Ants face a choice when danger confronts them: do they stay put and take up the fight or flee for safety? Some have been particularly ingenious and developed various specialized avoidance strategies: certain species snap their jaws on a hard surface to catapult themselves to safety, while a selection of tree-dwelling ants drop from the canopy to glide down safely to the forest floor. In a recent study, Donato Grasso and colleges from the University of Parma, Italy, report on a strategy that had previously not been described. When threatened on a sloping surface, Myrmecina grainicola ants curl into a ball and roll away. The team decided to analyze the behavior in their laboratory and tested the idea that ants only use this strategy in situations where they know it will get them out of a tight corner.

The researchers collected colonies of M. grainicola ants from natural nests and housed them in the laboratory. The team then provided the ants with a sequence of small platforms to roll off, so that they could investigate the fine details of the insects’ maneuvers. First, to understand which circumstances would trigger the ants to perform a roll, the researchers put the insects on a sloping surface, which they tilted gradually from horizontal to 45 deg, while they lightly tapped the surface to scare the ants into rolling. Next, the team tested how far the rolling motion would take the ants on different surfaces, ranging from earth and leaves to stone, which the ants encounter in their natural environment. Finally, the team performed a detailed step-by-step video analysis of the ants as they rolled down a slope inclined at 25 deg.

The researchers found that the ants only used the rolling strategy when the surface they were walking on was inclined at an angle of 10 deg or steeper. This suggests that the ants only began rolling when the slope made rolling an effective strategy to get away from danger. What's more, the surface affected the effectiveness of the roll: on stone and leaves, the ants rolled distances of 17 and 8 cm, while rolling on earth only carried them a distance of 2 cm. To begin rolling, the ant put its head followed by its bulbous hind-segment on the ground, then its entire body curled up before it finally kicked its hindlegs to speed up the roll. Most impressively, the ants achieved an extraordinary speed of 40 cm s⁻¹ while rolling, compared with their leisurely walking pace of just 0.5 cm s⁻¹.

In an effort to find out how the ants used rolling to their advantage when under attack, the team pitted an ant against a competitor of another species in an arena that was either flat or inclined at 25 deg. After the pitched battles, it was clear that the incline allowed the M. grainicola ants to roll away from their opponents, improving their chances of living to fight another day: only 10% of the ants suffered injuries on the sloped surface, while 63% sustained injuries on the horizontal surface. As the ants only began rolling on the inclined surface, this suggests that the ants only resort to rolling when they know it provides an effective form of escape.

Grasso and colleagues have reported an unusual strategy that M. grainicola ants employ to get away from danger. These ants join a select group of only a few other animals, including some spiders, caterpillars and salamanders, that use rolling as a form of motion. This provides us with the opportunity to gain a better understanding of how rolling is used by a range of ingenious animals that have all reinvented the wheel by becoming one themselves.