Three simple rules guide fire ants in building towers

When faced with rushing floodwaters, fire ants are known to build two types of structures. A quickly formed raft lets the insects float to safety. Then, once they find a branch or tree to hold on to, the ants might form a tower up to 30 ants high, with eggs, brood (including larvae) and queen tucked safely inside. Neither structure requires a set of plans or a foreman ant to lead the construction, though. Instead, a new study shows, both structures form by the same set of three simple rules.

Those rules initially came from a study of fire ant rafts. First rule: If you have an ant or ants on top of you, don’t move. Rule two: If you’re standing on top of ants, keep moving a short distance in any direction. Lastly: If you find a space next to ants that aren’t moving, enter it and link up.

When fire ants are in water, these rules compel them to build rafts, notes Sulisay Phonekeo. “The same rules dictate them to build towers when they are around a stem [or] branch,” his team’s new data show. Phonekeo is a mechanical engineer at the Georgia Institute of Technology in Atlanta. He led the new study, published July 12 in Royal Society Open Science.

Tiny builders

Phonekeo and his Georgia Tech colleagues collected the fire ants that they studied from roadsides near Atlanta. While covered in protective gear, the researchers dug up ant mounds and placed them in buckets. They lined those buckets with talc powder so the insects couldn’t climb out. Being quick was a necessity because “once you start digging, they’ll … go on attack mode,” Phonekeo points out. The researchers then slowly flooded the bucket until the ants floated out of the dirt and formed a raft that could be easily scooped out.

Back in the lab, the researchers placed some of these ants in a clear petri dish with a central support. Then they watched — and filmed — as the insects formed a tower. The support had to be covered with the non-stick chemical (Teflon), which the ants could grab onto but not climb without help. Over about 25 minutes, the ants would form a tower stretching up to 30 millimeters (1.2 inches) high. The ants themselves are only 2 to 6 millimeters (0.07 to 0.24 inch) long.

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The structures they erected looked like the Eiffel Tower or the end of a trombone, with a wide base and narrow top. And the towers weren’t static, like the rafts of ants are. Instead, videos showed that the towers of ants were constantly sinking and being rebuilt.

Peering into the transparent petri dish from below revealed that the ants had built tunnels in the base of their tower. They used those tunnels to exit the base before climbing back up the outside.

“The ants clear a path through the ants underneath much like clearing soil,” Phonekeo says. Ants may be using the tunnels to remove debris from inside the towers. And the constant sinking and rebuilding may give the ants a chance to rest without the weight of any compatriots on their backs, he adds.

To find out what was happening inside the tower, the researchers fed half their ants a liquid laced with radioactive iodide. It emits X-rays. They then filmed the insects using a camera that recorded those X-rays. In the film, radioactive ants appeared as dark dots. The researchers could see that some of those dots didn’t move, but others did.

The team then turned to the three rules that fire ants follow when building a raft and now realized that they also applied to towers.

But there was also a fourth rule: A tower’s stability depends on the ants that have attached themselves to the rod. The top row of ants on the rod isn’t stable unless those ants form a complete ring. So to get a taller tower, there needs to be a full ring of ants gripping to the rod and to each other.

That such simple rules could form two completely different structures is inspiring to Phonekeo. “It makes me wonder about the possibilities of living structures that these ants can build if we can design the right environment for them.”