It's hard to keep your footing in a steep tunnel made of loose dirt while others are scrambling around and over your body. Harder still in pitch blackness. That's why fire ants build tunnels that will catch them when they fall—a strategy human engineers might want to steal.
"Slips and missteps are likely a constant, recurring feature of life underground," says Nick Gravish, a graduate student in Daniel Goldman's rheology and biomechanics lab at Georgia Tech. Yet ants have to traverse their tunnels quickly, especially when there's a colony emergency like a flood or destruction by a gardener's spade.
To study how ants engineer their tunnels, Gravish brought the fire ant Solenopsis invicta into the lab. Invasive to countries around the world and packing a nasty sting, these South American ants deal out plenty of hardship. But Gravish was interested in how they handle adversity themselves.
First, the ants were put into "laboratory soil" (actually tiny glass balls) to dig. Researchers took x-ray CT scans of the resulting tunnels and found that no matter the moisture of the "soil" or the size of the glass beads, ants dug circular tunnels of approximately the same diameter. That diameter was just a little bit more than the length of their bodies, not counting legs or antennae.
This suggested that the diameter of the tunnel was crucial to the fire ants. To see how well the ants moved within these tunnels, the researchers recorded video of them climbing as fast as they could. ("We startled them into climbing at high speed by exhaling gently into the nest," Gravish says.) They saw that ants were able to navigate their tunnels quickly, reaching speeds of more than 9 body lengths per second. They also saw that sometimes the ants slipped and had to recover their footing.
In addition to their tunnels, the researchers recorded ants climbing in vertical glass tubes. To get a better idea of how ants corrected their falls, the scientists jolted the tubes to knock the ants off the walls while they were climbing. (If you enjoy videos in the falling-bugs genre, this study generated several new additions. Here's one video of several ants falling and stopping themselves.)
Now the reason ants build tunnels so close in diameter to their own body length became clear. Ants responded to a fall by spreading all their appendages wide and waiting until they jammed to a stop. "One of the coolest things we found was that fire ants used their antennae to brace themselves," Gravish says. While falling, the ants turned these delicate sensors into extra load-bearing limbs.
When the glass tube width increased to 1.3 times the ants' body length, the strategy began to fail. The tunnels ants built themselves had an average diameter of just 1.06 times their body length, the authors report in PNAS. It seems fire ants put most of the responsibility for stopping falls on the tunnels themselves. After that, all a plummeting insect has to do is stretch out its limbs.
Gravish likens this strategy to the way humans build stairs. Steps are engineered to fit our bodies. If they're too tall or short, we struggle to use them (or maybe just fall down them). But with the right design, our environment works with us to get us where we're going.
This strategy could inspire how we design robots for confined spaces such as search-and-rescue zones, Gravish says. For instance, "falling is usually considered a failure mode for a robot." But fire ants seem to use little falls to descend more quickly through their tunnels. If engineers knew the size of the cracks and crevices in a disaster area, they might be able to send in many inexpensive robots designed to tumble through those spaces—rather than one very expensive robot built to keep its footing.
What about humans ourselves: would we benefit from building tunnels that were only as wide as our head-plus-torso length, like the ants? Gravish points out that fire ants often fall many body lengths before catching themselves, making this not such a great strategy for people. "Ants have a robust exoskeleton," he says. "We humans are quite soft in comparison." Images: ant in tunnel by Laura Danielle Wagner; ants falling by Gravish et al. Gravish, N., Monaenkova, D., Goodisman, M., & Goldman, D. (2013). Climbing, falling, and jamming during ant locomotion in confined environments Proceedings of the National Academy of Sciences DOI: 10.1073/pnas.1302428110