Harvester ants of the genus Pogonomyrmex collect and deposit many items on top of their nests. Although these deposits, or depots, may be colossal there is very little known about them. This research is a case study of the depots in the Florida harvester ant, P. badius, describing the depot contents, distributions of different components within the depots, the ontogeny of the depots, a stimulus for their creation, testing hypotheses of their function, evaluating their effect on a measure of colony level fitness, and providing an overview of information on depots of other species throughout the genus.
The depots of P. badius consist mostly of small charcoal fragments, while those of other species are primarily pebbles. Mature colonies can have millions of objects in their depot, each of which is the result of a single foraging trip by a worker. 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, increasing proportionately with the number of mature workers in the nest. Moreover, the amount of charcoal per colony size varies with season (they are larger in fall compared to spring), and site. The absence of the depot stimulates collection of non-food objects. The depots do not appear to function as territorial markers, roofs (preventing chamber collapse), or mulch (preventing water evaporation). Depots had a moderate effect on temperature (2°C, but only in the top 2cm), but it is not known whether the ants exploit this temperature difference. And finally, the effect of charcoal on the seasonal fitness of colonies was not statistically significant, but colonies with charcoal depots did produce more sexuals and workers, indicating that the effect of charcoal on fitness may accumulate over multiple seasons.