Passage IV

NATURAL SCIENCE: The following passage is adapted from an article by Lucia Jacobs titled "Cache Economy of the Gray Squirrel" (©1989 by American Museum of Natural History).

In the eighteenth and nineteenth centuries, travelers in North America reported seeing armies of squirrels, sometimes numbering in the thousands, making their way across fields, forests, and rivers. These movements continued to amaze and appall the citizenry until the squirrels' habitat, deciduous forest, was greatly reduced in the late 1800s.

The gray squirrels' migrations were undoubtedly a response to changes in their food supply. Their species evolved in a virtual ocean of food trees一the primeval deciduous forest that once covered the North American continent from the Atlantic Ocean to the Mississippi River. The annual changes in the squirrels' diet must have been much the same then as they are now. In the spring, squirrels eat tree buds, tree flowers, and early seeds, such as maple samaras. These provide energy and key nutrients, such as calcium and sodium. But by midsummer, food is scarce, and by late July, the squirrels lose weight, resorting to such inferior foods as mushrooms. With the coming of fall, however, the mainstay of the squirrels' diet—acorns and other nuts—appears.

But even when the forests were at their most extensive, and oaks and hickories in seemingly endless supply, the fall harvest was unpredictable. Oaks and hickories produce huge crops of acorns and nuts, but only at odd intervals. During some autumns, often those following a late spring frost, which may kill the trees' flowers, there is virtually no seed crop at all. Compiling information from newspapers and travelers' accounts, A. W. Schorger showed how squirrel migrations correlated with years of poor seed production. His work, published in 1949, showed that other species were also affected by the trees' crop failure; in acorn-poor years, for example, newspapers reported free-running hogs starving to death in the forest.

Innumerable changes in the North American landscape eventually brought an end to the great squirrel migrations, although an occasional small exodus has been observed in the twentieth century. The gray squirrel is still, however, very much affected by fluctuations in nut crops.

As a rule, tree squirrels keep alive from fall until spring by eating stored foods, while ground squirrels hibernate. (Some squirrels, such as the eastern chip munk, do both.) But some tree squirrels have an easier time of it than others. Those that live in evergreen forests, such as the red squirrel of eastern and northern forests and the chickaree of western forests, generally gather pine cones and hoard them near their nests. Such pine cones may not be abundant every year, but if cut green and stored in a moist spot, they may last several years. But gray and fox squirrels must replace their stored food supplies every year, for acorns and nuts germinate the same year they are buried. When trees fail to produce many nuts, these squirrels' reproductive success, as well as survival itself, is jeopardized.

Given the gray squirrel's life-and-death dependence on stored food, its manner of storing may seem oddly inefficient. Most food-storing mammals maintain either a single larder or a mix of central larder and many spread-out smaller caches. Eastern chipmunks, for example, keep a large underground store in their burrows and, as this is depleted, turn to scattered surface stores to replenish it. The gray squirrel's habit of burying each nut separately—called scatter-hoarding—seems cumbersome. Why does this squirrel avoid putting all its "eggs" in one basket?

Christopher Smith, of the University of Kansas, has suggested that the distribution of nuts and acorns and competition from other seed eaters force gray squirrels to scatter-hoard. Because gray squirrels may not know which trees will produce a big crop, the argument goes, each squirrel must forage over a large area each year. Such a large area cannot be exclusive, as other squirrels inevitably arrive to harvest seeds from the masting trees. For that matter, even small areas cannot be defended against an influx of competing squirrels, so the only chance is to cover a lot of territory.

Support for this theory comes from Helen Lair, of the University of Laval, who observed red squirrels in Quebec and found that they put food in larders only when it was abundant and easily harvested from an area small enough to be subsequently defended. Otherwise, they acted like gray squirrels, scattering many small, hidden, undefended caches.