Desert ants construct mounds to help them navigate the featureless landscape of the North African salt pan.
Desert ants construct mounds to use as navigational landmarks, which help them find their way home in the otherwise flat Saharan landscape.
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| A desert ant (Cataglyphis fortis) on its nest mound Markus Knaden, Max Planck Institute for Chemical Ecology |
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 desert-dwelling ants like Cataglyphis fortis, which must find their thumbnail-sized nest entrance 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 in the desert after noticing their varying heights. Mounds at nests near the shrub-covered edges of the salt pan were barely noticeable, while those in the centre were closer to knee-high, suggesting that the hills served a purpose.
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.
The researchers then removed the mounds near some ant nests and followed the insects again. Without the mounds, fewer ants made a successful journey home and 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.”
“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.
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