Available space, symbiotic fungus and colony brood influence excavation and lead to the adjustment of nest enlargement in leaf-cutting ants

Abstract

The size of underground ant nests positively correlates with their worker number, suggesting that during colony growth, workers respond to increased space demands by enlarging the existing nest. In leaf-cutting ants, the presence of brood and a growing symbiotic fungus are expected to generate spatial demands workers should respond to. We investigated to what extent the presence of in-nest stores, such as brood and fungus, and actual space availability influence the digging behavior of leaf-cutting ants and lead to the adjustment of nest enlargement. In the laboratory, we offered worker groups of Acromyrmex lundi a nest site consisting of either a tunnel, representing reduced available space, or a tunnel and a small chamber, representing ample available space as a starting point for nest enlargement. Workers could move and store offered brood and fungus into the nest structure, and enlarge it as needed. The presence of brood and fungus influenced digging activity as well as the architecture of the nest. In the presence of relocated brood, digging activity was higher in reduced space than in ample space, suggesting that high worker density stimulated excavation. When workers relocated fungus and brood into ample space, digging activity was similar as in their absence, yet a larger chamber and fewer tunnels were excavated. During the process of nest enlargement, workers were observed to initially excavate space in excess, which was refilled with part of the removed soil pellets, thus leading to a reduction of available space. Pellet deposition seemed to be opportunistic, with workers refilling unused space. Results indicate that the size of a leaf-cutting ant nest does not simply depend on the number of inhabiting workers. Rather, workers adjust the enlargement of their nest space depending on the current space available and the presence of in-nest stores.