A swarm of fire ants cling to a chain link fence and floating debris. (Chris O’Meara/Associated Press)

When facing floodwaters, ants use their babies as life preservers, sticking them at the very bottom of the life rafts that they build with their own bodies.

Findings described in a paper published in PLOS One reveal that ant rafts have a structure that maximizes the group’s buoyancy and thus its chances of survival. But this structure exposes the young ant brood to hungry fish and the risk of drowning.

“It was an interesting contribution. No one had really looked at this idea of the brood as a flotation device,” said David Hu, a mechanical engineer at the Georgia Institute of Technology, who was not involved in the study. “It adds a level of sophistication to the rafts that was previously not understood.”

Researchers have long marveled at the incredible organizing powers of ant colonies. Like hives of bees and nests of termites, these super-organisms exhibit what’s known as “swarm intelligence,” able to act quickly in coordinated ways.

Ants use their bodies to build bridges that other ants can walk over. They surround intruders and kill them with their body heat. And when a flood hits, they can link up and form rafts that can float to higher, drier ground. One study found that the invasive fire ant Solenopsis invicta actually traps air pockets to form a protective layer that helps keep them afloat.

But researchers at the University of Lausanne in Switzerland wondered if there was even more complex organization to these ant rafts. After all, who were the unlucky ants forced to the bottom of the raft? Was there a reason they were chosen for the job? Were they submerged and thus at higher risk of drowning? The researchers also wondered whether the ants would place the most valuable or vulnerable members, like the queen and the young brood, away from the water.

To test their hypothesis, the scientists used Formica selysi ants from colonies along the bank of the Rhone River, putting them in groups of 60 workers with queens and giving some of them 10 young brood ants to care for. Then they started to slowly raise the water level and watched how these mini-colonies reacted.

Sure enough, the worker ants placed the queen in the center of the raft, protected from the water on all sides. But, to the researchers’ surprise, the ants took the young, helpless brood — the larval and pupal ants — and lined their bodies along the base of the raft, where they would be most exposed to water.

This seemed counterintuitive, but after submerging both adult ants and their brood young in water, the scientists found that the pupae and larvae were actually more buoyant than the adults. This played out in the overall success of the brood-
bottomed rafts, which seemed to do better than rafts held together only by adult worker ants. Once the rafts disembarked, the ant rafts with no brood took less time to disassemble, but they had more “nonresponsive” workers that needed reviving, the authors found. The rafts with brood, on the other hand, took a little more time to disassemble, but there were far fewer on their crew who needed CPR.

The ant young seemed to suffer no long-term ill effects from being put to work like this, the researchers found: They reached adulthood at the same rate as brood that hadn’t been rafted.

Los Angeles Times