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 CO2 concentration as an orientation cue when selecting a place to locate their fungus garden, and whether they show preferences for specific CO2 concentrations. We also evaluated whether levels preferred by workers for fungus-rearing differ from those selected for themselves. In the laboratory, CO2 preferences were assessed in binary choices between chambers with different CO2 concentrations, by quantifying number of workers in each chamber and amount of relocated fungus. Leaf-cutting ants used the CO2 concentration as a spatial cue when selecting places for fungus-rearing. A. lundii preferred intermediate CO2 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 CO2 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 CO2 levels, which were otherwise avoided. Workers’ CO2 preferences for themselves showed no clear-cut pattern. We suggest that workers avoid both atmospheric and high CO2 concentrations not because they are detrimental for themselves, but because of their consequences for the symbiotic partner. Whether the preferred CO2 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.
Highlights
Symbioses, through evolutionary processes, have shaped the biology of many organisms on this planet because they enable the associated organisms for instance to occupy new ecological niches, to gain access to alternative food sources or to attenuate environmental stress
We investigated whether leaf-cutting ants (A. lundii) use the CO2 levels inside the nest as an orientation cue for the selection of places to relocate their fungus, and quantified CO2 choices for fungus cultivation
Colonies of the leaf-cutting ant A. lundii inhabit shallow subterranean nests, located 30–50 cm underground [65] in heavy clayish soils, where the CO2 concentrations range from 1–3%
Summary
Through evolutionary processes, have shaped the biology of many organisms on this planet because they enable the associated organisms for instance to occupy new ecological niches, to gain access to alternative food sources or to attenuate environmental stress. The defense against environmental stressors, be they biotic or abiotic, can be one of the major benefits of a mutual symbiotic association. Leaf-cutting ants are a classical example of a successful symbiosis because of their association with a fungus, which rendered them the primary herbivores of the Neotropics [9, 10]. They forage large quantities of live plant material, on which they grow a basidiomycetic fungus as a food source to raise the colony’s brood [11]. Biotic and abiotic stressors continuously threaten this successful antfungus symbiosis
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