Stilts for ants make case for pedometer.

Gluing pig bristles to ant legs to lengthen their strides or trimming the insects' legs to shorten their steps distorts their judgments of distance, say researchers.

The distortions offer the first experimental evidence in any animal of a built-in odometer based on strides, says Herald Wolf of the University of Ulm in Germany.

Wolf and his colleagues studied the ant Cataglyphis fortis of the Sahara. Even though the landscape looks flat and featureless, foragers move this way and that until they find food and then make a more direct trip home.

Past research revealed that the ants judge direction by sun position and light polarization. As long ago as 1904, theorists proposed that ants measure distance by their own strides.

In Tunisia, Wolf and his colleagues tested this idea for the first time. They trained ants to trot along a 10-meter-long runway between a nest and a feeder. Once ants were familiar with the route, the researchers caught some at the feeder and modified the length of their legs. Researchers then put the test ants on a different runway parallel to the training course. The ants resumed their normal behavior, quickly accepting a crumb and heading in the direction of home.

Those with shortened strides underestimated the distance, starting to look for the nest when they had traveled only 6 m instead of the usual 10 m. Ants on pig-bristle stilts, however, scurried some 15 m before looking for the nest.

After a day or two on the new legs though, the ants could gauge the distance correctly, as if they'd reset their odometers, researchers report in the June 30 Science.--S.M.