Fungus garden structure in the leaf-cutting ant Atta sexdens (Formicidae, attini)

Leaf-cutting ants have a mutualistic relationship with a fungus, which they cultivate on fresh plant material. For optimum efficiency, this 'fungus garden' must have a structure that combines a large area for the production of ant rewards ('staphylae'), with the smallest chamber volume in which it can be maintained, and with accessibility for workers. We investigated the structure of a fungus garden of Atta sexdens (L.) by sectioning. It contained many small cavities, most of which (74.7%) were only accessible to small 'minima' workers, excluding larger sizes. These cavities provided a large internal surface area, 74% of the total surface area of the garden examined. Internal surfaces had more staphylae per unit surface area than external surfaces, suggesting a heavy harvesting pressure from large workers on the latter. The problem of producing a garden structure capable of yielding large crops of staphylae may have been important in the evolution of the characteristically small minima workers, which have access to the smallest cavities. We also examined staphyla production in fungus gardens. Numbers of staphylae present increased with garden age, but few were lost with discarded substrate. This suggests that workers remove all staphylae before removing substrate, or that the oldest garden produces few staphylae anyway.