Mammals of the Southwest Mountains and Mesas

by George Olin

These were the Mountain Men, a hard-as-nails breed of frontiersmen in a class by themselves. In the period from 1820-1854, when a large part of the Southwest became part of the United States through the Gadsden Purchase, they roamed the plains and mountains of the American Desert. Their roster includes such legendary figures as Bill Williams, Pauline Weaver, Kit Carson, and James Pattie. Their argosy was a quest for the rich, brown beaver pelts which were a golden fleece indeed when presented to the fur traders in far-off St. Louis. In time, their moccasined feet beat a broad path across the western plains—a path then known as the Santa Fe Trail, but identified today as U.S. 66, the “Main Street of America.”

Today, many of the streams which supported beaver colonies in the desert places have vanished entirely, and others have been so effectively harnessed for irrigation and power that there is no place for beavers in them. In the higher mountains, however, there are many streams remote from civilization where clear ponds still sparkle in the sunlight, and the splash and dripping of busy beavers can be heard on quiet, summer evenings. Because beavers quickly become established under any conditions which are at all favorable, they have been reintroduced into numerous places where they had been extinct for many years. Usually this is good conservation practice, but under some conditions, it may prove a mistake. Ecologically speaking, beavers probably are the most important creatures in any animal community of which they are members. This is because these busy engineers not only impose a tremendous drain on the surrounding area for material, very often they also radically alter the character of the terrain to fit their own needs.

beaver

The life history of the beaver is one of the most interesting of all mammals. It has been studied for centuries by naturalists in both the New and Old Worlds, for the beaver, with but few differences, is native to both. All this study and observation notwithstanding, the habits are still only partially known. This is because the beaver is mainly a nocturnal creature which spends most of its daylight hours in the concealment of a lodge or burrow. Then, too, in the northern latitudes where the ponds are covered with ice throughout the long winters there is little opportunity to observe this phase of its existence. There is but one species of beaver in North America but about two dozen subspecies. The northern types and those which live in the mountains of the Southwest seem to be dam builders who live in beaver “lodges.” Those which inhabited rivers of the lower desert were mostly “bank” beavers which lived in burrows in the banks of streams. This latter type is rare today.

Perhaps the best way to understand the ecological importance of the beaver is through watching the rise and decline of a typical colony. Picture if you will a small, shallow stream flowing gently down a narrow valley in the mountains. Bordering the low banks is a thicket of alders. Back of them a thick growth of aspens extends to the edge of the valley and mingles with the spruce trees on the slope. Down this slope comes a young male beaver at a clumsy gallop, his broad tail striking the ground with an audible thump at every lope. This emigrant has struck out for himself because the colony to which he belongs has become crowded. He finds the stream and, since the water is too shallow to conceal him, crouches under an overhanging bank until darkness falls.

As soon as it becomes completely dark, he hunts for a suitable place to build a dam and soon finds a site to his liking. On one side of the stream a thick clump of alders projects from the bank, and on the other a water-soaked log is half buried in the bottom of the creek. From these anchor points, he begins his dam, building toward the middle from each side. The work calls for a great deal of the alder brush to be cut and sunk in the bed of the stream. There it is weighted down with rocks and mud until secure. Additional brush is brought and interwoven with the first; gradually the structure grows until in a few days it converts the stream into a quiet pool deep enough to hide the beaver, should an enemy appear. As the water rises it covers the bases of the alders, which begin to die in the pond.

The beaver next turns his attention to building a lodge. Selecting a point to one side of the current entering the pond, he begins as he did with the dam by sinking brush to the bottom and weighting it down with rocks. As he builds, he cleverly fashions several underwater entrances to the house that will be. When he has finished, the house projects several feet above the water, and the materials are so thoroughly interlaced and plastered that even the most determined enemy would despair of gaining entry to the living room. Debris from the construction has floated downstream to become lodged in and on the dam, making it more secure and watertight that it was when first built.

With the dam and the lodge both completed, the next task is to collect a food supply for the following winter. This is carried on intermittently during the autumn. It consists of cutting down aspens, whose bark the beaver dearly loves, sectioning the branches and small trunks into pieces which may be handled conveniently, and dragging them to the pond. Once in the water, they are weighted down and will remain in good condition for a long time. The beaver is joined in this task by a female which has also migrated from an overcrowded colony. Two need more food than one, consequently their trails begin to head a little farther into the aspen forest as they work through the crisp autumn nights. These trails converge as they leave the forest and approach the pond, and end in a few well-developed mud slides that enter the water. Constant traffic of the wet beavers leaving the water keeps the slides moist and slippery.

As winter settles in on the mountains, a thin skim of ice begins to form on the edges of the quiet water on cold nights. Then one night it freezes completely over. This causes the beavers no inconvenience at all because if on one of their underwater excursions they should wish to surface for air, they have but to swim to a shallow place with firm bottom, and with one quick lift of their powerful muscles break a hole through the ice with their backs. They can break surprisingly thick ice in this way. The beavers live in comfort and plenty throughout the winter. The living room of the lodge has been furnished with comfortable beds of the cattails that have already become established along the edge of the pond. The lodge, although tightly built, still admits enough air for the beavers and food is stored in plenty on the bottom of the pond. As the bark is gnawed from the aspen branches, the bare poles are added to the bulk of the house or used in further construction of the dam. Before long, the mild southwestern winter merges into spring.