Nature’s Ultimate Bankers

It’s late January as I crunch over frosty, fallen leaves on my way to Coyote Bench. Almost immediately I hear the high-pitched whine of a female gray squirrel in a mating chase. Four male squirrels are after her, but one male fends off the others. Once the female turns and faces him at the end of a branch and he retreats. Then she enters a tree cavity and pokes her head out, nipping at any male squirrels that try to enter.

Her actions are designed to send the males into a greater frenzy and they do. A dominant male busily defends the cavity from above and below while the other males try to breach his defense. When he drives away all but one other competitor, she emerges and races off in what scientists call a “breakaway.”

A Johnny-come-lately runs up the tree she had been in, following his nose and emitting the low, grunting calls of a questing male. He sniffs around the rim of the cavity, thrusts his head inside, and then runs back down the tree trunk.

At this point, the female, pursued by five males, streaks up from the ravine and across the road. The last I see of her she has six males on her tail.

According to an excellent new book North American Tree Squirrels by Michael A. Steele and John L. Koprowski, this courtship day started shortly after dawn and will continue for eight hours. In that time the female will probably mate three or four times with two to four males.

Although female squirrels are only receptive to mating for about eight hours, males are active from November until August. Every day during that period, they spend the first couple hours visiting the nests and home ranges of adult females. Cautiously they sniff the rear ends of each female they encounter. If they find one within five days of her receptive period, they follow her. By the big day, males have congregated outside the female’s nest from as far away as half a mile. Although P.D. Goodrun watched a record 34 Texan gray squirrels in a mating bout back in 1961, Steele and Koprowski have observed up to 22 and my personal record is 10.

If the food sources are good, female squirrels may mate twice a year, according to Michael A. Steele, who is an associate professor of biology at Wilkes College. During a telephone interview, Steele told me that Koprowski wrote the chapters on social behavior and reproduction and he wrote those on habitat, diet, patch use, food caching, and seed dispersal.

Steele has several northeastern Pennsylvania forest sites where he conducts his extensive research on food selection, habitat use, and seed production by small mammals, but one of his favorite and most visible sites is Kirby Park along the Susquehanna River in Wilkes-Barre.

“We get a lot of questions from visitors,” Steele says. And no wonder. Does anyone in Wilkes-Barre recall seeing people vacuuming up the remains of acorns in Kirby Park? As part of a study Steele and his students conducted of the caching and feeding behavior of gray squirrels in autumn and winter, they used a handheld, portable vacuum to collect pieces of acorns that gray squirrels dropped after they ate, Steele writes in North American Tree Squirrels. They needed all those pieces to figure out why squirrels seemed to eat less efficiently in winter, when they needed the extra energy to survive the colder temperatures, than in autumn. What they found was that early in autumn, they feed quickly and sloppily, dropping edible portions of the acorns, and in winter they eat slowly and carefully and drop very few, if any, small, edible pieces. They concluded that in autumn they eat quickly so they can cache as much food as possible when it is abundant and in winter they spend more time eating every bit of the acorn and carefully storing whatever other nuts they can find.

This is just one of many studies Steele has performed on the habituated gray squirrels of Kirby Park. “Tree squirrels,” he and Koprowski write, “are model organisms for testing and exploring important questions in behavior and ecology.” So Steele live-traps them and uses dye to mark all the gray squirrels in the park with letters or numbers. Over the years, people have asked him and his students why they bother to study such common, well-known animals.

“They don’t realize that many of the experiments done at this park have become famous,” [in scientific circles], Steele says. Furthermore he maintains, “How little we really know about the world around us. These are critters we see every day,” and yet they engage in complex behavior still not fully understood by ecologists.

One of Steele’s newest studies in the park asks why squirrels bury smaller acorns close together and under the canopy and larger acorns farther apart and beyond the canopy. After all, squirrels risk predation by hawks and use more energy to bury the larger acorns. However, those acorns are less likely to be pilfered by other squirrels and chipmunks. The smaller acorns, which are not as desirable, are more easily discovered and dug up by other animals. Those animals keep a constant watch on each other and where they bury their stores.

But Steele has also found that gray squirrels are “masters of deception.” They frequently dig a hole, pretend to bury a nut, and cover it over. Then they move on and bury a nut somewhere else. This “functional deception,” as Steele calls it, “really confuses onlookers, such as chipmunks, blue jays, and other squirrels, as to where they’ve buried the nut.” They may pretend to bury an acorn anywhere from two to nine times before actually doing so. Sometimes they reverse this procedure by immediately burying an acorn and then pretending they are burying it somewhere else.

Tree squirrels, Steele hypothesizes, may be “nature’s ultimate bankers…moving and managing caches, much the way a financier will manipulate investments to maximize long-term returns…”

And the way they bury acorns helps to decide the future of oak forests. Steele and his colleague Peter D. Smallwood of the University of Richmond have been performing a series of studies to find out why eastern gray squirrels, flying squirrels, deer mice, white-footed mice, and fox squirrels usually store acorns from the red oak group and eat those of the white oak group. It turns out that the gray squirrels, at least, know that white oak acorns will germinate almost immediately and red oak acorns will not germinate until the following spring. If they do store white oak acorns, they first bite off their tips, which destroys the embryo and prevents them from germinating.

But how do they “know” this? “Likely there is a chemical cue in the shell of the acorns that tells squirrels to store red oak acorns,” Steele says, because after soaking the shells of both red and white oak acorns in acetone, which masked the difference between the species, the squirrels ate all the acorns. In Pennsylvania there are three times as many red oaks as white oaks. Could the way gray squirrels bury acorns have something to do with this? On average, Steele and Smallwood have discovered, the squirrels cache 15% of white oak acorns, usually near the acorns’ parent trees, and more widely cache 60% of red oak acorns. To do this, gray squirrels “scatterhoard,” meaning that they bury only a few or one item in many dispersed cache sites instead of storing all their food in one place as larderhoarders, such as chipmunks and red squirrels, do. Lately, Steele has been putting metal tags on individual acorns and then finding out later where they have been cached.

Such scatterhoarding does increase the chances that acorns may germinate on years when there is a bumper crop of nuts. This so-called “predation satiation hypothesis” speculates that trees “evolved to satiate seed consumers in food mast years and cull their population in poor years.” Unless they are overwhelmed by a crop, die, or disperse from their caching area, squirrels recover as many as 95% of their cached acorns because they are able to remember their exact locations. So far, scientists do not know how long they remember.

Tree squirrels appear to be important seed dispersers. “By transporting and scatterhoarding acorns…to individual sites just below the leaf litter, many tree squirrels reduce the probability of seed predation, seed desiccation, and seedling competition, and at the same time increase the chances of germination, root establishment, and winter survival…The cache sites of…the eastern gray squirrel may even be optimal for germination, survival, and growth of oak seedlings,” Steele writes.

Both Steele and Koprowski have studied tree squirrels for more than 20 years, but they make it clear that many questions about tree squirrels are still unanswered and that there is still much more that can be learned about their behavior and its effect on our forests. And because they are easy to observe, they are ideal mammals to study.

After reading North American Tree Squirrels and talking with Steele, I am more interested than ever in watching the behavior of gray squirrels. This is exactly the reaction Steele and Koprowski are hoping for from all the folks who read their book. “A little time with this book, and the reader should never view a squirrel in quite the same way again,” they write in their preface. “For the general reader, we seek to share not only our knowledge of the tree squirrels but also the sheer delight that comes with studying them…”

They have succeeded admirably.