Some fire ants build towers of their own bodies in an amazing display of acrobatics and collective intelligence. Even more surprisingly, ants circulate through the tower while keeping its overall shape constant, overcoming a tendency for it to sink, reports News Scientist.
The fire ants (Solenopsis invicta), which are found in wetlands, link together to build living rafts to keep the colony afloat during floods. When the water recedes, they cling to exposed plants and form a tower as a temporary shelter until they have a chance to build an underground nest.
Craig Tovey of Georgia Tech and colleagues set up a camera to study how the ants build such a tower, and accidentally left it rolling for an hour after it was built. Since the tower appeared to be static once built, they thought the footage would be worthless.
But when a PhD student watched it back at 10 times normal speed, he noticed the middle of the tower was slowly sinking. “When you speed it up, the ants on the surface are a blur and underneath the blur you can see the slow sinking movement of the tower,” says Tovey.
After further experiments, they realised the sinking was due to ants at the bottom moving outwards under the weight of the ants on top. Meanwhile, ants on the outside were perpetually rebuilding the tower by moving towards the top. “The rest of the tower is gradually sinking, while the ants at the top keep building it higher and higher,” says Tovey. “It’s kind of hilarious.”
The team’s previous research on ant rafts showed how, although no one is in charge and no ant can see the big picture, simple behavioural rules can lead to the creation of a resilient structure. The same rules guide the construction of the tower, with the added limitation of how much weight an ant can support.
Understanding how ants build structures with no executive oversight will have many applications in robotics, says Tovey. In search-and-rescue situations, it would be useful to deploy a fleet of small robots that can fit through small gaps, but also work together to get over obstacles and cross gaps.
“I would so love to be able to send a thousand little robots into a collapsed building after a fire or earthquake and find survivors,” Tovey says.