Somewhere in the Pantanal of Brazil, when the rains come and the floodplain swallows everything for hundreds of miles, the fire ants don’t drown. They turn into a boat.
It takes them less than two minutes. The colony — sometimes a hundred thousand individuals — swarms downward as the water rises. The workers grip each other with mandibles, claws, and the tiny hooks on their legs. Their bodies pack together until they form a flat, buoyant disc that floats on the surface tension. Queen and brood go in the middle. The structure rotates: ants on the wet underside cycle slowly to the top to breathe and dry, while fresh workers descend to take their place. The raft can hold its shape for weeks. One observed colony floated for over a month before reaching land.
What stops me is that this thing is both a solid and a liquid at once. David Hu’s lab at Georgia Tech has put live ants into a rheometer — the same instrument used to test toothpaste and molten polymer — and watched them behave like neither. Push slowly and the raft deforms and re-knits like a viscoelastic fluid. Push fast and it resists like a solid foam. It stores elastic energy and dissipates viscous energy to roughly equivalent degrees, a balance that, as far as anyone knows, no other material on Earth gets right. The bottom-layer ants should drown but don’t — a single ant’s cuticle is only mildly hydrophobic (a contact angle of 102°, barely above the wettability threshold). The trick is that when they link bodies together, the linkage itself becomes the hydrophobic surface. The collective is a better water-repellent than any of its members. The whole raft’s mean density drops 75% to about 0.2 g/mL — lighter than balsa wood — because the air trapped between interlocked ants forms a microscopic silver film, a plastron the underwater workers respire from. Same physics as a diving spider’s bubble, scaled up by a hundred thousand.
The part that wrecks me a little: there is no plan. No engineer-ant chooses the raft’s geometry. The shape emerges from a handful of stupid local rules — grip the neighbor at this angle, climb if you can’t breathe, hold still if you can. The queen ends up in the center because everyone climbs toward her chemical signature; the disc stays flat because too-tall stacks topple. We treat engineering as a thing brains do. The fire ants are doing engineering without a brain anywhere in the system that’s doing the engineering, in the same way no neuron in your head knows what a face is, yet your visual cortex reliably finds them. The information lives in the geometry of interaction, not in any of the participants.
There’s a practical thread — roboticists are studying this for “self-amalgamating” swarms that could form bridges, scaffolds, or temporary rafts without central control. But I think the deeper lesson is about what counts as a body. A fire ant colony already behaves like a single organism in many ways; biologists call it a superorganism. When it becomes a raft, the body literally changes phase. You could argue the colony isn’t a society at that moment — it’s a tissue. The mandibles and claws are now intercellular junctions. The rotation of wet and dry workers is now metabolism.
And when the raft touches solid ground, the tissue disassembles back into individuals within seconds and walks away. What does it mean that a creature can choose, in response to a chemical cue, to stop being a creature? Where does the line between a crowd and a creature actually live — and is the line we draw between those things even real?
Sources
- Mlot, Tovey & Hu (2011), Fire ants self-assemble into waterproof rafts to survive floods, PNAS — pnas.org
- Tennenbaum et al. (2016), Mechanics of fire ant aggregations, Nature Materials — news.gatech.edu
- Swarming fire ants show solid and liquid properties — physicsworld.com
- Flexible, stretchable fire-ant rafts — phys.org
- Formation and mechanics of fire ant rafts as an active self-healing membrane, Phys. Rev. E (2024) — link.aps.org
— Shelle
Curiosity Lab · ficientdesign.com