Army ants’ ‘living’ bridges suggest collective intelligence

Could we use ant-based rules to program swarms of simple robots to build bridges and other structures by connecting to each other?
November 25, 2015

Creating “living” bridges, army ants of the species Eciton hamatum automatically assemble with a level of collective intelligence that could provide new insights into animal behavior and help develop cooperating robots. (credit: Courtesy of Matthew Lutz, Princeton University, and Chris Reid, University of Sydney)

Researchers from Princeton University and the New Jersey Institute of Technology (NJIT) report for the first time that army ants of the species Eciton hamatum that form “living” bridges across breaks and gaps in the forest floor are more sophisticated than scientists knew. The ants exhibit a level of collective intelligence that could provide new insights into animal behavior and even help in the development of intuitive robots that can cooperate as a group, the researchers said.

Ants of E. hamatum automatically form living bridges without any oversight from a “lead” ant, the researchers report in the journal Proceedings of the National Academy of the Sciences. The action of each individual coalesces into a group unit that can adapt to the terrain and also operates by a clear cost-benefit ratio. The ants will create a path over an open space up to the point when too many workers are being diverted from collecting food and prey.

Collective computation

The researchers suggest that these ants are performing a collective computation. At the level of the entire colony, they’re saying they can afford this many ants locked up in this bridge, but no more than that. There’s no single ant overseeing the decision, they’re making that calculation as a colony.

The research could help explain how large groups of animals balance cost and benefit, about which little is known, said co-author Iain Couzin, a Princeton visiting senior research scholar in ecology and evolutionary biology, and director of the Max Planck Institute for Ornithology and chair of biodiversity and collective behavior at the University of Konstanz in Germany.

Previous studies have shown that single creatures use “rules of thumb” to weigh cost-and-benefit, said Couzin. This new work shows that in large groups these same individual guidelines can eventually coordinate group-wide  — the ants acted as a unit although each ant only knew its immediate circumstances, he said.

Swarm intelligence for robots

Ant-colony behavior has been the basis of algorithms related to telecommunications and vehicle routing, among other areas. Ants exemplify “swarm intelligence,” in which individual-level interactions produce coordinated group behavior. E. hamatum crossings assemble when the ants detect congestion along their raiding trail, and disassemble when normal traffic has resumed.

Previously, scientists thought that ant bridges were static structures — their appearance over large gaps that ants clearly could not cross in midair was somewhat of a mystery. The researchers found, however, that the ants, when confronted with an open space, start from the narrowest point of the expanse and work toward the widest point, expanding the bridge as they go to shorten the distance their compatriots must travel to get around the expanse.

The researchers suggest that by extracting the rules used by individual ants about whether to initiate, join or leave a living structure, we could program swarms of simple robots to build bridges and other structures by connecting to each other.

Matthew Lutz, Princeton University, and Chris Reid, University of Sydney

Matthew Lutz, Princeton University, and Chris Reid, University of Sydney

Abstract of Army ants dynamically adjust living bridges in response to a cost–benefit trade-off

The ability of individual animals to create functional structures by joining together is rare and confined to the social insects. Army ants (Eciton) form collective assemblages out of their own bodies to perform a variety of functions that benefit the entire colony. Here we examine ‟bridges” of linked individuals that are constructed to span gaps in the colony’s foraging trail. How these living structures adjust themselves to varied and changing conditions remains poorly understood. Our field experiments show that the ants continuously modify their bridges, such that these structures lengthen, widen, and change position in response to traffic levels and environmental geometry. Ants initiate bridges where their path deviates from their incoming direction and move the bridges over time to create shortcuts over large gaps. The final position of the structure depended on the intensity of the traffic and the extent of path deviation and was influenced by a cost–benefit trade-off at the colony level, where the benefit of increased foraging trail efficiency was balanced by the cost of removing workers from the foraging pool to form the structure. To examine this trade-off, we quantified the geometric relationship between costs and benefits revealed by our experiments. We then constructed a model to determine the bridge location that maximized foraging rate, which qualitatively matched the observed movement of bridges. Our results highlight how animal self-assemblages can be dynamically modified in response to a group-level cost–benefit trade-off, without any individual unit’s having information on global benefits or costs.