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How Fire Ants Build Incredible Writhing Towers

Using X-rays, researchers find simple rules help the ants raise each other up, which could be useful in robotics

smithsonian.com

Fire ants are resilient little creatures. Originally from the Brazilian Pantanal, the world’s largest tropical wetland, the insects can link together to form a living raft during floods, staying afloat for weeks at a time. But once the water recedes these versatile creatures can form towers reaching more than 30 ants high, which serve as a temporary shelter and gives them time to rebuild their underground dwellings.

Now, new research shows just how these towers can stand for extended periods without hurting the ants at the bottom, reports Sam Wong at New Scientist, and could help in designing new modular robots. The research appears in the journal Royal Society Open Science.

The find was accidental. A research team at Georgia Tech was studying how the fire ants (Solenopsis invicta) were building a tower, according to a press release. They only planned to record for two hours while the critters formed the tower—but the camera rolled for three.

They assumed that there would be nothing to see once the ants assembled the writhing tower. But as researcher Craig Tovey tells Charles Q. Choi at LiveSciencehis colleague Nathan Mlot “was too good a scientist to discard data." Even so, it seemed like a waste of time to watch an hour of nothing. "So he played the video at several times regular speed.”

While fast forwarding, Mlot noticed that the ants forming the tower were not stationary as the researchers believed. Instead, the tower was in very slow, constant motion with the column of ants slowly sinking, like butter melting.

To investigate further, Choi reports, the researchers fed some of the ants water laced with radioactive iodine. They then recorded the colony building the tower using X-ray videography, confirming that as some ants climbed around on the outside of the tower, the Eiffel-tower shaped mass was slowly sinking. 

To form the tower, the ants don’t appear to use any sort of plan or have any type of leadership. Instead, similar to the way they build their rafts, the ants follow a set of simple behavioral rules, crawling along their friends till they find an open spot, then linking themselves into the tower. Eventually, with all the ants following those rules, they form the tower with a thick base that grows increasingly narrow.

According to Wong, the sinking occurs because the ants on the bottom of the structure give up because the weight of the tower is too much. They walk away and climb up the sides of the tower,  eventually finding new position at the top of tower. This process happens over and over, constantly rebuilding the structure. “The rest of the tower is gradually sinking, while the ants at the top keep building it higher and higher,” Tovey tells Wong. “It’s kind of hilarious.”

Choi reports that in another experiment the researchers placed sheets of transparent plastic on the ants, finding that the little insects could toleate about 750 times their own bodyweight. In practice, however, the ants were happiest shouldering the weight of three comrades. More than that and they would leave their position in the tower.

The researchers also hope to investigate how the ants work together to form bridges, which they use to cross chasms, reports Choi. They hope these studies could help in designing modular robots, which use simple behavioral rules to work together. For instance, tiny bots could be used to move through the small spaces in collapsed buildings during search and rescue missions. Like ants, they could assemble to cross gaps or form towers to climb obstacles.

About Jason Daley

Jason Daley is a Madison, Wisconsin-based writer specializing in natural history, science, travel, and the environment. His work has appeared in Discover, Popular Science, Outside, Men’s Journal, and other magazines.

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