Fire Ants: How Expert Excavators Make the Best Invaders

Who are the best diggers around? Is it humans? Fantasy dwarves? Mole people? Nah... as far as the experts are concerned, the fire ant takes the cake. New research has revealed that one of the primary reasons these little guys are such successful invaders is that they are able to thoroughly excavate complex colonies regardless of where they decide to settle - whether it be in wet clay or coarse and difficult-to-shape sand.

That's at least according to a study recently published in The Journal of Experimental Biology, which details how invasive fire ants (Solenopsis invicta) are able to change their excavation technique depending on the type of soil in which they are digging.

Dan Goldman of Georgia Tech admitted in a recent release that he has an insatiable "interest in animals interacting with complex materials." In the past, he has closely examined creatures such as sidewinder snakes and sandfish lizards moving through and across sand.

So with fire ants being one of several insect species invading Georgia turf, he couldn't help turning a magnifying glass to them as well - not to burn, of course, but to study.

With the help of his colleagues, Nick Gravish, Daria Monaenkova and Sarah Sharpe, Goldman was able to simulate multiple types of soil in a controlled lab setting. Soils ranged from clay-like particles to ant head-sized grains of sand that were also either anywhere from perfectly dry to near-mud. Small colonies of about 100 fire ants apiece were introduced to these soil settings and were left to excavate over a 20-hour window. (Scroll to read on...)

Monaenkova was then tasked with imaging the work of each colony even while leaving them undisturbed. To achieve this she opted to use X-ray imagery, taking an estimated 400 shots per nest before painstakingly reconstructing theis excavation work with a 3D modeling program alongside her colleagues.

The fruits of this labor was an easy-to-compare suite of ant colony models. Interestingly, the team found that overall, the hastily excavated nests did not vary in level of complexity, despite the fact that it should be understandably harder for the tiny insects to dig through large grains or shape a dry tunnel. What was different was the structure of the colonies themselves, as if the architecture of each nest was dictated by the material used.

"It is just mind blowing how they can dig so well," Goldman said.

In order to determine what made them such excellent excavators, the team finally set up colonies in glass containers full of tiny glass beads. This allowed them to observe the ants work in real-time, even as they struggled to dig through the smooth beads; but dig they did.

"It was basically the case that, once you get above about 5% moisture content, they will dig tunnels," Goldman recently told BBC News, explaining how water particles were used in just the right way to ensure that 'soil' grains stuck together.

According to the study, when the soil was coarse, they grasped a single grain and shuffled backward up the tunnel, dragging it with them. However, when the soil was fine and the ants could grasp multiple grains, they gathered them into a pellet and marched upward - a sign that on some intrinsic level perhaps, they understood that different environments require a different strategy. It remains unclear, however, how they know this.

"They invaded [the US] 80 years ago and they have dug nests from Georgia to LA," Goldman added, admitting his awe for the tenacious insects. "They can dig in anything."

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