How Ants Make Rafts to Save the Queen

When it rains, it pours—and to escape the deluge, some ants build floating rafts.

Though scientists already knew about this strategy, little was known about the insects’ decisions behind their raft-building process.

Flood ants build a raft to escape a flood and protect the queen. Photograph by Jessica Purcell

Now a study published February 19 in PLOS ONE shows that flood ants (Formica selysi) that live in the floodplains of Switzerland work together to protect the queen by strategically placing larvae, pupae, and worker ants at the bottom of the raft. The queen was placed in the center of the raft to shield her from water and other dangers.

This arrangement initially perplexed the scientists, since the bottom layer of the raft can be eaten by fish and swept away by turbulent waters. (Also see “Pictures: Fire Ant Swarms Form Living Life Rafts.”)

“We were expecting the colonies to protect the most valuable members, the queen and young,” said study leader Jessica Purcell, an ecologist at the University of Lausanne in Switzerland.

The short answer: The young ants are used as floating devices that are more efficient—that is, fewer lives lost—than older ants, Purcell explained.

Buoyant Babies

By observing flood ants in the lab, the team found that when the water begins to rise, worker ants will grab any available larvae to use as a floatation device, and more ants pile on top of them. The queen is moved to the center early in the building process. (Watch how fire ants create rafts in the video below.)

Young ants are more buoyant than worker ants, and the team discovered the young insects have a lower risk of dying at the bottom of the boat than thought.

“The larvae and pupae are buoyant and not entirely submerged,” said Purcell. “They are also quite fat, which may be one characteristic that helps buffer them against cold-water conditions.”

Without larvae and pupae under the raft, 25 to 50 percent of the worker ants had at least partial contact with the water, putting more of them in danger.

Also, by strategically placing young ants at the bottom, the colony can quickly return to normal after the flood. That’s because it takes more than an hour for submerged adult ants to recover from water exposure. (See National Geographic’s pictures of fantastic ants.)

This isn’t the first study to document the use of young at the bottom of ant rafts—other ant species build similar rafts—but it offers a new cost-benefit analysis of the behavior.

“Placing young on the base of the raft is a very efficient means of creating a highly buoyant raft,” said Purcell, “and they achieve all this at minimal cost in terms of mortality.”

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Meet the Author
Angie McPherson is the Digital News Intern at National Geographic.