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Ants with artificially extended legs (top) walk too far when returning home; those with stumps (bottom) don't walk far enough.


Ants on Stilts

A tiny set of stilts is helping to solve the mystery of how ants navigate through the desert. When researchers gave ants a leg up, the insects overshot their target while trying to find their way home. The results suggest that ants keep track of how far they've marched by an internal pedometer--a first for the animal world.

The ants that live in the Sahara desert never seem to get lost. They wander across vast swathes of virtually identical-looking terrain, but once they find a piece of food they head right back to the nest in a straight line rather than retrace their steps. How do they do it?

Scientists have found one trick that desert ants use to navigate. By memorizing the position of distant landmarks, they can keep track of which direction they're facing (ScienceNOW, 1 July 2004). But this alone shouldn't be enough information to get home, because the ants must also know how far they've gone in various directions. According to one theory, the insects keep track of how many steps they've taken. Because their stride length is fixed, this would allow them to calculate how far they've traveled relative to home.

To test the idea, a team led by Matthias Wittlinger, a biologist at the University of Ulm, Germany, made modifications to desert ants that were worthy of a circus. After setting up an ant home outside the lab, the researchers let 25 ants take a 10-meter trip from their nest, then collected them. For one group, the team glued tiny stilts to the insects' legs. For another, they clipped the legs down to stumps. And for a control group they left the legs alone. Then the researchers gave each ant a piece of food and set it free. With morsels of food in their jaws, the ants immediately headed home. If desert ants do indeed use an internal pedometer, then the modifications should mess up their calculations.

Not only did the stilted and stumpy ants not make it home, but they also misjudged their distances exactly as the researchers predicted. The ants on stilts went about 5 meters too far before stopping to search for the nest, whereas the stumpy ants stopped about 5 meters too short, the team reports 29 June in Science. (Control ants got back home just fine.) After the modified ants were returned to the nest, they were able to go out and get back home just as accurately as normal ants, which should be the case if they're keeping track of the number of steps.

This is "convincing evidence," says Mandyam Srinivasan, a biologist at Australian National University in Canberra. "The challenge now would be to discover exactly how the nervous system senses and integrates the limb movements to provide the ant with a sense of how far it has traveled."