Scurrying across the scorching desert floor, Cataglyphis ants spend most of the day searching for food. However, having roamed significant distances in search of a tempting treat, the intrepid insects are faced with the tricky task of finding their way home. According to Rudiger Wehner, from the University of Zurich, the ants rely on a host of navigational systems,including local landmarks and their own internal map of the path taken (path integration). But with a selection of navigational strategies to choose from,how do they use this information to find their way home? Do they combine and process information from each system, coming up with a single homing strategy before they embark, or do they use several navigational systems simultaneously, relying to varying degrees on each as they proceed along?Wehner knew the only way to find out was to put two of the ants' navigational systems into conflict and see how the insects fared(p. 1868).
Travelling to the Tunisian desert, where he has been studying ant navigation for over 30 years, Wehner, Patrick Bregy and Stefan Sommer set about testing the insects' homing strategy. First they trained individual ants to shuttle back and forth between their nesting site (marked with a large black cylinder landmark) and a feeder stocked with tempting biscuit crumbs. Once an insect had got the hang of foraging at the feeder, the team transported it in the dark to an identical test site several hundred metres away; only this time the cylinder had been removed too. Releasing the insect,which thought it was still at the feeder, Wehner was pleased to see that it headed straight towards the site where the nest should have been. Deprived of landmarks, the insect had resorted to following its `path integration'internal map. But what would happen if the team messed about with the ant's landmarks by moving the cylinder?
Repositioning the cylinder to one side of the ant release site, the team released another insect and charted its route. The ant headed off in the direction where it expected the nest to be, but veered off towards the cylinder before arriving at the location where the nest should have been. Repeating the test with 30 more insects, the team found that the ants' courses always deviated towards the cylinder before they successfully returned homewards. And when the team tested the insects with the cylinder located 5,10 and 15 m along the ants' homeward route, the insects consistently drifted toward the landmark before finding their way home.
The team realised that instead of deciding on a single strategy before embarking on their journey, the ants were simultaneously using their path integration and landmark navigation systems as they returned home, relying more heavily on path integration than the landmark's location. `The ants didn't neglect the landmark, even when it was in the wrong place' says Wehner.
