By Rebecca Morelle Science reporter, BBC News

Scientists compared normal ants (left) with ants on stilts (right) Enlarge Image

Desert ants use an internal "pedometer" to measure exact marching distances, according to a study.

Researchers knew foraging insects could navigate using light from the sky, but were puzzled by the animals' ability to gauge the length of ground covered.

By manipulating the ants' leg lengths to give them longer and shorter strides, a Swiss/German team found the ants "counted" steps to judge distance.

The research is published in the journal Science.

The long and short of it

Cataglyphis fortis, otherwise known as foraging Saharan desert ants, travel great distances over flat, sandy terrain searching for food.

The creatures have the remarkable ability to return to their nest using a direct route rather than retracing their outbound path.

To perform this feat, the ants need to judge directions and distances. But while they rely on the sky for orientation, their means for measuring distance had remained a mystery.

To investigate, scientists from the University of Ulm, Germany, and the University of Zurich, Switzerland, set some ants off on a foraging trip along a straight tunnel, but once they had reached the food their legs were manipulated to either make them longer by adding stilts, or shorter by partially amputating them.

The ants were then returned to the same spot to begin their homeward-bound journey. However, the researchers discovered the ants with longer legs overshot the nest entrance, while those with the shortened legs undershot it.

The long and short of it

They found when the ants performed both the outward and homeward-bound journeys with manipulated legs, they judged the nest-distance almost exactly, suggesting that stride-length was the key factor.

They also looked at ants with shorter legs

Professor Harald Wolf, an author on the paper and neurobiologist from the University of Ulm, said: "This means the animals are 'counting' their strides - like a pedometer.

"If you shorten leg length and it takes them a thousand strides from the nest to the feeder, they would of course assume that they needed to take another 1,000 strides to return to the nest.

"And if they take the 1,000 strides with shorter legs, this will take them over a shorter distance, and if the legs are extended it takes them over a longer distance.

"Our next step will be to scrutinise what is known about the control of leg movement in leg walking, to see if there is something which could act as an odometer (distance calculator) in these animals."