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See Inside Scientific American Volume 310, Issue 5

Ants Save Their Hides by Floating on Their Children's Backs

A species of ant escapes floods on a raft made of its babies


SWARM: Some ant species build living rafts or bridges, such as this overpass constructed by Eciton burchelli.


ALEX WILD

“In the ant's house, the dew is a flood,” an old proverb tells us. Yet for floodplain-dwelling ants, a little dew is nothing. When a real flood arrives, some ant species are known to evacuate their nest and self-assemble into rafts that float to dry ground. Swarm behavior is common in ants: some species even build living bridges to let their kindred march atop.

When it comes to raft building, the behavior has been observed in fire ants, but scientists at the University of Lausanne in Switzerland have discovered a peculiar design in living rafts of another species, which builds boat bottoms with its young. Using babies as flotation devices is not as much a threat to propagating ant genes as you would think.

This species of floodplain-dwelling ant, Formica selysi, nests throughout the Pyrenees and the Alps. The queen ant lives for about 10 to 15 years and experiences a Noah's ark–like flood an average of two or three times in her lifetime. During a flood, the worker ants collect the brood—immobile eggs, larvae and pupae—into a pile, and then another three or four layers of workers climb atop them and hold onto the babies with their mandibles. The queen assumes her place in the protected middle of the raft.

Placing the brood on the bottom, where it is most at risk of drowning, seems like a bad idea. After all, along with the queen, offspring are the most valuable members of the colony because their survival will determine its evolutionary success. “The conventional wisdom would be that the workers would put the brood in the middle of the raft with the queen,” says postdoctoral student Jessica Purcell, who led the research.

Purcell and her colleagues mimicked flood conditions in their laboratory with F. selysi ants they collected along the Rhône River banks in Switzerland. All ants rafted in artificial flood conditions, regardless of whether or not they had a brood; those with no babies at hand built their boat base out of worker ants instead. After the flood subsided, the raft without a brood had more unresponsive worker ants and they took more time to recover, which may explain why this species recruits its buoyant youth.

Surprisingly, the ant babies did not appear to suffer at all from their watery chores. Those that made up the raft bottom survived just as successfully as the brood control group kept on dry land. The tremendous buoyancy of ant babies, most likely the result of high fat content, prevents them from sinking when they have to carry their parents on their back. So in the ant world, offspring are not so useless: it's the mommies and daddies who are the hangers-on.

This article was originally published with the title "A Pontoon of Pupae."

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