Army ants make complex bridges with their own bodies in tropical forests

Army ants overcome challenging engineering problems by constructing complex bridges – using their own bodies – to create shortcuts and span gaps in the floor of Central America’s tropical forests, says an international team of scientists from the United States and Germany.

One of the lead authors, Dr. Christopher Reid, who works at the New Jersey Institute of Technology’s Department of Biological Sciences, and colleagues from Princeton University, George Washington University, Harvard University, the Max Planck Institute for Ornithology, and the University of Konstanz, wrote about their study in the academic journal Proceedings of the National Academy of Sciences (PNAS).

Ants – which are nomadic species – relocate their colonies on a regular basis throughout the rainforest. Moving up to 1 million individuals on the extremely uneven forest floor can prove very challenging.

Army ants creating a bridgeTeam of army ants creating a ‘living bridge’. (Image:

In this study, the authors said they observed several of the ant workers using their own bodies to plug holes the colony was travelling through.

Army ants made ‘living bridges’

In order to span bigger gaps, the workers were seen attaching themselves to each other, effectively making living bridges made of many dozens of insects in some instances.

The ants were able to build and disassemble the living bridges within a few seconds, allowing the members of their colony to travel at high speeds across new, unknown and unpredictable terrain.

Dr. Reid said:

“These bridges change dynamically with the traffic pattern on the trail. 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.”

The authors noticed that after starting at intersections between lianas (woody climbing plants that hang from trees) or twigs travelled by the ants, the bridges gradually moved away from their starting point, creating shortcuts and progressively lengthening by adding new workers, before stopping, suspended in mid-air.

Ants aimed to make shortest route possible

Dr. Reid, who was surprised by this stopping, added:

“This stopping was a complete surprise for us. 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.”

The authors discovered that, while the ants gained great benefit from the shorter routes thanks to their living bridges, they also incurred a cost by sequestering workers that could have been used for other vital tasks, such as brood transportation and capturing prey.

When constructing their bridges, army ants clearly choose to meet this cost-benefit tradeoff, and therefore could not build longer bridges between distant parts of their trails without risking a shortage of workers elsewhere.

Swarms of robots could use ants’ systems

Reid said:

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

Artificial systems made of several independent robots using the same principles as the army ants could construct large-scale structures as required.

Robot swarms could accomplish remarkable tasks, such as creating plugs to repair structural breaches, building bridges to navigate complex terrain, or supports to stabilize a structure that is starting to fail.

These systems could be used to help robots function more effectively in complex unpredictable settings where human presence would be problematic or dangerous, such as in natural-disaster areas.

Citation: “Army ants dynamically adjust living bridges in response to a cost–benefit trade-off,” Chris R. Reid, Matthew J. Lutz, Scott Powell, Albert B. Kao, Iain D. Couzin, and Simon Garnier. Proceedings of the National Academy of Sciences (PNAS). Published online before print November 23, 2015. DOI: 10.1073/pnas.1512241112.

Video – Army ants make living bridge