But Myrmecina graminicola — an ant about the size of a sesame seed — doesn’t want to get into all that. According to research published last week in Scientific Reports, if one of these ants encounters danger while it’s on a slope, it makes a practical choice: It tucks itself into a little ball and rolls away.
It is the only ant known to move in this way, and one of few rollers in the animal kingdom over all, said Donato Grasso, the paper’s lead author and an ant ethologist at the University of Parma in Italy.
Dr. Grasso and his colleagues first spotted this unique behavior while scanning the forest floor during a trip to one of their field sites in Fornoli, Italy. (Many entomology discoveries are made this way: “When you are a biologist interested in insects, it is impossible not to look at the ground,” Dr. Grasso said.)
The team found a few colonies of M. graminicola, which are so small and elusive they often go unnoticed. When the insects were menaced by spiders and other ants, “they curled their bodies and disappeared” into the leaf litter, Dr. Grasso said. “They rolled away.”
The research team decided to take some of the ants back to the lab. It was difficult to find them, and when the researchers picked the ants up, they would sometimes somersault out of their hands. But eventually, they caught some living inside fallen tree galls.
In the lab, the researchers used slow motion video to tease out the ants’ choreography. Roughly: A ready-to-roll ant tucks in its head and pulls its abdomen forward to form a ball. It then lifts its legs up and tips itself forward to rest on its mandibles and antennae, which balance it like arms, Dr. Grasso said. A final push with the hind legs, and it’s off.
On smoother surfaces like stones and leaves, the ants traveled at about 15 inches per second — about 80 times faster than their average walking speed. They could move themselves about 6 inches, or about 50 body-lengths.
Such a distance is “pretty impressive,” and shows that the ants’ rolling form must be very efficient, said Nicholas Gravish, an engineer who studies ant locomotion at the University of California, San Diego and was not involved in the new research.
Dr. Grasso’s team also set out to learn exactly what prompts the ants to make this unique exit. They placed the ants on increasingly steep gradations, from zero degree all the way up to 90.
As the ants walked the slopes, the researchers exposed them to stressful stimuli. The ants modulated their reactions depending on the context. If they were on a relatively flat surface when the researchers bothered them, they simply froze.
But once the slope reached 10 degrees, some of the ants started to tumble away. And at inclines above 25 degrees — about as steep as a beginner-level ski trail — all the ants tucked and rolled.
These and other tests show that “these ants behave this way only in specific circumstances,” and that rolling wasn’t a byproduct of other defenses like curling into a ball, Dr. Grasso said.
While humans enjoy assisted rolling, it is rare that other animals take part. There are spiders that cartwheel across the desert, and some salamanders back-flip down boulders. If you give a mother-of-pearl moth caterpillar a good poke, it will spin away like a little green coin.
But in nature, rolling is not nearly as popular a tactic as, say, running away.
It’s also high-impact, which could be “catastrophic to larger animals,” said Glenna Clifton, a scientist in Dr. Gravish’s lab.
But if you can pull it off, it is effective. In a later experiment, the researchers exposed M. graminicola to one of its enemies, a different ant species. On flat ground, the M. graminicola casualty rate was 63 percent. But when they tussled on a slope, it dropped to 10 percent.
“Fighting is not always the best way to survive,” Dr. Grasso said. “Sometimes the best way is to escape.”