Carbon dioxide sensing in an obligate insect-fungus symbiosis: CO$_{2}$ preferences of leaf-cutting ants to rear their mutualistic fungus

Abstract

Defense against biotic or abiotic stresses is one of the benefits of living in symbiosis. Leaf-cutting ants, which live in an obligate mutualism with a fungus, attenuate thermal and desiccation stress of their partner through behavioral responses, by choosing suitable places for fungus-rearing across the soil profile. The underground environment also presents hypoxic (low oxygen) and hypercapnic (high carbon dioxide) conditions, which can negatively influence the symbiont. Here, we investigated whether workers of the leaf-cutting ant Acromyrmex lundii use the CO$_{2}$ concentration as an orientation cue when selecting a place to locate their fungus garden, and whether they show preferences for specific CO$_{2}$ concentrations. We also evaluated whether levels preferred by workers for fungus-rearing differ from those selected for themselves. In the laboratory, CO$_{2}$ preferences were assessed in binary choices between chambers with different CO$_{2}$ concentrations, by quantifying number of workers in each chamber and amount of relocated fungus. Leaf-cutting ants used the CO$_{2}$ concentration as a spatial cue when selecting places for fungus-rearing. A. lundii preferred intermediate CO$_{2}$ levels, between 1 and 3%, as they would encounter at soil depths where their nest chambers are located. In addition, workers avoided both atmospheric and high CO$_{2}$ levels as they would occur outside the nest and at deeper soil layers, respectively. In order to prevent fungus desiccation, however, workers relocated fungus to high CO$_{2}$ levels, which were otherwise avoided. Workers’ CO$_{2}$ preferences for themselves showed no clear-cut pattern. We suggest that workers avoid both atmospheric and high CO$_{2}$ concentrations not because they are detrimental for themselves, but because of their consequences for the symbiotic partner. Whether the preferred CO$_{2}$ concentrations are beneficial for symbiont growth remains to be investigated, as well as whether the observed preferences for fungus-rearing influences the ants’ decisions where to excavate new chambers across the soil profile