Fire ants assemble into living waterproof rafts

By Ed Yong | April 25, 2011 3:00 pm

What happens when you dump 8,000 fire ants into a tray of water? Nathan Mlot from the Georgia Institute of Technology wanted to find out. Mlot scooped the ants into a beaker, swirled it around to roll them into a ball, and decanted them into a half-filled tray.

Over the next three minutes, the ball of ants slowly widened and flattened into a living, waterproof raft. By trapping air bubbles trapped among their interlocking bodies, the ants boosted their natural ability to repel water and kept themselves afloat. Humans build rafts by lashing together planks of wood or reeds; the fire ants do so by holding onto each other.

The experiment might seem odd, but it mirrors conditions that the fire ant (Solenopsis invicta) regularly has to cope with in its natural environment. The ant hails from the Brazilian rainforest floodplains of Argentina, where rising water regularly submerges their nests. They respond by weaving their own bodies into rafts. The ants also come together to construct bridges, ladders and walls, but the rafts are the longest-lasting of these living structures. In this form, they can float and sail for months.

Even though ants are denser than water, individuals can stand on it thanks to surface tension, the property that makes the surface of a liquid behave like an elastic sheet. But surface tension can only support small objects. An ant’s foot has no problem, but it’s harder to see how a writhing ball of thousands of ants could avoid sinking.

Ants are covered in a waxy layer that repels water. If you put a drop on an ant’s head, it sits there as a bead, rather than flattening out and covering the insect. A drop of water on a group of ants is even better at retaining its spherical shape.

This is because rougher surfaces are even better at repelling water than smooth ones because they trap more air pockets between the surface and the water. Together, a group of ants forms a rougher surface than any individual. By their powers combined, the ant raft repels water even more effectively than a single ant.

The surface of the lotus leaf is another natural structure that excels at repelling water. It too is rough, and studded with small bumps. But the leaf is a single surface – the ants, by comparison, assemble a similar surface by linking their own bodies.

By trapping bubbles around and between their bodies, the ants also ensure that they can breathe and float. The bubbles slash the density of their self-made raft by a whopping 75%. An individual ant may be denser than water, but a raft of ants is far less dense. That’s why it floats. Even if Mlot pushed the raft down with a stick, it still refused to sink. The ants ‘dented’ the water, but they wouldn’t go under it.

To see how the rafts are formed, Mlot dumped balls of 1,000 to 8,000 ants in water. They behaved like a spreading drop of fluid. It took just a few minutes for even the largest colonies to reorganise themselves into a pancake-shaped raft, spreading outwards just as a drop of dye would do.

The size of the raft is finely calibrated. If Mlot removed ants from the top layer, those on the bottom moved up to replace them, keeping the raft at a constant thickness. In fact, this jostling between ants on the top and bottom layer determines the size at which the raft grows, and its eventual size.  Ants on the top are free to walk around, and they bounce off the edges until they are caught at one. Ants on the bottom layer are pinned there by the sisters around them, holding them down with jaws and claws, and those walking on top of them.

The raft is a miracle of biological engineering. It assembles itself in minutes with no equipment, it floats and repels water, and it can hold millions of passengers with zero casualties. Mlot is interested in replicating these features with robots.

Reference: Mlot, Tovey & Hu. 2011. Fire ants self-assemble into hydrophobic rafts to survive floods. PNAS

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Comments (12)

  1. kirk

    I first heard this story 30 years ago from my Aunt in S. Texas. Rain flooded a stock tank (Texan for pond) and as the water rose over each FAR AINT bed, a ball of unspeakable evil floated to the surface.

  2. A lovely write-up as usual, Ed.

    One small correction, as you’ve copied one of the authors’ errors. The fire ant doesn’t actually hail from the Brazilian rainforest.

    Rather, the bulk of the evidence indicates that S. invicta came from the Paraná flood plains of northern Argentina (pdf), a natural history tidbit that rather strengthens the context of this paper.

    Anyway, it’s a minor nitpick about an otherwise excellent study.

  3. Lovely stuff. Having lived with them and been bitten by them, however, I really wish these films showed them being horribly drowned. Alas.

    Most annoying thing about these ants (well, one of many): They love any trace of sweat. So if you lay your shirt aside (could be worse; underwear) for the night, then put it back on the next day, it may harbor a couple or couple dozen fire ants, who will then bite the hell out of you.

    Lovely creatures.

  4. Hooper T

    I didnt know fire ants were that smart

  5. Michael Sternberg

    Hooper: The behavior is an emergent phenomenon.

    The ants are not smart as such, evidenced by the similarity of their swarm behavior to that of liquids, to which we typically do not attribute any smarts. Each ant probably follows simple (and evolved) rules.

  6. Dunbar

    Wouldn’t our minds also be an emergent phenomenon?

  7. dgp7856

    If you spray the island with soapy water from a spray bottle or a squirt gun the entire island will sink like a rock and they all drown. The soap seems to break the water tension.

  8. dgp7856

    If you spray the island with soapy water from a spray bottle or a squirt gun the entire island will sink like a rock and they all drown. The soap seems to break the water tension. I have done this many times when we get a heavy rain and they start floating toward my house.

  9. @dgp7856 – Yep, soap ruins their waterproofing. Some caterpillars do roughly what you did – they vomit detergents onto ants:

  10. I’ve seen these a lot when I lived in Florida during heavy rains from tropical storms and hurricanes. I’d always been told they floated on the dead bodies of those that drowned – so cool to learn the truth, which is even more impressive.

    For the record, if you see one of these, don’t look up close at the marvel of its ingenuity. These rafts are nasty. I’ve seen firsthand what happens when such a raft encounters a solid object that is above the water. The ants respond by rapidly ascending the object en masse, whether it’s a tree, wall, or, in the case that I saw, a very unfortunate friend of mine.

    A tropical storm hit my college with sudden ferocity one day when I was in class, and the teacher realized that rising waters had overcome the nearby seawall and the area under the building where a student’s thesis project was being kept. We all ran downstairs and battled the winds and rain to rescue the plants he’d been growing for months before they were wiped out by the imminent flood waters. We were in maybe 6 inches of water when one of these bioengineering masterpieces floated into my friend’s leg. Within a second his leg was covered in hundreds of ants, which were not very pleased at his attempts to remove them. His leg was covered in bites, and the pain was so excruciating he could barely walk on it.

  11. RockyABQ

    “The raft is a miracle of biological engineering.”

    I wince whenever the “m” word is used in conjunction with real science.

    Very interesting stuff though!

  12. Kevin

    Is this specific to this species or do other species do it? Non-ant species?


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