A raft of fire-ants (Reuters)
A raft of fire-ants: Practically unsinkable, and now we know why. Photo by Reuters
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Reuters
Closeup of ant leg, as seen by CT scan. Photo by Reuters

Fire ants are among nature's greatest engineers, famously working together to form rafts with their own bodies that are almost unsinkable. Researchers have long wondered how they do it and now, helped by a freezer and CT scanner, scientists at Georgia Institute of Technology have unraveled part of the mystery, which could potentially be useful to human engineers as well.

First of all, why would fire ants (Solenopsis invicta) evolve the ability to form living rafts? Because they originate in the Brazilian rain forest, which is prone to flooding. When danger appears in the form of heavy rain, they link their legs, forming a living raft that floats away, bearing their precious queen.

How they came to evolve that behavior is one question; another is how they stay afloat, given that each ant is more dense than the water, and would sink.

The answer: "The ants connect with themselves very, very, very well, more than we had thought," says assistant professor David Hu of Georgia Tech. "Imagine you have a hundred ants, which means 600 legs. Ninety-nine percent of those legs will be connected to a neighbor so they're very, very good at maintaining that network." Before this, scientists had assumed the inter-ant links to be much less efficient.

The ants even use their jaws to stay connected with one another, the scientists found. Once they're firmly connected, they stretch their legs slightly in order to increase the surface area of the raft, improving its buoyancy especially in rough waters.

In order to peer into the secrets of fire-ant raft-building, the Georgia Tech scientists froze entire rafts in the lab, and also scanned the individual insects using computerized tomography. Their results were published in the Journal of Experimental Biology.

"They do it systematically, with larger ants forming central hubs for their smaller neighbors to hold onto," says Hu. "As a group, they also maintain spaces between their bodies, giving the raft greater buoyancy."

The ants have lessons to teach human engineers, says Hu. "There's a lot of interest in active materials, small robots or materials that can configure themselves to build larger, more capable objects and now we know it's quite possible for scientists to do so, if we just follow what the ants are doing."

One thing CT scans can't reveal is how the ants know where to go in the raft and what to do. Hu says that is a mystery still to be solved.

It bears mention that in February, a group of scientists published a study in Plos One on flood ants (Formica selysi) in Sweden, showing that the ants place their queen and larvae in the center of the emergency rafts they form, on top of worker ants and larvae – using the relatively more buoyant young'uns as flotation devices. Not everything in ant engineering bears emulation, it would seem.