The Effects of Squirrels' Storing Behavior

In a process known as scatter-hoarding, some species of squirrels store food for future use in many different locations. Using this process, the squirrels will store hundreds if not thousands of seeds and nuts per individual squirrel. Early observations led to the long-held notion that squirrels store more than they need and then forget where many seeds and nuts are stored, allowing some of them to germinate(begin growing into new plants) and become established(grow roots). More recent research, however, suggests a far more complicated picture, one in which the squirrels are considerably more efficient at seed and nut recovery and in which germination and establishment occur under far more limited circumstances than previously thought. Nevertheless, scatter-hoarding squirrels are critical for seed dispersal and, in many forest systems, they may serve as its principal agents. Experimental evidence strongly suggests that individual squirrels remember precise locations of their stored nuts, most likely based on spatial information, as is also shown for the scatter- hoarding corvids (birds such as rocks, jays and crows).This is not to say that they don't steal from one another, pilfering is common and sometimes even extreme. Yet in many situations these scatter-hoarders may have control over their own scatter-hoards.

The process by which scatter- hoarding squirrels disperse seeds, nuts, and fruits is rather involved. It is perhaps best demonstrated by the eastern gray squirrel, which resides predominantly in oak forests throughout much of the central and eastern United States. Numerous experimental studies now demonstrate that this species is highly selective with respect to the nuts that are eaten and those that are stored. Acorns of red oak species, for example, are significantly preferred for scatter hoarding over those of white oak. Typically, the acorns of red oak species are higher in fat content and tannin levels that reduce the tastiness and digestibility of plant foods, but they also exhibit dormancy (a period in which development is temporarily halted to conserve energy) prior to germination. White oak acorns, in contrast, have lower tannin and fat levels and germinate rapidly in the autumn, sometimes while still attached to the tree. Behavioral experiments by Michael Steele, Peter Smallwood, and others show that in the autumn, gray squirrels selectively consume white oak acors, but at the same time they also selectively store red oak acorns, primarily because of their delayed germination schedules and reduced perishability. Tannins and fats secondarily influence the squirrels' eating preferences, but overall the primary determinant of their fall caching(storing) decisions is the germination pattern.

Indeed, early germination in white oak acorns appears to have exerted a strong selective pressure on eastern gray squirrels. Several decades ago, John F. Fox demonstrated that when faced with heavy crops of white oak acorns, gray squirrels hide these more perishable food stores, but, when doing so, they carefully cut-off the small embryo at the pointed end of the acorn with a few quick scrapes of their front teeth to prevent the acorn from germinating. Moreover, in experiments with captive-raised eastern gray squirrels having no previous experience with acorns, they also attempt embryo-excision-removal). But they often perform it incorrectly or on the wrong part of an acorn. Thus the behavior may be largely inborn. Although numerous other rodents- such as mice, chipmunks and southern flying squirrels--selectively store red oak acorns, these species do not appear to perform embryo removal. Hence this excision behavior may be unique to a few lineages of squirrels that regularly scatter-hoard acorns.

Numerous other nut characteristics.., nut size, weight, and insect infestation) also influence the food-hoarding decisions of scatter-hoarding squirrels in many predictable ways. Ultimately, many of these behavioral decisions, in turn, affect where nuts are hidden and the likelihood that the nuts will germinate and establish if they are not recovered. We currently know that the sites frequently selected by eastern gray squirrels for scatter-hoarding- are often coincidentally optimal for germination. And these are also the sites where acorns store well. We also know that the probability of seeding establishment increases during years with high yields, when animals store large quantities of seeds and nuts and some are not recovered.