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May 13, 2011

The incredible floating fire ant


Brian Vastag's April 25, 2011 Washington Post front page story reported on a study by mechanical engineers at the Georgia Institute of Technology investigating fire ant raft structure and function, published April 20, 2011 in the Proceedings of the National Academies.

Excerpts from the Post article follow.


[Caption for video above: "Side view time lapse (10x) of an ant raft composed of about 3,000 ants spreading out on the surface of water.]

When in danger of drowning, a colony of the critters — thousands of them — will save themselves by joining forces and forming a raft. They pile together and lock legs and jaws.

So bound, an ant raft can survive for months.

Engineers studying animal oddities now report that together, the ants aren’t just stronger. They’re floatier. Airtight, even.

"Water does not penetrate the raft," said Nathan Mlot, a mechanical engineer at the Georgia Institute of Technology and lead author of the ant-raft report published in Monday’s Proceedings of the National Academies. Even the bottom layer of ants stays dry, he said.

And although individual fire ants have been thoroughly studied in the lab, until now no one had bothered to figure out how ant rafts float.

The uneven, hairy surface of the ant’s skin explains this phenomenon. Bumpy, bristly, or otherwise rough surfaces repel water because of something called the Cassie-Baxter law of wetting.

This physical law states that as a surface gets rougher, water has a tougher time touching it. Duck feathers also repel water because of their tiny bumps.

In the lab, Mlot deposited colonies of 500 to 8,000 ants in large beakers. When gently swirled, each colony spontaneously formed a sphere.

[Caption for video above: "Top view time lapse (15x) of an ant raft composed of about 8,000 ants spreading out on the surface of water."]

Mlot dropped these spheres into water, and the amazing social behavior of ants became evident. The ants on top of the ball crawled down to the water and grabbed onto other water-level compatriots. The next layer of top ants then crawled to the edge, and so on. In about a minute and a half, each ant-sphere flattened into a dome, then flattened further into a pancake shape — a raft.

This togetherness pushes each ant’s individual air bubble against the next ant’s bubble. The bubbles join, protecting the whole raft.

"If water can’t come into contact with one ant, or the next, or the next, air can't get through," said Mlot.

The rafts are so buoyant that the engineers had to push them eight inches underwater before any water leaked through. As the rafts submerged, the ants pulled together even more tightly, working together to maintain their watertight communal craft.

When building these structures, no single ant is in charge. Yet what looks like chaos is really communal organization. The individuals act as one, becoming what entomologists call a super-organism.


The Ant Raft web page has three videos and four pictures of various aspects of ant raft formation and function.

The Georgia Tech Ant Lab YouTube channel features seven videos.

[Caption for video above: "Panning across an ant raft of about 3,000 ants on the surface of water. This view also shows the undersurface of the raft. Note how the bottom layer of ants remains on the water's surface so that water is prevented from penetrating the raft."]

Here is the abstract of the study published in the Proceedings of the National Academies.


Fire ants self-assemble into waterproof rafts to survive floods

Why does a single fire ant Solenopsis invicta struggle in water, whereas a group can float effortlessly for days? We use time-lapse photography to investigate how fire ants S. invicta link their bodies together to build waterproof rafts. Although water repellency in nature has been previously viewed as a static material property of plant leaves and insect cuticles, we here demonstrate a self-assembled hydrophobic surface. We find that ants can considerably enhance their water repellency by linking their bodies together, a process analogous to the weaving of a waterproof fabric. We present a model for the rate of raft construction based on observations of ant trajectories atop the raft. Central to the construction process is the trapping of ants at the raft edge by their neighbors, suggesting that some “cooperative” behaviors may rely upon coercion.


May 13, 2011 at 10:01 AM | Permalink


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Flautist, one word: Aardvark. http://bit.ly/iUu6At

Posted by: 6.02*10^23 | May 13, 2011 7:24:50 PM

My front yard's full of their architecture. Two summers ago, someone, not wanting to litter up the street, I'm sure, decided to put a half-eaten candy bar in my mailbox. When I went to retrieve the mail at 10pm, in a split second I had fire ants up to my chin - I shall never forget the sensation. I hope the little f*****s drown. And may the candy bandit be visited by fire ants, brown recluses, Asian giant hornets and bedbugs all at once.

Posted by: Flautist | May 13, 2011 2:32:51 PM

They don't do this on Trout streams.

Posted by: 6.02*10^23 | May 13, 2011 2:07:45 PM

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