Fire ants assemble into living waterproof rafts

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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