Leaf-cutter ants pick up the pace when they sense rain

In Central America’s rain-drenched forests, leaf-cutting ants collect pieces of leaves on which they grow fungi for food. But the rain can hit hard, especially for a small ant. When leaf-cutting ants sense an incoming shower, they hoof it back to their nests, says a study in the May Insectes Sociaux.

Researchers from Argentina, Mexico and Peru tested how one species of leaf-cutting ants, Atta cephalotes, in Costa Rica deals with rain. The scientists placed hollow boxes filled with wet cotton on ant trails in the forest. When A. cephalotes walked through the boxes, they experienced higher relative humidity, as if it were about to rain. In another experiment, the researchers poured water on plants beside the trail to simulate falling raindrops. Both situations caused the ants to scramble to their nest up to 30 percent faster than normal, from about 1.21 meters per minute to 1.49.

The researchers think that leaf-cutter ants speed up to keep their cargo and themselves dry, with good reason: A wet leaf fragment weighs more than double a dry one. In the study, when the ants or leaves got wet, the insects readily dropped their harvest and returned to the nest. But by hurrying along at the first hints of rain, the ants could stay dry and hold onto the leaves.