A desert ant’s short life is passed under constant threat from the cruel sun or hungry insects. Yet the foraging desert ants, Cataglyphis fortis, would rather risk the sun than end up on someone else’s diner plate; they run right down the middle of any obstacles they encounter on a foraging sortie. Rudiger Wehner realised that these ants were behaving exactly like bees, who never deviate from the central path when they fly along a tube to their hive. When Mandyam Srinivasen originally noticed this behaviour, he called it ‘centring’. Wehner wanted to know if his ants were using the same navigation system as the bees to navigate along an ant channel. After several months of baking in the Sahara desert with his ants, Wehner realised that they had come up with an entirely different strategy. The ants were judging the height of the channel’s walls on either side and taking a path that made the walls appear to be the same height from the ant’s perspective, even if they were different in reality.
Wehner and his student, Daniel Heusser, headed down to their desert research station to watch Cataglyphis scurrying along a channel. After training the ants to run back and forth between their nest and a well-stocked larder, they captured individuals just as they began their return journey, and transferred them to a parallel home run, to see how they reacted as they rushed home along the channel.
Wehner knew that bees balance visual information from both eyes as they fly along a tube to maintaining a central course. Srinivasen had made the bees think that one wall was moving faster by projected moving patterns onto the walls of the tube. The bees thought that they were closer to the ‘faster’ wall and diverted towards the ‘slower’ wall to balance the wall’s speeds and hopefully keep an equal distance from both. Wehner and Heusser designed an ant channel with moving walls to see if they would make one wall seem closer than the other, and drive the ants away from their central path. But no matter how Heusser and Wehner tried to visually confuse the ants, they never deviated from the central path.
So the ants had developed a different strategy for navigating a channel. Wehner wondered if the ants were judging the height of the two walls, plotting a path that made both walls appear the same height from the ant’s eye view. If he made one wall higher then the other, it should drive the ants towards the lower wall. Which it did.
After a season in the desert, Wehner and Heusser returned home. When they revisited the desert a year later, the channels left from the previous year’s experiments had sprouted two lines of bushes, with one bush wall slightly taller than the other. Wehner and Heusser decided to see how freshly trained ants would react to this relatively natural setting. Sure enough, the ants always deviated towards the shorter bushes, because they believed that was the central path between the bush barriers.
