Researchers Tackle The Nutty Truth On Acorns And Squirrels

"Are squirrels dispersers and planters of oak forests or pesky seed predators? The answer is not a simple one," says Steele, associate professor of biology at Wilkes University in Wilkes- Barre, PA. "Gray squirrels may devour many acorns, but by storing and failing to recover up to 74 percent of them, these rodents aid regeneration and dispersal of oaks."

The two researchers are studying why squirrels cache only certain acorns of the 32 species of oaks across eastern North America. The studies by Steele and Smallwood, associate professor of biology at the University of Richmond, could have a significant impact on oak forest regeneration.

"Tree squirrels are one of the most important animals for helping oaks spread, because they store acorns in the ground, practically planting baby oak trees," says Smallwood. The researchers note that evidence is accumulating that along with blue jays and a few other small animals, squirrels are important in maintaining and regenerating second-growth oak forests, and may even have been responsible for spreading the vast stands of oak throughout North America.

During the autumn and winter months their main diet consists of nuts and seeds. Active throughout the year, the squirrels store large quantities of seeds and nuts--usually acorns-- to see them through the winter.

"To a squirrel, the acorn is a package of energy that can be easily opened and eaten in less than half the time needed for other, harder nuts or stored for use months later," says Smallwood.

But all acorns are not alike. In more than 1,500 feeding trials, the professors recorded whether the acorn was eaten or stored, the distance it was dispersed, and the amount of time the animal took to eat or bury the acorn. The found that about 85 percent of white oak acorns were eaten shortly after discovery, while about 60 percent of the acorns of red oaks were stored.

In another group of experiments to determine the squirrel's role in dispersing trees, the researchers tagged thousands of red and white acorns with small metal labels and, after the animals dispersed the seeds, used metal detectors to recover them.

"Again, the results were clear. That red oak seedlings were more widely dispersed through forests, while white oak seedlings were more likely to be found right next to the parent tree," says Smallwood. This fact is important in understanding forest regeneration, suggesting that red oak trees will be the first oaks into new forests.

Early in their studies, the researchers also observed a particularly puzzling behavior pattern. Squirrels would pry off the caps of red oak acorns, bite through the shells to get at the nutritious inner kernels, and then discard them half eaten. Moments later, they would size another red oak acorn and repeat the routine.

The two major groups of oaks--red and white--have seeds that differ generally in chemical makeup, says Smallwood. Red oak acorns are rich in fats but are laced with tannins, the compounds used to tan hides. White oak acorns are less fatty and lower in tannins. Red oak acorns lie dormant in winter and sprout in spring; white oak seeds usually sprout soon after falling to the ground in autumn.

The researchers found that in those red oak acorns that were eaten, frequently only the top half--the end farthest from the embryo--were consumed. Because the embryos were not destroyed, these damaged red oak acorns also germinated.

The germination and the bitter-tasting tannins might explain why a white oak acorn is more likely to be consumed on the spot and why only the top half of a red acorn is eaten.

"The higher levels of tannins are located near the bottom of the acorn, where the embryo is located. That factor may influence the squirrels' choices," says Steele. "If an acorn germinates before the squirrel can recover it, up to half its stored energy goes to the seedling rather than the squirrel."

Although the researchers are just beginning to grasp some of the complexities of the squirrels' relationship with oaks forests, they suspect that gray squirrels can strongly influence the distribution and range of various oak species.