Fire ants are some of the most incredible cooperators in the animal kingdom, with entire colonies tirelessly working together. But they're also fire ants, and the fact that they can float around on rafts of their own stinging bodies is both incredible and downright unnerving.
Since we're all most likely not swimming in the path of a fire ant pain raft, let's focus on the incredible part. How is it, exactly, that hundreds or thousands of ants can lock together to form sturdy structures out of their own bodies?
A new study from researchers at Georgia Tech aimed to find out, and the results are fascinating: Fire ant morphology is uniquely suited to building "highly interconnected networks, possessing as much porosity as known biomaterials such as bone," as the authors write in the Journal of Experimental Biology.
While the findings offer insight into how ants actually work, they have even broader implications for developing smart, self-healing materials.
"The movement of the ants results in materials that can contract, and response to stresses, by arrangement of individual ants," study coauthor David Hu told me in an email. "The result is a structure that is responsive yet highly interconnected."
Fire ants have been observed to create a variety of structures simply by linking their bodies together, including, as shown below from the study, a) rafts, b) hanging columns, c) bivouacs, d) escape droplets, and e) escape towers:
As you can see in d), individual ants are incredibly strong. But a raft can't only be strong, it also has to be light and porous. Of course, measuring the porosity of a swarm of living fire ants isn't exactly easy, so the researchers got creative. And by that, I mean they waited until study ants built a raft, and then froze the whole thing with liquid nitrogen.
Once they had a frozen, complete raft, the researchers were able to use micro-scale computed tomography (micro-CT) to scan the clump of ants and measure its network connections. What was once a jumble of dead insects thus became a 3D computer model of, well, dead insects:
At this point, the Georgia Tech team was able to really compare the network connectivity of the ant swarm versus a control group, which was created by taking individual dead ants and shaking them together in a tube to see how they'd agglomerate by random chance.
Morphologically speaking, fire ants come in three size groups: "A study by Wood and Tschinkel found 45% of ﬁre ant workers in a mature colony are small (head width up to 0.8 mm), 42% are medium (head width 0.8 - 1.0 mm), and 16% are large (head width greater than 1.0 mm)," the authors write, noting that the largest ant is about three times larger than the smallest. Being insects, they also have six legs each, with which they can grab onto each other.
The team found that each ant in the test rafts averaged an astonishing 14.3 connections to the network at large; in other words, each ant had its six legs attached to someone else (outbound connections), and had 8.3 other legs latching on to itself (inbound connections). Here's a visual from one individual, with its own leg connections marked in blue, and inbound connections marked in red:
In addition, the researchers found that the test group was packed 34 percent less tightly than the control group, which is the result of ants using their legs to space out their bodies to create a less dense mass.
This, of course, is crucial to their ability to float; while the ants' bodies are hydrophobic, a raft that was too tightly packed—or filled with too many holes, for that matter—would sink. These scanning electron microscope images show just how the ants' tiny little legs lock on to just about anything they can:
Aside from helping solve the mystery of floating fire ants, what does the work mean? Well, the finding that ants actively space themselves into large aggregations is fascinating in its own right, as it suggests a new facet of complexity in ant behavior.
"To control their arrangement, ants would require some level of cooperation, whose level still remains unknown," the authors write.
But beyond that, the high connectivity of ant aggregations sheds light on other highly-connected porous structures, or for materials science.
"Further investigation of ants actively rearranging their network may yield discovery of mechanisms for controlling arrangement and further inspiration for the development of biomimetic self-healing materials," the researchers write, which is surely one reason the Department of Defense was interested enough in the study to help fund it.
"This is the first example of a macroscopic self-healing material. Most self-healing materials as cited in the paper uses polymers or chemicals," Hu said. "These ants shows that this ability can be extended to a larger scale."