Food-Storing Strategies of Squirrels

The habit of storing food for later use is widely recognized as one of the most distinctive traits of squirrels. Perhaps best known for their mastery of food hoarding (food storage) are the territorial pine squirrels (i.e., red and Douglas squirrels). Common occupants of the evergreen, coniferous forests of Canada and the western and northern United States, these small but highly aggressive and vocal squirrels store huge quantities of conifer cones that they vigorously defend against competitors. Such extensive hoards (or "middens," as they are sometimes called), easily recognized by their conspicuous piles of cone cores and leaves under which new cones are placed, provide a cool, moist environment that is ideal for the storage of cones. Most middens contain enough food to last one to two seasons, and they are often passed on through several generations of squirrels.

Food storing is also a critical survival strategy for many of the hundred or more species of tree squirrels distributed throughout the temperate and tropical forests of the world. But unlike the pine squirrels, the tree squirrels have adopted an entirely different approach to the problem of stockpiling their groceries. Members of this group are far less likely to store all their food in a single location. Instead, they place one or a few food items just below the ground surface in many-sometimes hundreds-of widely dispersed cache sites (hiding places). The result is a spatial distribution of stored food that is impossible to defend against competitors, whereas the advantage is that competitors are less likely to steal from the squirrel's hoard.

Why the two different approaches? The nearly opposite food-hoarding strategies of the pine and tree squirrels - called larder-hoarding and scatter-hoarding, respectively - illustrate two extremes of a continuum of storing strategies employed by all food-hoarding animals. Most animals exhibit behavior well between the two extremes, and some vary their strategy with the environment, habitat, and conditions. On occasion, even pine squirrels will scatter-hoard and tree squirrels will larder-hoard, depending on the availability and type of food. But by and large the pine and tree squirrels each remain loyal to their adopted (evolved) strategy.

Most researchers believe that for food-hoarding animals in general the two strategies for storing food have evolved in response to one of four factors: (1) the time when the hoarder is active relative to potential cache robbers; (2) the aggressiveness of the hoarder relative to robbers; (3) the climate, which likely influences how well seeds will store and how easily they are recovered; and (4) the type of food that is stored. Among the squirrels, the fourth factor seems to have had a decisive effect. The larder-hoard of pine squirrels provides a cool, moist environment that prevents cones from opening, thereby preventing cache losses to the many mammals and insects that are unable to open the cones and get to the seeds. The location of the midden, close to the center of the owner's territory, also makes it energetically defensible against neighboring squirrels that might otherwise collect a free lunch.

But such a strategy would be ineffective for the nuts and fruits stored by many tree squirrels. These foods, high in energy value and easily consumed by many other species, would be virtually impossible to defend in an obvious larder-hoard. The alternative is to secrete each item in a hidden cache site and invest relatively little energy in defending it. Although the low density of food means it is probably indefensible, it is also less likely to be discovered by cache robbers.

These two strategies for storing food have far-reaching effects on many other aspects of squirrel behavior. The defensible larder-hoard of the pine squirrels contributes to their highly territorial social structure, which in turn dictates not only the animal's behavior but also many other aspects of its population biology. Similarly, for the tree squirrels, scatter-hoarding allowed the evolution of nonterritorial behavior, overlapping home ranges, and a pattern of dispersal of the young that is very different from that of the pine squirrels. A full appreciation of the behavioral decisions involved in storing food is key to exploring many other important aspects of the biology of tree squirrels.