Inside the Secret Lives of Squirrels and Their Half-Eaten Acorns

Learn about the eating habits of tree squirrels

Clearly, the gray squirrel was spending only a few seconds on each acorn. The animal removed the cap with its lower incisors, then proceeded to chip away at the outer shell and the nutritious kernel inside. But after a few seconds, the acorn was discarded and the behavior repeated. This continued a dozen times or more before the shadow of a passing red-tailed hawk forced the animal to seek thicker cover, and we were free to move in for a closer look. When we did, our suspicions were confirmed. The ground was covered with thousands of half-eaten acorns; and not one was eaten from anywhere but the top of the seed.

Why would any animal—especially one recognized for its ability to feed so efficiently—waste time and energy, and risk predation, only to leave a large part of each acorn uneaten? This simple question led to one of a series of experimental studies that are now beginning to unravel the complex interactions between tree squirrels and some of the plants on which they feed. The ecological and evolutionary interactions between squirrels and plants are complex. Indeed, Henry David Thoreau recognized the close interdependence of squirrels and plants more than a hundred years ago when he wrote: “Consider what a vast work these forest planters are doing! So far as our noblest hardwood forests are concerned, the animals, especially the squirrels and jays, are our greatest and almost only benefactors."

Today, some of these interactions are obvious and well understood; others are not. Many, we are confident, remain to be discovered. Increasingly, though, a number of studies have begun to reveal an intricate web of interactions that are strongly suggestive of a coevolutionary relationship, one in which each player strongly influences the other. 

It is perhaps stating the obvious to say that when squirrels eat tree seeds they exert a strong selective pressure on the plant. Seeds of course are the plant’s reproductive investment and the means by which its genes are passed on to the next generation. Consequently it follows that the activity of squirrels selects for specific seed traits that help protect or ensure their eventual germination. Squirrels on the other hand, adopt counterstrategies—through learning, evolution, or a combination of the two—that allow them to sometimes thwart these defenses. Here we consider the characteristics of the seeds and those of the squirrels that are central to this interaction.

The plants exhibit a number of adaptations to prevent losses to seed predators such as tree squirrels. Whereas some may have evolved specifically in response to tree squirrels, others may have evolved to protect against entire assemblages of seed consumers. Still others may have evolved for entirely different reasons but function secondarily as protection against squirrels. Regardless of their origin, three general types of seed characteristics reduce predation by tree squirrels. We refer to these as physical, chemical, and numerical protection.

Many plants exhibit protective devices around their seeds that reduce the probability of consumption by tree squirrels. Obvious examples include the woody covering of some hickories and walnuts in which <20 percent of the nut’s energy is devoted to the digestible kernel. Similar protection is common in conifers in which the small but nutritious seeds account for only a small portion of each cone. The remainder of the structure—as high as 99 percent by mass—is comprised of woody, sticky bracts that cover and protect the seeds. Such devices are not always successful against squirrels, but some are. The southern flying squirrel, for example, is able to open the heavy covering of walnuts, but the energetic costs for doing so exceed the energy acquired from the kernel; consequently, they avoid walnuts. Many conifer cones are also arranged in clusters around tree branches in a manner that makes them difficult to harvest.

Chemical protection against seed predators and other herbivores comes in many forms. For tree squirrels a few compounds seem most critical. Tannins are particularly important for protection of many squirrel foods, especially the acorns. Other compounds, such as lipids, carbohydrates, and proteins, vary considerably between various food items and are likely to affect food choices; yet, to date, the details of their influence are not well known. Conifers also often produce terpenes, which may deter squirrels from feeding on cones of a particular tree or tree species.

Last, we consider numerical protection, that is, the abundance of seeds. The number of seeds produced by a tree, relative to other trees around it and to the number of seed predators, may greatly affect the probability of seed predation. Many seed trees, including the oaks, hickories, and beech that are so favored by tree squirrels of the temperate forests, often undergo tremendous yearly variation in seed crops. This phenomenon—known as masting—involves the periodic and episodic production of seeds in some years, followed by low or nonexistent crops in others. Sometimes synchronized over large geographic areas, these boom-or-bust cycles can greatly affect the dynamics of forest communities. Although the mechanisms controlling these cycles are not yet understood, one potential explanation—the predator satiation hypothesis—holds that they evolved to satiate seed consumers in good mast years and cull their populations in poor years. And indeed it appears they may have this effect. Populations of tree squirrels and other small mammals frequently fluctuate wildly in response to these cycles in seed production.

