To cross streams or secure themselves against water currents, fire ants join forces to form rafts or build towers. Researchers have now worked out how the ants sustain tall structures without crushing their friends: they constantly circulate around the tower, behaving like a fluid.
Fire ants (Solenopsis invicta) have sticky pads on their feet that help them to assemble collectively into shapes.
Researchers had already worked out the secrets of fire ants’ raft-building techniques: the ants adhere to each other with their feet and orient themselves to create pockets of air, distributing their weight to form a buoyant structure. So a team co-led by Craig Tovey, a modelling mathematician at the Georgia Institute of Technology in Atlanta, sought to find out how the insects sculpt themselves into towers.
In a laboratory, the team used high-speed cameras to record how the insects assemble around a slippery Teflon rod, and tagged half the colony with a radioactive tracer to see how the insects moved inside the tower structure.
The ants use trial and error to form a tower, continuously rebuilding weaker parts that collapse until the structure is sound. Each individual insect can support up to three other ants, the researchers found. And when an ant is overloaded, it lets go of its neighbours and sinks down the column until it emerges outside the base of the tower.
The result is a dynamic, bell-shaped structure that moves similarly to a fluid, and in which each ant carries an equal load. “The ants are circulating like a water fountain, in reverse,” says Tovey. The work is published in Royal Society Open Science1.