By: Mike Boston
Mike is an honor’s biology graduate of the University of Portsmouth and is the owner / operator of Osa Aventura, the premier guiding outfitter for Corcovado National Park, and has led groups into the Park for the past eighteen years. Contact him directly at firstname.lastname@example.org.
When I’m guiding almost invariably I come upon columns of ants marching along immaculate trails carrying fragments of leaves above their heads. I explain to whom I am guiding that these are leafcutter ants, known locally as zompopas, and are among natures’ first farmers. Indeed, they beat us human beings to the invention of farming by 50 million years!
Next to humans, leafcutter ants form the largest and most complex animal societies on Earth. And, like humans, these ants support the immensity and complexity of their societies on the high energy-yielding, intense cultivation of another living species – farming. But as we humans have discovered over millennia, farming is energy intensive and brings with it a slew of problems, most notably disease, pests, parasitism and predation. Over eons, leafcutter ants have solved these problems in ingenious ways.
In huge, multi-chambered, underground nests, and with colonies of up to 8 million individuals, leafcutter ants cultivate a fungus upon which they feed. In each of the many chambers worker ants prepare leaf cuttings, which other workers have harvested from surrounding vegetation, and intensively propagate their symbiotic fungus. It is a complex process requiring many tasks to be carried out simultaneously.
In essence, an ant colony is a matriarchal monarchy where a fertile female (the queen) reigns over her daughters (sterile females called workers): male ants (drones) have only a transient role in reproduction. About once a year the queen will produce winged males (from sterile eggs) and winged females (future queens), which leave their nests on a nuptial flight, intermingle and mate. The males then die. The newly fertilized queen casts her wings, begins constructing her own nest, and starts a fungal garden from fungal spores taken from her parental nest and producing daughters (workers) from fertilized eggs. In her 14 to 20 year lifespan the queen will produce up to 150 million offspring, the vast majority being workers.
Worker leafcutter ants come in three types or castes and perform the myriad tasks of a functioning colony: soldiers, the largest caste, whose main duty is to defend the nest against predators but also participate in trail clearance; the media, the medium-sized workers, perform a variety of heavy-duty tasks from nest excavation and maintenance to leaf cutting and carrying, refuse disposal and undertaking dead ants from the nest; the smallest caste, the minima, perform the remaining tasks, which include feeding the larvae and the queen, tending to the fungal gardens and preparing the incoming leaves for culturing the fungus, trail clearing and maintenance, cleaning incoming leaf cuttings of contaminating microbes and acting as decoys to parasitic phorid flies.
Because farming involves the concentrated propagation of another species for food, major problems arise from pathogens infesting and contaminating that food supply. The fungal gardens that leafcutter ants cultivate eventually become over burdened with pathogens to the point where their food fungus can no longer propagate. At this point a fungal garden has to be abandoned. Now, if leafcutter ants put all of their eggs in one basket, so to speak, this would spell disaster to the colony. A colony of leafcutter ants, therefore, will continually create new fungal gardens as they abandon old ones. But, it is important that the old fungal gardens do not contaminate the new gardens with their pathogens. Most of the 47 known species of leafcutter ants are thought to isolate their old, contaminated gardens underground. Atta colombica, however, dumps its fungal refuse above ground allowing us to observe the extent to which the ants partition labour to avoid contaminating incoming leaf cuttings from old garden refuse.
As a rule, within the worker casts of all ants, the oldest workers do the most dangerous tasks, and so it is with the media caste of leafcutter ants: as media age they switch from being foragers to waste disposers. Once engaged in waste disposal older media work in strict isolation from the rest of the ant colony to avoid contamination of the incoming leaf cuttings. Atta colombica waste dumps are made some distance from the nest at the opposite direction to the incoming leaves. An aside: Material from the Atta colombica waste dumps is not only an excellent fertilizer for garden plants, but acts as a repellent to foraging leafcutter ants that might attach those plants.
A particularly virulent pathogen to the lepiotician fungus the ants’ culture is another fungus, Escovopsis, which is the main culprit in the decline of a fungus garden. However, it and other harmful pathogens can enter the ant colony on the surfaces of the leaf cuttings foragers bring to the nest. A noticeable feature of the marching workers and their leafy burdens is minima riding piggyback on many of the leaves. It is thought that these minima are cleaning the leaf-cuttings of microbial contaminants, including Escovopsis. However, piggybacking is a perilous task as this exposes the minima to attack by parasitic, phorid flies. This may, in fact, be a secondary benefit to the functioning of the ant colony: it is better that the phorid flies target the piggybackers than the ants carrying the leaves – perhaps they are sacrificial lambs! The piggybackers are older minima that once tended the fungal gardens and larvae.
Avoiding contamination is one method leafcutter ants have evolved to protect their fungal gardens. Another method they use is biological control. On the underbellies of worker leafcutter ants are white powdery-looking colonies of bacteria that secrete powerful fungicides which destroy Ecovopsis and other fungal pathogens, but is harmless to the fungus the ants cultivate. A second microbe, black yeast, has recently been discovered on the bodies of leafcutter ants, which is thought to play a role similar to the bacterium in protecting the ants’ fungus and, perhaps, the ants themselves. Clearly, we are only scratching the surface of the complex, symbiotic relationships leafcutter ants have with microbes to maintain the health of their colonies and the food fungus upon which they depend.
Leafcutter ants are endemic to the tropical Americas where they are the dominant herbivores. These ants cycle huge amounts of vegetable matter through the Neotropical ecosystems and their symbiotic fungus fixes atmospheric nitrogen, a nutrient vital to the health of these systems. The complex societal structure of the leafcutter ants’ multi-million member colonies share many features in common with human cities. But for farming and its high energy yield, neither ants nor humans would be able to support their dense and complex societies. It is a little ironic, however, when leafcutter ants go to such lengths to protect their mushroom farms, that they are a major agricultural pest to us humans in this part of the world.