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Swarming Fire Ants Turn Themselves Into a Liquid by Playing Dead

How ... horrible.
Image: Georgia Institute of Technology

A fire ant is a discrete organism. A single ant unit comes complete with its own six legs, antennae, brain, and everything else—but a fire ant alone still doesn't count for very much. Its real power is as a member of an aggregation, a connected swarm of ants behaving not so much as one single super-ant, but instead as a biological super-material.

This super-material is of great interest to physicists, it turns out. Last week a group of researchers based at the Georgia Institute of Technology published a paper in Nature Materials describing the peculiar ability of fire ant aggregations to oscillate between arrangements analogous to the solid and liquid phases of matter. The result is a so-called active material, a substance whose constituent particles are all individual autonomous actors.

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As the GT researchers explain, fire ants are in a category of their own among active materials (flocks of birds, schools of fishes, other collections of self-propelled particles). This is due to the property of viscoelasticity. An aggregation of fire ants can both ooze and drip like honey and stretch and bend like rubber. To achieve this, each individual fire ant must serve to both store and dissipate applied energy.

In the group's experiments, the ants were first collected from their native dirt and, then, before each experiment, put inside a container, "which we gently shake to force the ants into close contact," the study explains. "The resulting aggregation clearly exhibits elastic behavior; when deformed externally by the application of a stress, it returns to essentially its original shape. It also exhibits viscous behavior; a lead sphere is able to sink through the aggregation very much like a solid object sinks inside a viscous liquid."

These properties can be measured using a rheometer, an instrument that quantifies the flow of various materials—more likely asphalt or beauty cream than insects—in response to an applied force. In the GT experiments, the device consisted of two Velcro-covered plates situated on top of each other separated by a 3 mm gap. Shear force was applied to the ants in between the plates by moving the surfaces in opposing directions. The resulting forces were then measured, offering information on the elasticity vs. viscosity of the aggregate ant substance. This was repeated for both live and dead ants.

The resulting phenomenon is known as "shear thinning." That is, as more shear stress is applied to the ant material, its viscosity actually decreases. It's a bit like wet paint, where the stress of the brush or roller lowers the paint's viscosity such that it can be easily and evenly spread around on the target surface. But, once applied, the paint thickens back up. The result is malleable paint that also doesn't just drip right off the wall.

In the case of fire ants, the transition happens right around the point where the applied force is great enough to just rip the individual ants apart. When a force is applied to a writhing ball of fire ants (again: a writhing ball of fire ants), the ants respond by grabbing hold of each other's limbs, which collectively offers a certain amount of resistance to the force. At the tipping point, however, they all start to play dead instead, resulting in a dense tangle of limp ants.

"Interestingly, experiments with dead ants reveal that the viscosity of dead and live ants is identical," the study notes. "When forced to flow, live ants seem to 'play dead', ceasing all active motion. We then hypothesize that the energy loss is caused by the friction in the leg joints of the ants; this friction must be overcome for an ant leg to give way and allow strain-rate-induced rearrangements within the aggregation." Poor fire ants.

Except not really: Fire ants are amazing and superbly adapted to their hypersocial ant-world. "Ants can maintain stresses enough that they can build a raft to support thousands of members, a bivouac that's 30 stories tall—for an ant—or a bridge," study co-author David Hu tells Physics World. "But when the going gets tough, they can just liquefy."