Scientists have known for some time that ants can use their bodies to form living structures when they get into a bind. They can build bridges to span gaps, group together to create a raft that will save them in a flood, and even form temporary shelters to house the queen and offspring in an emergency. Recent research suggests that groups of ants display physical characteristics that are similar to both liquids and solids.

But it seems that ants' miraculous physical abilities aren't their only redeeming trait. When army ants of the genus Eciton gather food in the Amazon, primarily by pillaging other insect colonies, they form living bridges to make the route to and from their own colony as straight as possible. But at a certain point, more ants in the bridges becomes less efficient because it means that fewer ants are actually gathering food, and the ants seem to know when they've reached this equilibrium point.

Christopher Reid/University of Sydney

In an experiment carried out in Panama by Christopher Reid of the University of Sydney, ants were given a zig-zagging path to follow (encouraged along by the pheromones they use to navigate) with gaps separating segments of the path. One ant would quickly sacrifice its body to bridge the small gap in the corner, and other ants would join to form a bridge that reduced the distance the line of gatherers had to march. As more ants latched on to one side of the bridge, improving the angle of the path for minimum distance, the ants on the unused side would untangle from the bridge and continue their march. The result is that the ant bridge moves down the gap, never widening or narrowing significantly, continuing to make the route shorter and shorter (check out the full video on Smithsonian.)

But at a certain point, they stop. As the gap they span lengthens, more ants are needed for the bridge, and fewer are available to carry food. But how do they know when they have hit the correct ratio for maximum food-gathering efficiency?

Reid hypothesizes that the ants decide whether or not to stay part of the bridge based on how often they are touched. If other ants are continuously flowing over them, they know to stay in the bridge. If they don't feel other ants frequently, they know they should let go and keep gathering food. This would also explain why ants on the far side of the bridge let go as the bridge travels away from them and the gatherers take the shorter route.

Reid plans to team up with Harvard computer scientist Radhika Nagpal to explore the possibility of developing tiny robots that could simulate the ants' behavior. "They could self-assemble into larger structures—bridges, towers, pulling chains, rafts," Nagpal told Smithsonian.

The findings from the Panama experiment are published in the Proceedings of the National Academy of Sciences (PNAS) .

Sources: University of Sydney, Smithsonian

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