The ability of squirrels to thwart many of the protective traits of seeds and nuts supports the argument that tree squirrels have evolved to deal with many of these characteristics. Tree squirrels, of course, possess a number of general characteristics that predispose them to a granivorous lifestyle. But in some cases the evolutionary responses of squirrels to various plant defenses may be more specific and strongly suggestive of a close evolutionary relationship between squirrel and tree. Smith’s 1970 study on red (pine) and Douglas’ squirrels in the Cascade Range of southwestern British Columbia offers a good example of this. West of the Cascade Range, the wet maritime climate results in moist forests with few lodgepole pines, and forest fires are rare. Here the pines produce soft, symmetrical cones, with many seeds per cone. These cones mature in a single season and are shed to germinate at the end of the growing season. However, east of the range the forests are quite different. In the rain shadow of the Cascades, the dry continental climate produces dry forests of lodgepole pine where lightning strikes and low-temperature fires are common. On this side of the range, the common fire-adapted pines produce hard serotinous cones that remain closed sometimes for many years until the advent of fire stimulates them to open and release their seeds. These cones exhibit a hard cone surface, strong woody attachment to the branch, asymmetrical shape, and many fewer seeds per cone. The pine squirrels in these dry forests east of the range are well equipped to handle these hard serotinous cones. They are larger in size and equipped with strong jaw muscles and a more robust lower jaw that enables them to pry off the bracts of these hard cones. In contrast, the closely related Douglas’ squirrels that reside to the west of the range are smaller in size and possess weaker jaw muscles and a weaker lower jaw.

In addition to morphological characteristics, tree squirrels exhibit considerable behavioral flexibility in dealing with various seed and plant defenses. Fox squirrels are quite efficient when feeding on cones of the longleaf pine despite the high costs of opening the cones. They sample cones from individual trees and then feed selectively in those trees that provide the greatest energy reward. Similar patterns of tree selection have been reported for the European red squirrel when feeding in a Scots pine forest in Scotland and pine squirrels feeding on cones in North America. Whereas such responses illustrate the squirrels’ ability to circumvent some of these cone defenses, they also suggest that the tree squirrels may exert strong selective pressures on individual trees. Fox squirrels, for example, quite often consume the greatest number of cones and seeds in those trees that have invested the greatest amount in reproduction. As a result, where fox squirrels reside, those trees sporting only moderate cone crops early in the season often have the greatest number of cones surviving to maturation. Among pine squirrels, cone selection can also occur within a tree on the basis of cone characteristics such as number of seeds per cone, cone hardness, and the arrangement of cones on the branches.

Perhaps one of the most interesting behavioral adaptations for dealing with seed predation is the act of embryo excision of white oaks by some tree squirrels. Although the rapid germination of white oak acorns is likely to be a diffuse evolutionary response to escape a diverse assemblage of mammalian and avian seed predators, the squirrel’s behavior of embryo excision of white oaks appears to be far more specific. To date, this behavior has been documented only for tree squirrels, despite numerous studies and observations on other seed predators of white oak. This suggests that the behavior evolved only in tree squirrels. And although refined with learning, the behavior is present at an early age and is therefore likely to be partly genetically predetermined. However, studies of pine squirrels suggest that young squirrels may learn feeding techniques by observing the mother. Clearly, some tantalizing questions remain about the genetic and evolutionary basis of embryo excision.

Just as squirrels can exert a strong negative impact on plants by consuming large numbers of seeds, they can also serve to disperse seeds and facilitate their establishment and survival. In some situations they are essential to this process. Dispersal, or movement of young away from the place of birth, is an important stage in the life cycle of many organisms. Plants are no exception. For many plant species, the advantages of dispersal include a decreased chance of inbreeding, a reduction in competition with sibling and parent plants, and an opportunity to establish in sites more suitable for germination or growth. But because they are sessile, that is, not free to move about, most plants rely on the movement of a specific life stage, such as a seed, by some agent of dispersal.

Many avian and mammalian species of seed consumers—tree squirrels included—are critical for the dispersal of some plant species. This is especially true when the seed consumer caches the seeds in the ground and then fails to recover some of these stored reserves. By transporting and scatterhoarding acorns, hickory nuts, or walnuts to individual sites just below the leaf litter, many tree squirrels reduce the probability of seed predation, seed dessication, and seedling competition, and at the same time increase the chances of germination, root establishment, and winter survival. The cache sites of at least one species—the eastern gray squirrel—may even be optimal for germination, survival, and growth of oak seedlings.

Why select cache sites that are suitable for germination? Certainly it is not the squirrels’ intention to actually plant the seeds. The squirrels’ motives are purely selfish. Their objective, when scatterhoarding, is to place the food out of reach of competitors, in a microenvironment that prevents rotting or drying. By coincidence, these conditions are often quite similar to those required for germination. On those occasions in which the squirrels fail to recover the seed, a new seedling becomes established.

But just how often is the squirrel likely to overlook its food stores? How effective are squirrels at retrieving their caches? Although answers to these questions are still elusive, a few generalizations are possible. There is now strong evidence that squirrels may remember the specific location of caches using spatial information. They are, by most estimates, quite efficient at cache recovery, sometimes revisiting and managing their caches. In some situations they are thought to recover >95 percent of their stores. Under many other conditions, however, a great number of seeds are likely to become established from these caches. This is especially likely when seeds are abundant and the need for stored reserves is relaxed or when the squirrel dies or disperses from its caching area.

There is growing evidence, that this process is far more involved than the above description indicates. A closer look at the complex interactions between oaks and squirrels serves to illustrate this point.

Read more in North American Tree Squirrels, which is available from Smithsonian Books. Visit Smithsonian Books’ website to learn more about its publications and a full list of titles. 

Excerpt from North American Tree Squirrels © 2001 by Smithsonian Institution