Researchers reveal the hidden secrets of ant rafts

A raft of fire ants measuring about a foot in diameter.
CC BY 2.0 Flickr user Maggie. A raft of fire ants measuring about a foot in diameter.

When threatened by flooding, fire ants have an amazing strategy to move their colony to safety. They assemble into a living raft, which is surprisingly buoyant even in rough currents. When the raft reaches land or a tree, the ants swarm out of the water to safety.

A new paper published in the Journal of Experimental Biology takes a deeper look—literally—at these floating colonies. To investigate the interior structure of the raft, researchers at Georgia Tech froze the raft and scanned it with a miniature CT machine.

In addition to gripping one another with their legs, fire ants also connect to their neighbors using adhesive pads and their mandibles. The researchers analyzed 440 ants, and found that on average a single ant makes 14 connections, attaching itself to 4.8 neighbors. The ants orient their bodies perpendicularly to one another.

"It's kind of like looking inside a warehouse and seeing the scaffolding and I-beams,” said Georgia Tech Assistant Professor David Hu in a press statement.

Although the ants can grip with a force that’s 400 times their body weight, they still maintain enough distance from one another to prevent the overall structure from sinking. "Increasing the distance keeps the raft porous and buoyant, allowing the structure to stay afloat and bounce back to the surface when strong river currents submerge it," said Nathan Mlot, a Georgia Tech graduate student who has previously published on the subject of ant rafts.

They also found that smaller ants will fill in holes around larger members of the colony, making the raft more water-tight.

Although we have new insight into the physical construction of the raft, researchers still don’t understand how the ants know where to go or what to do.

Hu suggests that better understanding of this ant behavior may one day help scientists make self-assembling robots, and indeed, insect colonies have already inspired engineers at Harvard’s Wyss Institute for Biologically Inspired Engineering.

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