The Squirrel Time Forgot

by Gary Turbak Photographs by Tom and Pat Leeson Here dwells on the North Rim of the Grand Canyon a diminutive prisoner of time, a sequestered witness to the mystery of evolution. Just as the ages have made the great chasm a marvel of geology, they have set the Kai bab squirrel apart from other living things. Apart literally and poetically.

Evolution at work: The Abert squirrel (LOWER LEFT) has a white belly and a dull gray tail. Its cousin, the Kaibab (LEFT AND BELOW) has developed a black underside and a white tail.

Millennia ago, a small squirrel with pointed hair tufts rising from its ears thrived in the woods on both sides of the Colorado River. The forest that spread north from the Grand Canyon provided ideal habitat for this treetop rodent with white belly and gray tail. But geologic and climatic forces had not yet worked their final magic on the continent. At a rate no squirrel (or man, had one been there) could perceive, the land around the North Rim rose to form what we call today the Kaibab Plateau. Gradually, too, the Great American Desert expanded to lap at the edges of this raised island of forest. To the south, the huge canyon stood as an impenetrable barrier. Eventually (but in an instant as geology and evolution mark time), the tassel eared squirrels on the Kaibab Plateau found themselves cut off from all others of their kind. Their isolation atop the Kaibab would be of little interest were it not for the curious course evolution has chosen to take among the exiles. Across the canyon and throughout the Southwest, the ori ginal species of squirrel has continued to flourish. Biologists call it the Abert, and its belly remains white and its tail a dull gray. But the Kaibab squirrel-sometimes just a few miles away from its estranged cou sin-has changed dramatically. Its under side has become black and much of its tail a prominent white, making it one of the most beautiful squirrels in the world. This, say scientists, is an excellent example of evolution in progress, even though biologists can't agree on the reason for the change. One theory is that the color shift was an adaptation to an as yet undetermined habitat condition on the plateau. It's more likely, however, that the reversal of belly and tail colors occurred as random mutations that soon became part of the genetic heritage of the isolated squirrels. Given enough time, the Kaibab squirrel could evolve eventually into a separate species from its Abert progenitor.

Presently, the two are classified as parallel subspecies of tassel-eared squirrel. Aside from color, the two animals are much the same. Both are dependent on the ponderosa pine, which they must have for food and cover. Both exist on a diet of seeds stripped from pine cones and inner bark peeled from twigs. And both of them sport prominent and inexplicable ear tufts. No one knows how many Abert squirrels exist throughout that subspecies' large range, but biologists estimate that the Kaibab population fluctuates between 17,000 and 28,000. The U. S. Forest Service, which manages much of the Kaibab Pla teau, has promised to preserve the dense ponderosa habitat the rodent requires. Time may have forgotten the Kaibab squir rel, but mankind has not.

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