Army ants build living bridges by linking their bodies to span gaps and create shortcuts across rainforests in Central and South America. An international team of researchers has now discovered these bridges can move from their original building point to span large gaps and change position as required.

The bridges stop moving when they become so long that the increasing costs incurred by locking workers into the structure outweigh the benefit that the colony gains from further shortening their trail. Bridges dismantle when the ants in the structure sense the traffic walking over them slows down below a critical threshold.

Co-lead author Dr Christopher Reid, a postdoctoral researcher at the University of Sydney's Insect Behaviour and Ecology Lab and formerly with the New Jersey Institute of Technology, said the findings could be applied to develop swarm robotics for exploration and rescue operations. By analysing how ants optimise utility, researchers may be able to create simple control algorithms to allow swarms of robots to behave in similar ways to an ant colony.

The paper, 'Army ants dynamically adjust living bridges in response to a cost-benefit trade-off', is being published in the journal Proceedings of the National Academy of Sciences (PNAS).

The team of researchers - from the Max Planck Institute for Ornithology (Konstanz, Germany), University of Konstanz, and the United States's New Jersey Institute of Technology, Princeton University and George Washington University - found the bridges can assemble and disassemble in seconds. They can also change their position in response to the immediate environment.

The dynamic nature of the bridges has been found to facilitate travel by the colony at maximum speed, across unknown and potentially dangerous terrains. Prior to the study it was assumed that, once they had been built, the bridges were relatively static structures.

"Indeed, after starting at intersections between twigs or lianas travelled by the ants, the bridges slowly move away from their starting point, creating shortcuts and progressively lengthening by addition of new workers, before stopping, suspended in mid-air," said Dr Reid.

"In many cases, the ants could have created better shortcuts, but instead they ceased moving their bridges before achieving the shortest route possible."

The researchers discovered that, although ants benefitted from shorter travelling distances because of their bridges, they also incurred a cost by sequestering workers that could be used for other important tasks. When building their bridges, army ants had to balance this cost-benefit trade-off.

Dr Reid said the findings had implications for other self-assembling systems, such as reconfigurable materials and autonomous robotic swarms. "Artificial systems made of independent robots operating via the same principles as the army ants could build large-scale structures as needed," Dr Reid said.

"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.

"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."