Fire ants self-organise to build towers to reach safety

Many animal social groups demonstrate amazing abilities in self-organisation and collective action, efficiently accomplishing complex tasks without the need for designated leaders despite convoluted decision-making or even individual understanding of the end goal. When it comes to animal architecture, though, fire ants (Solenopsis invicta) might be in a league of their own.

The ants have evolved two very distinct collective abilities to survive the periodic flooding of their home habitat in the Pantanal wetlands of western Brazil: the first is to link their bodies together into stable ant-rafts that keep the colony afloat until they find firm ground; the second is to build towers, to scramble up to a potential safe haven, as a temporary shelter from rain and water, and as an operational base for building the underground tunnels of a permanent home.

How the ants build these towers is the subject of new research, published in Royal Society Open Science, by scientists from the Georgia Institute of Technology, in the southern US city of Atlanta.

The team, which brings together expertise in biology, engineering and physics, has previously examined how fire ants build their rafts. “Why wouldn’t we study these processes?” asks study co-author Craig Tovey. “Engineers and scientists don’t always know what our findings will lead to, but bio-inspired design can be a powerful tool to make our world more efficient.”

Part of the scientists’ research involved doping their subject ants with radioactive iodine then X-raying the towers as the ants built them. This revealed the extent to which the towers are dynamic forms, constantly “sinking” and being reformed – a distinct structural difference to the static nature of the rafts, in which the ants making up the “hull” assume fixed positions.

“To the best of our knowledge, this is the first example of two very different structures being built by a swarm,” the researchers write, “where individuals follow the same simple rules, and the difference arises from the environment and mechanical properties.”

As to how the fire ants know what they are doing, the researchers’ conclusion is that they don’t: “According to our hypothesis, the tower results from fire ants following the same set of rules that were previously used to accurately predict the shape and growth rate of a different structure, the ant raft formed when the ants are in water. The ants may simply be moving randomly atop other ants until they occupy an empty space adjacent to a stationary ant.”