Living in a flood zone is never easy, especially if you're a tiny insect with few avenues of escape. But the ants living in the floodplains of the Alps and Pyrenees (a species called Formica selysi) have developed a way to survive when waters rise: they grab onto each other and form rafts that allow them to float away without being harmed. An important element of those rafts, according to research published Feb. 19 in the journal PLoS One, are the youngest members of the colony. These pupae and larvae have more buoyancy than full-grown ants, and their involvement allows more of the colony to survive.

These "ant rafts" have been observed before, but researchers from the University of Lausanne and the University of Neuchâtel in Switzerland wanted to understand them better. They gathered ants from several locations and subjected them to laboratory flooding experiments. In each case, the worker ants quickly assembled into rafts, always placing the apparently helpless and immobile young brood pupae and larvae on the bottom. The worker ants then distributed themselves throughout the raft, while the queens always took the safest position in the center. (The ants in the experiments also had the option of selecting pieces of buoyant wood but always opted to use their young relatives instead.)

The researchers write that this "swarm intelligence" takes advantage of brood buoyancy while preserving colony integrity and allows the vast majority of the colony to survive the flood. Not only did the worker ants survive, the majority of the brood did just fine and later matured without showing any signs of stress or injury from the floods.

You can see this swarm intelligence in action in this video, shot from below as the floodwaters approached:

Formica selysi- Raft formation with sexual brood, filmed from below from Jessica Purcell on Vimeo.

Now these were small experiments of only about 60 ants at a time. As David Hu, a Georgia Institute of Technology mechanical engineer who was not involved in the study told the Los Angeles Times, ant rafts in the wild can carry 100,000 or more members of a colony. The floods in the lab also only lasted a few minutes; in nature they can submerge colonies for days at a time. All the same, the researchers write that this is an interesting example of social organisms making quick decisions based not on altruistic self-sacrifice but the "functional properties of individuals."

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