Hurricane Harvey caused tremendous hardship and damage in parts of Texas and Louisiana. Among the other debris and pollutants, floating masses of fire ants were observed drifting in the floodwaters. How does a swarm of ants float?
Fire ants are native to South American rainforests, where they live in large subterranean nests. Floods are frequent, so fire ants have evolved a system to survive and disperse to new areas by forming into an interlocked, floating mass called a raft. These rafts can stay afloat for months, whereas individual ants in water can only stay afloat for a short period of time.
To understand how these remarkable structures work, a team of researchers from Georgia Tech filmed the formation of ant rafts and even flash froze some rafts to study their structure. They found that the rafts functioned by building on the natural water repellency of ants. Ants are naturally somewhat water resistant, which prevents dehydration. Clumping together enhances the ability of the overall structure to force water on the surface to bead. The ants hold themselves together by linking their mandibles to the legs of other ants, and locking leg to leg. When using mandibles, the ants are careful to avoid hurting each other. The overall structure attracts and holds a layer of air called a plastron layer tightly around the raft. The plastron layer enhances buoyancy and allows even the ants on the bottom of the raft—who are underwater—to breathe. Consequently, ants in rafts rarely drown.
The rafts can form quickly, presumably so that ants can rapidly evacuate a nest. Even a group of thousands of ants can completely raft up in minutes. Generally, each raft will be shaped like a pancake with most ants on top and a small layer of ants on the bottom holding up the others. If ants are removed, the whole structure will quickly redistribute to maintain the overall raft integrity. The ants on the bottom change places with those on top, but infrequently. The ants must closely cooperate to form and maintain a raft. It’s unclear what makes an ant stay on the bottom, but it’s possible that any given individual’s role is not completely voluntary.
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The behavior of the rafts is so extraordinary that the Georgia Tech team considered the ant swarms as a fluid. Each ant is almost like a molecule in a drop of solvent spreading across water. When the ants finally reach dry land, the rafts immediate break apart and the ants set about their next job: making new nests.