Together with human beings, leaf-cutter ants form the most complex animal societies on planet Earth. Within a few years the central mound in their underground nests can grow more than thirty metres in width, with smaller mounds around it which extend out in an eighty metre radius, occupying anything from thirty to six hundred square metres, and hosting eight million ants.
Their societies are based on an ant-fungus symbiosis and different species use very similar species of fungus (perhaps one sole species, Leucocoprinus gongylophorus), but all the fungi used by the ants are members of the Agaricaceae family, previously Lepiotaceae. The ants actively cultivate their fungi, feeding them with freshly cut plant matter (generally leaves and flowers) and keep them free of infestations and mould. This symbiotic relationship appears to be even more complex due to the apparently symbiotic participation of bacteria that grows on the ants and secretes chemical products – essentially the ants are portable antimicrobial agents. However, this bacteria-fungi association does not seem to be specific. Leaf-cutter ants are sufficiently sensitive to recognise the reaction of the fungi to different plant matter, apparently detecting the chemical signals emitted by the fungi. If a specific species of plant is toxic for the colony’s fungus, it stops collecting it. The fungus cultivated by the adults is used to feed the larvae and adult ants. The fungus needs the ants to remain alive, and the ants need the fungus to survive. The only other two groups of insects to have developed fungus-based agriculture are the Ambrosia Beetle and Termites.
These ants form a disciplined army consisting of thousands of workers, capable of transporting through the jungle and on their heads pieces of leaves with which to build the fungus that serves as their home and shelter for their queen.