Scurrying under the baking sun is a hazardous business for Cataglyphis fortis desert ants. They must return swiftly to the nest or risk death in the blistering heat. But these ants have a suite of navigational tricks on hand to help them locate home quickly. Keeping track of every twist and turn on the outbound journey, they speedily calculate the most direct path home when they are ready to return. Yet, tiny errors in the ant's tallying system mean that their return route can miss the nest by centimetres, so the tiny explorers use local landmarks to navigate visually once they know they are near the nest. However, Markus Knaden from the Max Planck Institute for Chemical Ecology, Germany, noticed that the desert is a very aromatic place. ‘When walking through the salt pan I realised that it smells quite different in some places from others, even though visually it looks the same,’ says Knaden. Having already found that the ants use their sense of smell to get their bearings when close to the nest, Knaden and his colleagues Kathrin Steck and Bill Hansson began testing whether the ants combine visual and scent cues to find home faster ().
‘We use long aluminium channels to train and test the ants,‘ says Knaden. ‘We use the channels because the desert is smelly and we do not want them to have contact with the smell of the ground. We wanted them to learn the odours that we provided,’ he explains. Connecting the nest entrance to a long channel before the ants were active, Knaden and Steck placed two pieces of black cardboard either side of the nest's entrance in the channel for the ants to use as visual landmarks. Then the ants scampered to a well-stocked feeder before returning to the nest. Allowing some ants to visit the feeder only once, and others 5, 10 and 15 times, one of the scientists then collected each ant and transplanted it to an identical channel – complete with cardboard landmark but lacking the familiar nest entrance – to see how long it took the ants to become confident that the landmarks indicated the nest's entrance.
According to Knaden, it took the ants 15 training journeys to the feeder to be convinced that the cardboard marked the nest entrance. And when the duo replaced the cardboard landmarks with a familiar odour from the insects' surroundings, it took the ants a similar number of training trips with the odour marking the nest for them to trust the cue.
However, when Steck repeated the experiments with both cues at the nest entrance, the duo was in for a surprise. The ant scurried directly to the landmarks and stood still. ‘It didn't want to go away. Usually they walk continuously but there it really stopped,’ says Knaden. Somehow the combination of the odour and visual landmark was enough for the ant to learn the nest's location in a single foraging trip. And when Steck tested the insect's reactions to the individual landmarks after training with both cues, the association was still as strong: ‘They would never have learned this fast from the single cues alone,’ says Knaden.
Having shown that the combination of visual and odour landmarks is a more powerful navigation aid for desert ants than each cue individually, Knaden is keen to find out how nest odours waft through the desert to guide ants home.
