One paw washing the other: Oak trees and squirrels have evolved to help each other


An Eastern Gray Squirrel chews the coating off an acorn from a Drummond Red Maple tree. (Jason Fochtman/AP)
April 8

If you think the squirrel needs the oak tree more than the oak tree needs the squirrel, Michael Steele has some news for you: They both need each other.

Steele is a professor of biology at Wilkes University in Pennsylvania. He has studied all sorts of animals, but he keeps coming back to squirrels, those bushy-tailed rodents that frisk and frolic in back yards and city parks. The reason? He’s curious about how animals and plants interact.

Eastern gray squirrels, the most common squirrels in the Washington area, are opportunistic eaters. That means they will eat just about anything. But they especially like acorns, which are the seeds from which oak trees grow.

Acorns grow best when they’re away from the tree they dropped from. If they stay right underneath, the branches of the parent tree will block the sun. Of course, acorns can’t get up and walk on their own. That’s where squirrels and other animals come in.

Oak trees have an interesting way of encouraging squirrels to help them. There are more lipids — a type of tasty fat — at the top of the acorn. There are more tannins — a bitter-tasting chemical — at the bottom. The bottom has the acorn’s embryo, the part that grows.

“The top is more like a fruit that attracts the animals,” Steele said. “The bottom is better protected.”

The first thing a squirrel does when it picks up an acorn is roll it. Then it shakes it.

At first scientists didn’t even know that squirrels were shaking the acorns. The animals did it so quickly that it was only by watching slow-motion video that the shaking could be detected.

Why the shake? For the same reason you check the expiration date on a yogurt before eating it.

“They’re assessing the seed quality,” said Steele, co-author of “North American Tree Squirrels.” “They can tell whether it’s a sound seed or not.”

Acorns can become filled with tiny insects known as weevils. Shaking the acorn gives the squirrel an idea of what’s inside. If it has weevils, the squirrel will eat the acorn right away, weevils and all. It doesn’t want to give the weevils time to eat everything. If it’s free of weevils, the squirrel will cache the seed — that means bury it to eat later.

Gray squirrels are known as scatter hoarders. They bury their seeds in lots of places. That makes them different from larder hoarders, such as chipmunks, which stash a lot of seeds in one place. (The good thing about being a larder hoarder is that you don’t have to go far for a meal. The bad thing is that if another animal finds your nuts, it will probably take all of them.)

Gray squirrels are pretty picky when it comes to burying the acorns they have collected. Researchers have discovered that they like burying crummy, less desirable seeds closer to the tree. But nicer seeds will be buried farther from the tree, as far as 200 feet away.

There’s a trade-off: The closer the acorn is to the tree, the better the chance another animal will find it and eat it. But burying the acorn farther away exposes the squirrel to predators such as hawks.

Sometimes a squirrel will bury an acorn far away and not come back for it. When that happens, the seed may sprout. That means another oak tree. More oak trees, more food for squirrels.

Said Steele: “In my mind, squirrels are a model organism for studying the close evolutionary interaction between plants and animals.”

— John Kelly

Sketch a squirrel

Squirrels are pretty cute — gray, brown, black, whatever their color. KidsPost would like to see your version of the fluffy-tailed critters. Send us your artwork by April 21, and we may feature it in a future KidsPost. If we publish your drawing in the paper, you will receive a KidsPost prize package.

In order to enter our contest, which is open to ages 6 to 13, you need to have an adult (a parent, guardian or teacher) send us your entry. On each entry, the adult should include your name, age and home town, as well as the name and phone number of the adult submitting the entry. A note from the adult giving permission for you to enter the contest also is required. Send entries to kidspost@washpost.com with “squirrel” in the message field or mail to KidsPost, The Washington Post, 1150 15th St. NW, Washington, D.C. 20071.

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