Although it looks like these ants are playing Twister, they're actually doing something far more interesting. Some ants fill potholes in the road with their own bodies so that other ants can go faster! How kind of them! But why do they do it?

Eciton burchelii, a type of army ant in Central and South America, lives in highly organized colonies of up to a million members. Each colony is a single family. At dawn the ants begin their daily foraging raids.They chop up their prey and carry the bits home over a rough terrain of twigs and leaves on the rainforest floor. And some of the ants lie motionless across gaps in the leafy roadway, like tiny bridges, letting others run over them. "When the ants bump into a hole they cannot cross, they edge their way around it and then spread their legs and wobble back and forth to check their fit," explains Dr. Scott Powell, of the University of Bristol. "This carries on until an appropriately sized ant plugs the hole."

The living bridges-"bridgets"?-stay in place hours at a time while the others continue foraging. At the end of the day when the traffic comes to a stop, bridgits climb out and follow their nest mates back home for dinner. The bridgits probably have fewer good stories to tell around the dinner table, but their role turns out to be surprisingly crucial. Researchers have concluded that if about 5 percent of the foragers stop foraging and instead dedicate themselves to becoming road-filler, the day's food intake goes up by 25 percent! If too many or too few ants prefer bridging over hunting, food intake goes down. So what's the "magic number"? Does some Einstein ant calculate it and then assign jobs accordingly? Ants are not that smart. We need another explanation: evolution.

Let's say that millions of years ago there was a mix of ant colonies, with different numbers of "natural born" bridgits. Some colonies have a lot, some a few, and some in between.This does not depend on any magic or smarts.The colonies with a not-so-bad number of bridgits will get slightly more food and raise slightly more young.The next generation of colonies will be similar to, but not exactly the same as, their parent colonies, so the offspring of our "notso-bad" colony might end up with a number of bridgits that works even better. Their colony will get even more food and grow even bigger. The successful ants reproduce and spread out; the unsuccessful ones die out. This simple process repeats over and over again, until all we see is that every existing ant colony has exactly the right number of bridgits!

To us it looks like the ants are geniusesand that's just how it looks when things are fine-tuned by natural selection over millions of years. And when you put together hundreds of thousands of average and unmagical ants, each performing one out of a dozen highly specialized tasks, what emerges? The twisted behaviors that you see in an ant colony.

-Richard Young

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