Army ants build bridges to shorten journeys through the rainforest
Army ants construct complex bridges from their own bodies to span gaps and create shortcuts in the floor of the tropical forests of Central America, according to a new study published in the Proceedings of the National Academy of Sciences.
Army ants are nomadic species. They relocate their colonies throughout the rainforest on a regular basis. In order to facilitate the movement of their large population — a colony can have up to 1 million individuals in some species — on the very uneven forest floor, some of the ant workers use their own bodies to plug holes along the path traveled by the colony. These workers can also attach to each other to span larger gaps, effectively building living bridges made of several dozens of ants in some instances. These bridges can assemble and disassemble in a matter of seconds, allowing the ant colony to travel at high speed across unknown and unpredictable terrain.
A collaborative team of researchers from New Jersey Institute of Technology (NJIT – Newark, N.J.), Princeton University (Princeton, N.J.), George Washington University (Washington, D.C.), Harvard University (Cambridge, Mass.), and the Max Planck Institute for Ornithology and the University of Konstanz (Konstanz, Germany) has recently discovered that these bridges move on their own from their original building point to create shortcuts across large gaps.
"These bridges change dynamically with the traffic pattern on the trail," says Dr. Christopher Reid, one of the lead authors of the study. "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." Now working at the University of Sydney in Australia, Dr. Reid performed the work cited in the study while he was a postdoctoral researcher at NJIT.
Indeed, after starting at intersections between twigs or lianas traveled 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.
"This stopping was a complete surprise for us," adds Reid. "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."
Reid and his collaborators discovered that, while ants benefit from shorter traveling distances thanks to their living bridges, they also incur a cost by sequestering workers that could be used for other important tasks, such as prey capture or brood transportation. When building their bridges, army ants have to meet this cost-benefit tradeoff, and therefore cannot build long bridges between distant parts of their trails without risking lacking workers elsewhere.
"Our work has implications for other self-assembling systems, such as reconfigurable materials and autonomous robotic swarms," says Reid. Artificial systems made of independent robots operating via the same principles as the army ants could build large-scale structures as needed. Such swarms could accomplish remarkable tasks, such as creating bridges to navigate complex terrain, plugs to repair structural breaches, or supports to stabilize 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.
This study by Reid et al. was published in the Proceedings of the National Academy of Sciences on November 23, 2015 and can be accessed at: http://www.pnas.org/content/early/2015/11/18/1512241112.abstract
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