Growth Dynamics of Symbiotic Fungus and How it is Affected By Proportion of Minor Workers (Hymenoptera: Formicidae: Atta)

Leaf-cutting ants cut and load leaves into their nests to grow the obligate symbiotic fungus, the ants’ main food source and shelter for their offspring. Atta, one of the four extant genera of leaf-cutting ants, occurs in most of Brazil. The relationship of these ants with their fungal cultivars represents one of the most successful symbiosis. Cultivating symbiotic fungi on fresh vegetation depend on specific allocation and division of tasks among individuals. This study aimed to understand the growth dynamics of a colony of Atta sexdens, based on the relationship between the growth of the symbiote fungus and the proportion of the minor worker subgroup. The dynamics of fungus growth with 100%, 50%, or 10% of the normal numbers of minor workers present were studied. The behavior of individuals in the foraging chamber and waste disposal was evaluated. Fungal growth in the three treatments fluctuated over time. However, the final fungus-garden volume was similar in all three treatments, likely indicating that homeostatic mechanisms act on the colony growth in response to a loss of workers. We also found variations in behavior and changes in the degree of aggregation of the remaining individuals in response to worker loss. Waste disposal did not differ significantly among the treatments. This investigation not only emphasized the ecological-behavioral importance of the minor workers, primarily involved in internal activities in the nest, but highlighted aspects related to high resilience, nest homeostasis, and growth dynamics of young colonies of leaf-cutting ants.