of agricultural land. This land soon reverted to the forest habitat ideal for beavers. Slowly at first, and then with increasing rapidity, the beavers returned to their old haunts.

The restoration of the beaver has had many beneficial effects. Countless wetlands, once largely dried up, now dot the landscape, helping to hold back high water and recharging underground aquifers. Even more important, perhaps, is the vast amount of habitat that beavers create for other species of wildlife. Indeed, in terms of benefits conferred on other wildlife species, beavers have to be awarded the palm as our single most important wild mammal.

Biologists credit beavers for their important role in the remarkable resurgence of moose populations in areas such as northern New England. Moose thrive on—indeed, require—the summer nourishment provided by aquatic vegetation in shallow ponds, and many beaver ponds are ideal for the growth of those plants. Further, moose need water where they can stay cool in the heat of summer and escape, at least temporarily, the swarms of biting insects so common throughout their range.

Moose are by no means the only beneficiaries of the beaver’s work. Muskrats, otters, and other mammals utilize beaver flowages. Many waterfowl species nest on the ground along the shores of beaver ponds and raise their broods on the ponds. Cavity-nesting birds, including wood ducks and golden eyes, find nest sites in standing dead trees killed by the flooding from beaver flowages. Herons, bitterns, egrets, and other wading birds find fertile hunting grounds along the shallow margins of these myriad bodies of water.