Object Depots in the Genus Pogonomyrmex: Exploring the “Who,” What, When, and Where

Abstract

Harvester ants of the genus Pogonomyrmex collect and deposit many items on top of their nests. The depots of P. badius consist mostly of small charcoal fragments, while those of other species are primarily pebbles. Mature colonies can have hundreds of thousands of objects in their depot. In P. badius, the distributions of midden and charcoal about the mound are not completely overlapping, but are positively correlated in areas of overlap. Charcoal depots are isometric with colony size, but the amount of charcoal per colony size varies with season and site. The absence of the depot stimulates collection of nonfood objects. Recruitment to objects only occurs in the presence of food, and foragers choose objects based on size, but not color. An overview of current knowledge concerning depots in the genus indicates that depot formation is likely the ancestral state in the North American species. Furthermore, it is likely that selection is operating indirectly on these depots through selection on many dependent tasks.