Liquid foraging behaviour in leafcutting ants: the lunchbox hypothesis

Abstract

Optimal foraging theory makes clear predictions about the benefits of maximizing energetic returns per unit of foraging effort. However, predictions become less clear when animals belong to symbioses that would be destabilized by such foraging decisions. For instance, leafcutter ants are dominant herbivores in Neotropical ecosystems that harvest fresh vegetation and convert it into compost used to cultivate specialized fungus for food. Individual foragers have long been assumed to supplement their fungal diets by harvesting liquid nectar outside the symbiosis, although this has not been demonstrated in the field, and would probably destabilize the fine-tuned farming systems. By dissecting liquid storage organs in foragers of four sympatric Panamanian leafcutter ant species we found that liquid foraging is not a general strategy in the field. Moreover, while over 40% of these foragers returned to their nests without leaf fragments, these unladen ants were not more likely to carry liquids. Instead, we found support for a newly formulated ‘lunchbox hypothesis’ because most workers exited nests for foraging trips with midguts full of liquids that were depleted (assimilated and transferred to hindguts) if workers returned with a leaf fragment in the field or transported a load in laboratory experiments. Thus, in contrast to the destabilizing effects of external nectar foraging, these results provide a novel mechanism promoting symbiotic stability, as fungi provide fuel for foragers to harvest more substrate for fungal crop production.