The physics that floats a bug's boat
Fire ants know how to survive when the waters rise: They turn their bodies into life rafts.
A new study explores the physics that keeps fire ant lifeboats, waterborne colonies sometimes containing tens of thousands of bugs, afloat. Linked together, the ants can form a watertight seal that keeps them from drowning, engineers from the Georgia Institute of Technology in Atlanta report the week of April 25 in the Proceedings of the National Academy of Sciences.
And the whole is bigger than the sum of its parts, says Julia Parrish, a zoologist at the University of Washington in Seattle: "The properties the group displays are not necessarily predictable by just looking at one individual."
Fire ants (Solenopsis invicta), an invasive species around much of the globe, are well-prepared for disaster. When their Brazilian homes flood, entire colonies — including queens, workers and workers carrying larvae — take to the sea. "They have to stay together as a colony to survive," says study coauthor Nathan Mlot of Georgia Tech. Their double-decked rafts — about half the ants float on the bottom holding the rest up — can bob along for days or even weeks.
The ants' seafaring success comes down to both small and big properties. On the small scale, single ants can walk on water, at least to a degree, similar to a floating pin or a water-striding insect. When wet, fire ants can also capture tiny air bubbles, probably thanks to the thin layers of hair covering their bodies, giving these intrepid mariners added buoyancy.
On the large scale, individuals weave together so tightly by biting onto their neighbors' legs that water can't sneak through the cracks. The ants that make up the hull of this lifeboat touch the water but never sink.
And the ant rafts are flexible enough to withstand some jostling. Mlot and his colleagues filmed the emergency raft construction by dropping congealed balls of up to 7,000 ants into water. No matter how many bugs clung to the rafts, these structures adopted the same 8-millimeter-thick "pancake" shape. When the team plucked ants off the top, others from the bottom crawled up.
These dynamic structures emerge entirely out of basic cues, Mlot suspects. In other words, ants don't think about what they're doing. If a space opens on top, bugs simply climb up. "Each ant is operating on a few simple rules of engagement," he says.
With these simple rules, ants have been able to build extraordinary structures, says Eric Klavins, a computer scientist also at the University of Washington. Many human engineers, on the other hand, build robots in their own images: Bulky and with a lot of processing power. But by studying how ants interact, engineers may be able to design robots that run on smaller processors and can work in concert to build things like emergency bridges, he says: "Man, ants are cool."
When floods strike, ants ride out the storm in a self-assembled ant raft. In this study, ant groups of 1,000 to 7,000 bugs spread out in the water to form a raft of uniform thickness.
CREDIT: Nathan Mlot/Georgia Tech
N.J. Mlot, C.A. Tovey and D.L. Hu. Fire ants self-assemble into hydrophobic rafts to survive floods. Proceedings of the National Academy of Sciences. Published online the week of April 25, 2011. doi: 10.1073/pnas.1016658108.