The aggressive red fire ant Solenopsis invicta has become a scourge of the southern United States, spreading like wildfire and delivering burning stings to unfortunate victims. Now, researchers spying on the ants' underground chambers have learned that their tunnel-building follows a simple, perhaps universal, rule.
The discovery, published Monday by Proceedings of the National Academy of Sciences, could provide insight for engineers building search-and-rescue robots.
Native to the Pantana wetlands of South America, the ants somehow hitched a ride to Mobile, Alabama about 80 years ago. From there they spread throughout the Southeast and then jumped to other countries through the United States. Though it's unclear how they first got to the U.S., the ants have a remarkable ability to form a ball of sorts in times of flood, allowing them to ride waterways to new homes.
"Nobody likes fire ants," said study coauthor Daniel Goldman, a Georgia Tech physicist who has taken to kicking the above-ground mounds atop a fire ant nest when he jogs around the neighborhood.
The bugs have become ubiquitous (though they've recently been getting overshadowed by a nastier species, the "crazy ant" Nylanderia fulva). And yet not much is known about their underground homes, which can go six feet deep and hold more than 160 feet worth of tunnels. And that's pretty crucial information for an ant colony, said lead author Nick Gravish, a Georgia Tech physicist, because the nest acts like an extension of the ant colony "superorganism."
To study how they build their tunnels, the researchers took shovels and cracked open hives to pull out thousands of ants, tucking their pants inside their socks to avoid getting bitten.
"Getting bitten's OK -- getting stung's the part that really hurts," Gravish said.
The researchers watched the ants burrow into a small cylinder of material and captured their developments with an X-ray CT machine. They found that while the tunnels' depth depended on the moisture and grain of the soil, the tunnels' diameter seemed to remain essentially the same.
So they then forced the ants to go through a series of glass tunnels of different diameters, from 1 millimeter to 9 millimeters. They noticed that, with a slight exception for the narrower tunnels, the ants' speed didn't vary. Surprisingly, the ants seemed to use their antennae as limbs too, helping to bear their weight as they navigated the uneven terrain.
They also shook the ant tunnels to make them fall, exerting a force of 27 Gs — far stronger than anything they'd feel from jostling from other ant bodies in their normal environment, the authors said. Even then, the tenacious ants stumbled only 48% of the time — and the wider the tunnel, the more likely they'd take a tumble.
The Goldilocks width — just about the length of their insect bodies — seemed to offer the perfect formula: enough space to move around, but small enough that they could easily find purchase as they sped through their underground highways.
The findings could prove useful in the future for engineers looking to build effective search-and-rescue vehicles, Goldman said.
"People would like to make devices that can move in complex, unstable, shifting environments," Goldman said. "And we think that having some insight into how you can make stable structures beneath the ground — how you can get through them rapidly — could help future robots."