For an ant scurrying up a dark, nearly vertical tunnel, one tiny misstep can mean a long tumble down the steep passageway. At least, that’s what happens if the ant doesn’t perform some acrobatic feats to catch himself mid-fall. So ants, a new PNAS Early Edition paper has concluded, design their tunnels to maximize their ability to stop falls. And to catch themselves, the researchers found, ants not only stick out their 6 legs to brace themselves against the tunnel walls, but use their antennae as well, which were previously thought to be used only for sensory information.
“We wanted to know how animals that create their nests below ground move around in those nests and prevent themselves from falling and stumbling in subterranean environments,” says physicist Daniel Goldman of the Georgia Institute of Technology.
To study this idea of the biomechanics of ants falling—or not falling—in confined tunnels, Goldman and graduate student Nick Gravish collaborated with biologist Michael Goodisman, also at Georgia Tech, who specializes in insect behavior.
The scientists began by observing the behavior of fire ants in an ant farm with see-through walls. The ants stumbled often, but caught themselves as long as they fell close to a tunnel wall. To determine exactly what size tunnel was required for such mid-fall catches, the team then designed an array of varying size glass tubes that ants could crawl through.
As the glass tunnels got narrower (down to 1 millimeter in diameter) or wider (up to 9 millimeters across), the ants maintained the same climbing speed—around ten body lengths per second. But when the researchers gave the whole system a shake, the diameter of the passageways influenced whether the ants fell and could catch themselves.
In the smallest diameter tunnels, ants most often didn’t fall at all after the quick shake. In the largest diameter tubes, the ants fell and were unable to catch themselves, as their appendages weren’t long enough to reach the sides of the tunnel as they plummeted. But when the tunnels were in the middle—around 3, 4, or 5 millimeters in diameter—the ants caught themselves as they fell.
Slowed-down videos of the falls and catches let the researchers see exactly when and how the ants caught themselves, sticking out their legs and antennae to span the sides of the tubes. Within a hundred milliseconds of the fall, the ants had most often made this contact with the walls, and that time was even shorter—mere tens of milliseconds— when the ants fell head-first, allowing the antennae to find walls faster. The importance of the antennae surprised them, Goldman says, since no one had previously reported this type of biomechanic role for ant antennae.
Next, the researchers studied the width of tunnels that fire ants dig on their own in different materials. They found that they always had diameters within that middle range, an average of 3.7 millimeters across.
“These animals are master diggers and can really dig in anything,” says Goldman. “And we saw in the laboratory that no matter what kind of soil they are digging in, they always dig the same size tunnels.”
Smaller tunnels could prevent even more falls, Goldman adds, but would impede traffic in the ant farm—the insects wouldn’t be able to pass each other in tighter passages. So he thinks the tunnel size is an optimal trade-off between minimizing the challenges of keeping traffic moving in a large colony and the risk of falling.
The research not only sheds light on the biomechanics of ants, but could inspire new kinds of tunneling robots, Goldman thinks.
“If you have big piles of rubble, like after the tornado in Oklahoma, you may want to send in a robot instead of first responders or canines. So how do you get into the rubble in an effective way? How do you create tunnels in the rubble to transport food or supplies down to people who are trapped?”
Studying how ants and other burrowing optimize their tunnels can help answer these questions and create better robots, the scientists think. “There’s a tremendous diversity of organisms that inhabit the ground beneath our feet and it’s really an unexplored world,” Goldman says. “By looking at these organisms in their natural environments you find beautiful behaviors which are scientifically interesting to look at but also could lead to practical advances.”