Army Ants Link Up To Build Dynamic Bridges [TIME-LAPSE]

Ants are known to form collaborative teams that work together to avert crisis and predation. Now the New Jersey Institute of Technology (NJIT) has revealed that army ants can even construct complex bridges using their own bodies to navigate obstacles.

"These bridges change dynamically with the traffic pattern on the trail," Dr. Christopher Reid, one of the study's lead authors and a postdoctoral researcher at the University of Sydney's Insect Behaviour and Ecology Lab, formerly with the NJIT, said in a news release. "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."

Army ants (Eciton burchellii) are considered a nomadic species because they frequently relocate their colonies. The ants are also known for "swarm raids" – times when they fan-out rapidly across the forest floor in search of prey, destroying all arthropods in their path.

Army ants form self-made bridges to create shortcuts or to cross gaps encountered along Central America's tropical forest floors. But what happens when the ants meet the tipping point – when more of their number are required to make the bridge than are available to forage for food?

That's the question researchers from NJIT sought to explore in partnering with scientists from Princeton University, George Washington University, Harvard University, the University of Konstanz and the Max Planck Institute for Ornithology.

They found that the bridges ants create incorporate new ants to lengthen the bridge progressively, thereby shortening the distance their comrades had to travel. They stopped building and reinforcing precisely at the tipping point – when the ants realized their resources had been exhausted and it was more important for those that followed in line to begin foraging.

"This stopping was a complete surprise for us," Reid added. "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."

For their experiment, researchers set up a zigzagging path with gaps separating segments of the route ants were supposed to follow. Instead of following all the twists and turns around gaps, the ants opted to creat their own short cut by spanning. A single ant would sacrifice its body to begin the bridge and other ants would join in until they made it to the other side. As reinforcements arrived, ants on the far side would untangle themselves and continue on their forraging mission. 

It was always assumed that ant bridges were relatively static structures, but the study revealed that they are extremely dynamic structures that are responsive to changing conditions.

"Our work has implications for other self-assembling systems," Reid notes, "such as reconfigurable materials and autonomous robotic swarms."

The study was recently published in the Proceedings of the National Academy of Sciences. A time-lapse of the ants' bridge-building skills and methods can be watched online. 

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