Temporary prey storage along swarm columns of army ants: an adaptive strategy for successful raiding?

Abstract

While pillaging brood of other social insects, Eciton army ants often accumulate prey in piles (or caches) along their foraging trails. Descriptions scattered throughout the past 100 years link this behavior to foraging-related migration. However, no empirical work has yet investigated its adaptive value. Here we asked whether caches facilitate prey flow from foraging fronts to temporary nests (or bivouacs) in the hook-jawed army ant, Eciton hamatum. We counted workers arriving at caches with prey from foraging fronts and departing caches towards the bivouac, quantifying their prey loads. While more workers carrying single-item prey loads arrived at rather than left caches towards the bivouac, ants carrying multiple-item prey loads arrived at and departed at the same rate. This probably resulted from raiders depositing prey in safe locations and rapidly returning to the foraging front, while other workers safely transported prey to the bivouac in multiple-item loads. This cache-mediated traffic partitioning probably allows maximizing the prey collection rate, and may be a counter-adaptation to the strategies prey colonies deploy to defend their brood from army ants.