March 29, 2013 | 2
The above proverb adorns the Web page of Nathan Mlot, a graduate student in mechanical engineering at Georgia Institute of Technology. Mlot studies ants, specifically how they self-assemble/self-organize. He made a splash in the media a couple of years ago when he reported that, based on his experiments, fire ants can actually link together to form life rafts should, say, their nest flood. Any single ant has a certain amount of hydrophobia — the ability to repel water — and this property is intensified when they link together, weaving their bodies much like a waterproof fabric.
They gather up any eggs, make their way to the surface via their tunnels in the nest, and as the flood waters rise, they’ll chomp down on each other’s bodies with their mandibles and claws, until a flat raft-like structure forms, with each ant behaving like an individual molecule in a material — say, grains of sand in a sand pile. And they can do this in less than 100 seconds. Plus, the ant-raft is “self-healing”: it’s robust enough that if it loses an ant here and there, the overall structure can stay stable and intact.
In short, the ant raft is a super-organism and the ants become a kind of granular medium. Even weirder, when they all link up like that, the ants start exhibiting more fluid-like properties — so much so, that it’s possible to “pour” them from a teapot into a teacup.
Mlot is just one of the people doing interesting interdisciplinary work on ants at Georgia Tech — two other ant researchers from the same institution reported on their latest work at the annual March Meeting of the American Physical Society in Baltimore a couple of weeks ago. Not only is the collective motion of the ants themselves a form of granular media, but so is the soil in which they dig to construct their large underground nests.
Fire ants — so named because of their fiery stings — are an invasive species in the region, thanks to a US cargo ship that docked in Mobile, Alabama in the 1930s, leaving behind a colony of Solenopsis invicta. And I do mean invasive: per Wikipedia, more than $5 billion is spent annually on controlling fire ant populations — and treating those suffering the aftereffects of its sting and the ants do costly damage to crops and livestock to boot.
A well-constructed fire ant nest can last a good ten years. You pretty much have to kill them all in one fell swoop, or you’ll find the infestation bouncing back in no time. The only country that’s been remotely successful at beating back a fire ant invasion is Australia, which managed to wipe out 99% of its fire ant population by 2007 — for a cost of $175 million (Australian currency).
Fire ants build large underground nests by grabbing particles of soil in their mandibles and transporting it to the surface. They navigate their nests via a series of narrow tunnels connecting larger chambers, according to Daria Monaenkova, a Georgia Tech physics postdoc who spoke at the APS meeting.
It’s tough to monitor the critters in the wild, so Monaenkova recreated ant-friendly environments in the lab. She placed her fire ants in artificial soil to track their progress, tweaking the conditions to determine which variables contributed to the ideal conditions for a colony of fire ants to set up camp in your backyard, monitoring their activity from various angles via a homemade x-ray CT scanning system.
Per Monaenkova, the reason the ants thrive in more temperate climates is that the soil can’t be too cold if the ants are to successfully dig their nests. It also needs to be wet. That’s due to the nature of granular media. A certain degree of wetness enhances cohesion, so the nest walls are less likely to collapse.
Can any of this apply to controlling invasive species? That’s a tougher nut to crack. Even pouring hot water into the nest to kill the ants isn’t foolproof, because some always survive, and eventually they come back and rebuild the nest. But the ants need moistened soil, so keeping your backyard dry could discourage them, and hopefully drive them into your neighbor’s yard instead. Droughts are our friend when it comes to discouraging fire ants.
That’s because dryer soil lacks cohesion — the individual grains don’t stick together as well, making it harder for the ants to dig stable tunnels. If the soil cohesion is too low, or lacking altogether, the ants dig less because it’s so exhausting; their digging rate decreases in inverse proportion to the excavation force required.
As for those tunnels, it turns out there are good reasons why they are so narrow. Monaenkov was joined at the APS meeting by her Georgia Tech colleague Nick Gravish, a graduate student. He noted that if ants were human-sized, their tunnels would be equivalent to our tallest buildings, yet the ants are incredibly productive: they build far more tunnels than we build skyscrapers.
Mostly, Gravish is interested in what he terms locomotion in confined environments — that is, how fire ants navigate those narrow tunnels at such high speeds. Environmental engineering simplifies subterranean locomotion, he explained, often limiting studies to a flat, featureless surface, which doesn’t shed much light on how ants cope in real-world conditions. He wanted to observe the fire ants in situ to determine how physical confinement — e.g., their vertical tunnels — influences the ants’ ability to move rapidly and stably.
So he built several sets of glass tubes with different diameters to see how width affects the ants’ movement. He found that when an ant falls, it knows how to arrest its fall using its sensory appendages (antennae), which serve as a stabilizing mechanism. Gravish tested this by performing “active perturbation experiments” with his tubes — that is, he shook them really hard to see if/when he could dislodge the critters. Per Gravish, “extreme perturbation” proved necessary because “ants have a very robust adhesive system.” It turns out that the ants’ ability to arrest their fall is highly dependent on the size of the tunnel with respect to the ants’ body size. If the tunnel is too wide, the ants can’t “jam” their bodies against the sides with their antennae.
Next he decided to see what size tunnels would be built if he just let his ants dig freely. He found they dug tunnels with diameters proportional to their body size — ideal for ensuring the ants could arrest their fall. He is now investigating how tunnel constraints affect the ants’ collective motion.
Working so closely with fire ants is not without risk. Fire ant stings are nothing to sneer at; if you happen to be highly allergic to the venom, they can even be fatal. At best, they are extremely painful, and if you scratch at the bite, it can easily become infected. When asked how often they had been stung by their experimental subjects, both Gravish and Monaenkova ruefully admitted they’d lost count. Fortunately neither of them is allergic. You’ve really got to love your research to risk constant bites by fire ants, is all I’m saying.
Gravish, Nick et al. (2012) “Effects of Worker Size on the Dynamics of Fire Ant Tunnel Construction,” Journal of the Royal Society Interface, August 2012 (online).
Gravish, N. et al, (2012) “Dynamics and robustness of fire ant nest construction.,” Journal of the Royal Society Interface (in review).
Gravish, N., Monaenkova, D., Goodisman, M.A.D., and Goldman, D.I., (2013) “Climbing, falling and jamming during ant locomotion in conﬁned environments.,” Proceedings of the National Academy of Sciences (in review).
Mlot, Nathan; Tovey, Craig A.; and Hu, David L. (2011) “Fire Ants Self-Assemble Into Waterproof Rafts To Survive Floods,” Proceedings of the National Academy of Sciences 108(17).
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