Squirreling away acorns for winter

Acorns are falling. I swept the deck yesterday afternoon, and by this morning a few dozen more acorns dotted the surface. They’re a sign that autumn isn’t far off, and for squirrels, they’re the start of the fall harvest.

Red oak acorns

    Gray squirrels are so much part of our suburban surroundings that we tend to overlook their activities. To remedy this, I opened North American Tree Squirrels, by biologists Michael Steele and John Koprowski. Steele and his colleagues and students designed clever experiments to elucidate the feeding behavior of squirrels they found on their campus at Wilkes University in Pennsylvania.

Eastern gray squirrel (Sciurus carolinensis)

    For gardeners who like oaks, it was good news that acorns can be half eaten and still germinate and produce seedlings. Steele noticed that in early fall, when acorns were plentiful and the weather was still warm, squirrels usually ate acorns of white oaks but cached red oak acorns for later. White oak acorns are programmed to germinate in the fall.

 

Squirrels may pretend to bury acorns in decoy locations

    The red oaks play a longer game. Their acorns contain higher levels of tannin, a nasty-tasting chemical that protects seeds from predators. They’ve evolved to germinate in spring after a necessary cold period. The researchers observed that later in the fall, squirrels started removing the acorn caps to eat just the tops of the red oak acorns, which contained more high-calorie lipids and less tannin. As winter set in, they were willing to eat the whole red oak acorn, tannin and all. Apparently like us, squirrels balance tastiness versus health factors when they choose food.

Health food or tasty treats? It depends on the time of year

    Do gray squirrels remember where they hide their acorns? It was long thought that they didn’t and had to sniff out buried acorns. Lucia Jacobs, then at Princeton University, showed through an ingenious experiment that squirrels found their own cached acorns more readily than those hidden by other squirrels, although others’ caches might be quite nearby. 

Squirrels remember where they hid their nuts--photo by Juraj Patekar

    As a directionally challenged navigator, I’m awed by squirrels’ ability to recover their hidden nuts. Scientists asked whether squirrels were just good at retracing their steps—I can usually do that—and were finding their caches that way. Or were they constructing a three-dimensional mental map that allowed them to locate their hidden acorns? 

    Another of Jacobs’ lab experiments confirmed the latter. If she trained squirrels to find a nut in a maze and then kept the nut in the same location but changed the way to get there, the squirrels still found the nut. They’d retained its location, not just the directions for getting there.

 

    Although squirrels recover most of the acorns they store, they do serve red oaks by dispersing their seeds. A study of white versus red oak seedlings found that most of the white oaks sprouted close to the parent trees, whereas red oak seedlings appeared three to six times farther from the mature trees, their acorns having been moved by squirrels and other animals. In this way, squirrels contribute to the development of our forests.