Desert ants are famous for their wayfinding skills, and many travel long distances to collect food to bring back to their colony. But these foraging trips are an especially daunting task for ants like Cataglyphis fortis, which live in salt flats in Tunisia and so must find the thumbnail-sized entrances to their underground nests without the aid of landmarks like plants, hills and water features.
Markus Knaden at the Max Planck Institute for Chemical Ecology in Germany and colleagues decided to investigate the purpose of mounds built by C. fortis after noticing their varying heights. Mounds near nests at the shrub-covered edges of the salt pan were barely noticeable, while those in the centre could reach taller than 25 centimetres, suggesting that they were important for navigation.
The researchers began by following the insects’ locations with GPS and found that they face high mortality rates. On the longest journeys, which were more than 2 kilometres, around 20 per cent of the ants failed to make it home and died in the baking heat.
The researchers then followed ants at 16 nests. At some of them, they removed nearby mounds and at others, the researchers left the area completely alone. They found that removing the mounds increased the chances of ants failing to find their way home by between 250 and 400 per cent. In nearly all cases, the foragers’ nest mates quickly began rebuilding the missing structures.
When Knaden and his team replaced the mounds with artificial landmarks – black cylinders the size of large fire extinguishers – they found that the ants didn’t rebuild. “It’s an enormous effort to build such a nest hill. There are hundreds of ants building the whole night,” says Knaden. “So they don’t do it if they don’t have to.” With the artificial landmarks, ants took more direct paths home like they did before their mounds were levelled.
“We are used to discovering myriad ways that insect foragers use clever tricks to help with their efficient navigation, but I was a little taken aback when this stretched to nest architecture,” says Paul Graham at the University of Sussex in the UK.
The remaining mystery is how the colony keeps track of when it needs new landmarks. C. fortis colonies have divisions of labour, so older foraging ants could be communicating to the young ants responsible for construction that they need landmarks. Or it could be an initiative younger ants take when they see their older colony mates struggling to return.