The effect of transporting hitchhikers on the stability of loaded leaf-cutting ants

Abstract

Since leaf-cutting ants’ food sources are often far from their nests, efficient leaf transport is key to improving the colony’s food intake. We examined whether the presence of hitchhikers on the carried load, which has a known positive effect by protecting foragers against parasitoids and cleaning the leaves, also has potential negative effects by adding extra weight and causing imbalance for the carrier. We tested this idea in two nests of the leaf-cutting ant Atta laevigata in an Amazonian Forest in northern Brazil. In a 50-cm long section of the main ant trails, we selected 184 foragers returning to their nests with loads of similar sizes carrying a variable number of hitchhikers. Within this section, we counted the number of hitchhikers on each side of the carried leaf and recorded how many times the carrying worker lost stability. We calculated an imbalance index asthe difference in the number of hitchhikers on both sides of the carried leaf. The number of times a forager dropped its load increased with the number of hitchhikers, but it did not show a consistent change with variations in the imbalance index. The presence of hitchhikers may help explain why larger ants carry leaf fragments below their maximum load capacity. Our results also suggest that transporting hitchhikers above a certain threshold adds extra weight that can cause load drops andforaging delays. However, in the field, the number of hitchhikers often fell below this threshold, allowing them to perform their beneficial roles without significantly adding extra weight. This illustrates how natural selection may influence foraging behaviors to maximize benefits while minimizing costs.