Carbon dioxide levels and ventilation in Acromyrmex nests: significance and evolution of architectural innovations in leaf-cutting ants.

Martin Bollazzi,Daniela Römer,Flavio Roces

doi:10.1098/rsos.210907

Abstract

Leaf-cutting ant colonies largely differ in size, yet all consume O2 and produce CO2 in large amounts because of their underground fungus gardens. We have shown that in the Acromyrmex genus, three basic nest morphologies occur, and investigated the effects of architectural innovations on nest ventilation. We recognized (i) serial nests, similar to the ancestral type of the sister genus Trachymyrmex, with chambers excavated along a vertical tunnel connecting to the outside via a single opening, (ii) shallow nests, with one/few chambers extending shallowly with multiple connections to the outside, and (iii) thatched nests, with an above-ground fungus garden covered with plant material. Ventilation in shallow and thatched nests, but not in serial nests, occurred via wind-induced flows and thermal convection. CO2 concentrations were below the values known to affect the respiration of the symbiotic fungus, indicating that shallow and thatched nests are not constrained by harmful CO2 levels. Serial nests may be constrained depending on the soil CO2 levels. We suggest that in Acromyrmex, selective pressures acting on temperature and humidity control led to nesting habits closer to or above the soil surface and to the evolution of architectural innovations that improved gas exchanges.

Highlights

Ant nests offer protection against predators and climatic extremes, yet they can compromise the air exchanges between the nest royalsocietypublishing.org/journal/rsos R
When CO2 levels in the soil are lower than inside chambers housing fungus gardens, CO2 moves into the soil phase, yet diffusion would tend to be reversed at increasing nest depths, since levels of CO2 in the soil phase increase with depth [2]
Our experimental results showed that levels of CO2 inside fungus gardens of Acromyrmex nests are strongly dependent on their architecture, the nest connectivity to the outside, and the presence of a thatched mound, which can be regarded as the key architectural innovation that enabled improved gas exchanges by taking advantage of different ventilation mechanisms

Summary

Introduction

Ant nests offer protection against predators and climatic extremes, yet they can compromise the air exchanges between the nest royalsocietypublishing.org/journal/rsos R. All chambers of leaf-cutting ant nests exchange respiratory gases with the soil phase via diffusive flows, which strongly depend on the porosity of the soil. The direction of such an exchange would depend on the concentration of CO2 in the soil relative to the concentration inside the chambers. It results in a dynamic equilibrium that equals the concentrations of CO2 and O2 inside a fungus chamber with those of the local surrounding soil, as far as no other ventilatory mechanisms driving the exchange of air between fungus chambers and the atmosphere are involved [3]. It is known that external architectural features, such as the mound shape or the construction of nest turrets might promote gas exchanges through passive, windinduced nest ventilation mechanisms [3,8,9,10]

Methods

Results

Conclusion