Fire Ants Lock Arms to Keep Bridges From Falling

AUSTIN—Fire ants make for formidable arch villains. Working together in large colonies, they can kill a mouse and strip its skeleton clean in a matter of hours. They survive fires, quickly develop resistance to pesticides, and outswim floods by rafting together and floating away. They also use their bodies to build bridges across gaps in their paths. Now, a research team has examined what makes their bridges strong. Even without a leader, the ants are able to sense when their bridges are in trouble and react to strengthen these living structures, the scientists reported here today at the annual meeting of the Society for Integrative and Comparative Biology.

Understanding how ants perform this maintenance could lead to better self-repairing materials, says David Hu, a mechanical engineer at the Georgia Institute of Technology in Atlanta and lead author on the new study.

Natives of the flood plains of Brazil, fire ants cope with periodic downpours that require them to pack up and move. They and army ants have evolved the ability to clump together to form structures, such as rafts and bridges, to get to dry land again. "The formation of living structures such as the bridges is one of the most fascinating examples of the collective capabilities of ant colonies," says Simon Garnier, a complex systems scientist with the New Jersey Institute of Technology, who was not involved with this study.

Often the bridges lack sturdy supports and hang between leaves or reeds that vibrate in wind or water currents, so researchers long wondered how these seemingly flimsy structures stay up. Hu and his graduate student Sulisay Phonekeo collected wild fire ants and tested how their bridges survive vibrations.

The ants naturally clump together and can be pulled like taffy into a bridge. Phonekeo suspended these living spans between the ends of two funnels and used time-lapse video to observe how they survived shaking at different frequencies. When he subjected the bridge to less than 20 vibrations per second, nothing happened. But more intense shaking caused the ants to spring into action. To form the bridge, the ants link legs. With each shake, the ants pull themselves closer together, tightening their grips on one another and shortening the bridge, Phonekeo reported at the meeting. “The ants are changing the material properties of the bridge by pulling their arms in,” Hu explains. By becoming stiffer, the bridge can “support more weight.”

Ants strengthen shaky bridges in another way. Individuals scurrying along the bridge tend to change course and gather at the beginning and end of the bridge, helping dampen the effects of the vibrations. When holes or weak points do appear, ants mend the gap by linking together at that spot. In a sense, the ants become a very dynamic building material. “The ants don’t have an observer to point to a hole, they have to feel around and detect it,” Hu says. Next, he and Phonekeo will try to learn how the ants pinpoint weak spots.

"This study is an important step toward understanding better how ants [form] their living architectures in response to environmental challenges," Garnier says. Because the bridge is built and repaired in a completely self-organized manner, "the construction rules followed by the ants represent a formidable source of inspiration for people working on self-assembling robots and self-repairing materials."

*Clarification, 10 January, 10:49 a.m.: Although Simon Garnier's lab is on Rutgers University's Newark campus, he is part of the New Jersey Institute of Technology faculty, not the Rutgers one.