The mystery behind the remarkable ability of fire ants to turn themselves into a living, crawling life raft has been unlocked by scientists: The insects use air pockets that form around their bodies to keep themselves from drowning.
The analysis, published online Monday in the journal Proceedings of the National Academy of Sciences, could one day provide a useful model for building robots that can perform complex functions quickly and cooperatively, the study authors said.
Fire ants, named for the burning sting left by their bites, possess incredible powers of floatation when they work together, turning themselves into life rafts that can survive flash floods in their native Brazilian rain forests and travel for months before making landfall.
"They bring out the eggs, the young ants and provisions — everything that happens to be in their mandibles at the time," and carry them onto rafts made of their own bodies, said lead author Nathan Mlot, a third-year doctoral student in the mechanical engineering department at the Georgia Institute of Technology in Atlanta, Ga.
But no one knew how these insects, which flounder and struggle in the water as individuals and whose bodies are denser than water and thus should sink, could float when they banded together.
To catch the ants in action, researchers collected fire ants off roadsides in Atlanta and brought them to their lab. The ants, they knew, behaved like a fluid: A handful could be molded and even poured. So the scientists swirled the ants in beakers until they formed spherical clumps, then dropped those ant balls into containers of water.
Quickly, the ants spread out into pancake-like formations, forming rafts. The researchers froze one of the ant rafts with liquid nitrogen and studied its composition under an electron microscope. They observed that the ants had used their claws, adhesive pads on their legs and their mandibles — pincer-like jaws — to grip onto one another.
"At first it just looks like a tangle of bodies and limbs everywhere, but the longer you look at the picture, the more you're able to distinguish between different body parts and see the connections," Mlot said.
At such a high — and perhaps unflattering — resolution, the researchers also noticed how hairy the ants' water-repelling bodies were. Those hairs allowed the insects to trap air bubbles. With so many ants jammed together, their individual bubbles fused to form a protective air layer for all of them.
The ants could also dynamically respond and rebalance the raft if needed, and the researchers wondered whether there was a method to their movements.
They used single paintbrush hairs to apply blue, pink, green and other bright colors across the creatures' behinds so that they could track individual ant movements. Understanding individual, and not just mass, movement would help the researchers generate a mathematical model to describe the ant raft's development.
The researchers had originally thought that the ants on the bottom of the raft were underwater and would be unable to breathe, and thus that the ants must have been changing shifts to prevent drowning. Not so: The insects didn't need to change positions because the protective layer of air kept their bodies from sinking beneath the water's surface.
The scientists did see, however, that there was always a clear ratio between the top ants and the bottom ants, and that if anything happened to upset that ratio, ants would crawl to the sides of the ant pancake and move up or down to rebalance the raft.
The researchers also wanted to know how tightly the ants were gripping one another. To find out, they glued individual live ants to the bottom of a glass slide and allowed another ant to grip on to the glued ant. They harnessed the gripping ant with a tiny elastic band and pulled on it to measure the grip strength. That force, they determined, was more than 400 times the creatures' body weight. The ants could use this power to pull in tighter and become more rigid if they felt a threat, such as a prod from a researcher's twig.
The researchers created ant rafts containing as many as 12,000 ants. In the wild, where colonies of fire ants can number in the hundreds of thousands, the floating rafts could potentially be much more formidable.
The research sheds light on how the deeply social insects act together: almost as if they're part of a superorganism, scientists said.
"Some people almost think of ants as a fluid neural network: The colony itself is performing collective computations, but each ant is unaware of all these options," said Iain Couzin, a biologist at Princeton University in New Jersey, who was not involved in the study. "The individuals acting together create this awareness of the environment that no individual ant has, and that's what I think we find fascinating about these insects."