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How army ants use their own bodies to build better bridges

Researchers find that army ants appear to stop extending the length of their living bridges when the costs outweigh the benefits.

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A gap in an ant’s path might be a daunting obstacle for the insect’s small body. But army ants in tropical forests of Central and South America work together to make shortcuts.

Linking their bodies together, the army ants build bridges for their comrades to march across. 

That sounds like smart cooperation, but the ants go beyond that. Researchers have found that the ants seem to weigh the costs and benefits of making long, living bridges. If a shorter distance requires too many ant bodies, the ants appear to decide that it’s not worth it, according to a paper published Monday in the journal Proceedings of the National Academy of Sciences

Army ants frequently build bridges beginning at the intersection between sticks, vines, or other material the insects are walking on. They continuously inch the bridge along a gap, adding more bodies in order to span a larger and larger space and make a more efficient route. 

This new research finds the bridge stops moving when the ants sense they are actually slowing their progress overall by using so many bodies.

"This stopping was a complete surprise for us," study co-lead author Christopher Reid said in a news release. "In many cases, the ants could have kept the bridge moving to create better shortcuts, but instead they stopped before achieving the shortest route possible."

"These bridges change dynamically with the traffic pattern on the trail," Dr. Reid said. "Imagine if the George Washington Bridge between New York City and New Jersey would reposition itself across the river depending on the direction of rush-hour traffic."

To observe the army ants in action, Reid and colleagues put boards down on the rainforest floor, creating an angular gap in the ants’ path. Sure enough, they saw the ants one-by-one use their bodies to build and extend a bridge.

When new ants added their bodies to the collective bridge, they chose to be on the side of the bridge furthest from the crook in the gap. This moved the bridge down the widening wedge-shaped gap, carrying the bridge toward an ever-shorter route.

But then this shifting stopped.

“I would have just expected them to make the bridge that makes the shortest possible path,” Georgia Tech’s David Hu, who has previously researched fire ants' bridge building during floods but was not associated with the new study, told Smithsonian.com. “How do they know that this is the best bridge for them?”

Dr. Hu said this new study is the first step to examine the ants’ problem solving behavior, but the researchers just provide a rough answer to the question of why the ants stop. 

Still, the army ants’ bridge building could help inform technological systems, like autonomous robotic swarms. “We have nothing built out of robotics that has this combination of moving really fast and also becoming building material,” Hu said. “They go between the walking state and the bridge-building state so quickly that this thing seems to just morph.”

Reid also looks to the ants for such insights. "Artificial systems made of independent robots operating via the same principles as the army ants could build large-scale structures as needed," he said in a news release.

"Such swarms could accomplish remarkable tasks, such as creating bridges to navigate complex terrain, plugs to repair structural breaches, or supports to stabilise a failing structure," he said. "These systems could also enable robots to operate in complex unpredictable settings, such as in natural disaster areas, where human presence is dangerous or problematic."

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