Ants are bristling with defense weaponry. Different species might sting their enemies, bite them with powerful jaws or shoot them with jets of formic acid. Some even explode.

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.)