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Ants clutch each other to build an amazing life raft

"Fire ants are special engineers," Hu says. "They are the bricklayers and the bricks. Somehow they build and repair their structures without a leader or knowing what is happening. They just react and interact," David Hu says. (Credit: Georgia Tech)

In a flood, fire ants grip each other with their mandibles and legs at a force of 400 times their body weight to build waterproof rafts to stay alive.

Now, using a miniature CT scan machine, researchers have frozen ant rafts and taken a closer look at the strongest part of the living structure—the inside—to discover how opaque ants connect, arrange, and orient themselves with each other.

“Now we can see how every brick is connected,” says David Hu, assistant professor of mechanical engineering at Georgia Institute of Technology. “It’s kind of like looking inside a warehouse and seeing the scaffolding and I-beams.”

He found a lot of beams.

On average, each ant in a raft connects to 4.8 neighbors. Ants have six legs, but using their claws, adhesive pads and mandibles, each critter averages nearly 14 connections. Large ants can have up to 21.

Out of the 440 ants scanned, 99 percent of them had all of their legs attached to their neighbors. The connectivity produces enough strength to keep rafts intact despite the pull of rough currents.

Hu and colleagues also noticed the insects use their legs to extend the distances between their neighbors.

Small ants fill the gaps

“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,” says Nathan Mlot, a graduate student who worked on both studies.

Ant self-assemblages: a) raft, b) hanging column, c) bivouac, d) escape droplet, and e) escape tower. (Credit: Georgia Tech)
Ant self-assemblages: a) raft, b) hanging column, c) bivouac, d) escape droplet, and e) escape tower. (Credit: Georgia Tech)
Ants use legs and mandibles to form connections with neighbors. Here we have scanning electron microscopy images of a) a single ant tarsi, b) an adhesive pad to leg connection, c) a mandible to leg connection, and d) a combination of claw and adhesive pad to leg connection. (Credit: Georgia Tech)
Ants use legs and mandibles to form connections with neighbors. Here we have scanning electron microscopy images of a) a single ant tarsi, b) an adhesive pad to leg connection, c) a mandible to leg connection, and d) a combination of claw and adhesive pad to leg connection. (Credit: Georgia Tech)

The researchers found that smaller ants tend to fill in the spaces around large ants. This keeps water from seeping in and prevents weak spots in the raft.

The insects, large and small, arrange perpendicularly rather than parallel. This adds to the adaptability of the raft, allowing it to expand and contract based on the conditions. The same is true when ants build towers and bridges for safety and survivability.

One thing the CT scan can’t solve, however: how the ants know where to go and what to do. Their cooperation is a mystery the research team hasn’t yet figured out.

“Fire ants are special engineers,” Hu says. “They are the bricklayers and the bricks. Somehow they build and repair their structures without a leader or knowing what is happening. They just react and interact.”

Better understanding of this phenomenon could lead to new applications for people and machines, Hu says. For instance, he envisions robots that can link together to build larger robots or bridges made of materials that can self-repair.

“If ants can do it, maybe humans can create things that can too.”

The National Science Foundation partially funded the research, which is published in the Journal of Experimental Biology.

Source: Georgia Tech