Scurrying across the desert floor, the Sahara Desert ant, Cataglyphis fortis, spends its life foraging for food. But with few landmarks to guide them back to their nests, the insects are dependent on their internal navigation systems for a safe return home. One of their strategies is to record exactly how far they have strayed. Harald Wolf from the University of Ulm, Germany, explains that the ants sum the distances that they cover in each step so that they know how far they have gone. But whether they record distance by keeping track of the rhythmic electrical nerve impulses that control their leg muscles or monitor the positions and movements of their legs, wasn't clear. Curious to find out how robust the ant's pedometer is,Wolf and his colleagues, Matthias Wittlinger and Kathrin Steck, decided to see if they could effectively `break' it. Could they create a situation where the pedometer failed?

Travelling to Tunisia, Wolf and his colleagues put the desert ants through their distance measuring paces. Knowing that foragers often lose limbs and antennae in skirmishes with ants from other nests, and yet continue foraging for the colony, Wittlinger decided to test whether the insects' pedometers continue to function despite injury(p. 2893).

First, the team trained ants to run from their nest along a featureless 10 m channel to a foraging site well stocked with cookie crumbs. Once the ants had set the distance between the nest and crumb site in their pedometers, the team intercepted the insects, quickly removed the lower half of two of their four rear legs, allowed them to stock up on cookie crumbs, set them running along a parallel identical channel and asked them where they thought the nest was. If the insects' pedometers were damaged by the loss of their lower limbs,the foragers would begin searching for the nest in the wrong place, but if their pedometers were functioning accurately, the ants would begin searching for the nest 10 m from the crumb supply.

The ants always began searching for the nest after travelling the correct distance along the channel. And when the team completely removed two of the insects' four rear legs, the ants `returned' successfully to the site where their pedometers told them the nest should be.

So the ant's pedometer seemed to be unaffected by limb loss but how would the insects fare while trying to negotiate uneven terrain? The team inserted corrugations along the entire length of the channel leading from the nest to the cookie crumb store to see how the insects coped with being wrong-footed by the uneven surface. The distances between two corrugation peaks ranged from a little more than one stride up to as much as two strides. Would the ants returning along the flat parallel channel overshoot the nest's location because of the additional distance travelled or would they take it all in their stride?

Again the ants were unperturbed. Despite stumbling and losing their footings on the outbound journey, the ants never lost track of their distance from the nest.

So how do the Tunisian desert ants keep track of the distances that they have travelled? Wolf suspects that the ants monitor sensory signals from the legs that actively bear weight and is keen to find out where these signals originate. He also admits that he was surprised that the ants' pedometers could not be disturbed, no matter what the team did.

Steck, K., Wittlinger, M. and Wolf, H. (
). Estimation of homing distance in desert ants, Cataglyphis fortis,remains unaffected by disturbance of walking behaviour.
J. Exp. Biol.