Ready Cache

Natural History, Oct, 1999 by Peter J. Marchand

IN THE FIELD

Traipsing through a stand of aspen and lodgepole pine in Colorado's Arapaho National Forest one October day, I came across dozens of cone caches heaped at the bases of trees and carefully arranged beneath a downed tree. All in plain sight, the neat little piles of twenty to fifty cones were concentrated within an area perhaps 150 feet in diameter. Succumbing to my quantitative urges, I started counting, and within the next two hours I tallied well over a thousand cones--the diligent work of a red squirrel.

October is harvest time, and when fruits are ripe and the storage organs of plants are plump with carbohydrates, few animals can resist hoarding food. Chickadees and nuthatches hide seeds under flakes of bark and lichens. If you watch carefully, you'll see that some of their apparent foraging may actually be food caching. Deer mice and flying squirrels leave stashes of food in tree cavities or bury them shallowly under leaf litter. The squirrels also wedge nuts into cracks and branch forks. Pikas increase their "haying" activities, gathering and drying leafy vegetation beneath overhanging rocks (and fending off any pika neighbors that might attempt a cache robbery). Even in the arid Southwest, kangaroo rats and pocket mice, using their external fur-lined cheek pouches, gather the fall seed crop as insurance against later shortages.

What is most impressive about all this activity is the remarkable quantity of food cached by some individuals. John James Audubon once uncovered the autumn stores of an eastern chipmunk and measured one gill (approximately half a cup) of mixed wheat and buckwheat seeds, a quart of hazelnuts, two quarts of unmixed buckwheat seeds, and a peck (eight quarts) of acorns. No less impressively, six or more Alaskan taiga voles, each only a third the size of a chipmunk, have been known to cooperate in putting up thirty-six quarts of fireweed and horsetail rhizomes in one season. A dozen Brandt's voles in Mongolia have been credited with caching seventy pounds of dried leaves, stems, and roots for the winter. A pair of adult beavers can stockpile enough woody shrubs and tree branches underwater (up to 1,100 pounds of usable forage) to provide for their kits through a long winter of isolation locked beneath the ice. And a single red squirrel in a boreal spruce forest can stash upwards of 15,000 cones and hundreds of mushrooms before the snow falls.

The reasons for autumn food caching are mostly (but not always) obvious. Voles (mouse-sized animals that are common at mid- and northern latitudes) remain active beneath the snow all winter. Their use of communal food stores reduces the amount of energy expended in foraging and keeps more bodies in the nest for longer periods, thus conserving heat. But when male Richardson's ground squirrels cache food, it is to gain a reproductive advantage in the spring rather than to ensure survival. Profound hibernators, these animals easily accumulate sufficient body fat to get through winter in the western U.S. mountains without supplemental food stores. The female does not hoard, but the male Richardson's often puts away substantial quantifies of seed in its burrow. Why this difference? Males require several days after awakening to regain sexual competence for spring mating. Those that emerge early satisfy their energy needs with stores accumulated in the fall, thereby avoiding any risk of food shortage caused by late snow.

While hoarding may be the best hedge against future shortages, it is not without its drawbacks. In nature, as in human society, accumulated wealth must be protected. The threat of theft has led to various hoarding strategies, generally involving either the concentration of food in a central, easily defended larder or the scattering of food widely enough so that the discovery of one or a few small caches is unlikely to lead to the discovery of all.

Observing food hoarders in autumn quickly reveals which of these strategies an animal employs. The red squirrel, for example, is a conifer seed specialist that tends to cache food in conspicuous middens, often at the bases of large or densely clustered conifer trees near the center of its territory. Cones are buried whole and later dismantled for their seeds, leaving huge mounds of discarded scales--the midden--into which, generation after generation, thousands more cones are buried. One advantage of the red squirrel's strategy is easy defense of the food store. The gray squirrel, by contrast, is less territorial and caches food by "scatter-hoarding" items singly or in small clumps over a wide area. Where home ranges of neighboring gray squirrels overlap, chance discovery of another's food stores is possible but inconsequential.

By far the greatest challenge with scatter-hoarding lies in relocating caches at a later date, often after snow has masked their whereabouts. Birds like the Clark's nutcracker and the pinyon jay, which live in western North America, bury pine nuts in the soil each fall at thousands of locations. When they recover the nuts in spring, they rely not on smell, which is poorly developed in birds, but on memory (even for food-caching carnivores such as foxes, it turns out that a good memory is better than a sensitive nose). As remarkable as their recall is, however, jays and nutcrackers still hedge their bets, generally stashing about twice their caloric needs for the season ahead.