The cultivation of fungus by ants is thought to have begun around 50 million years ago, when ancestors of the modern Attini ant tribe dropped the hunter-gatherer lifestyle in favor of becoming farmers. Today, every one of the 200+ species of Attini cultivates some species of fungus, but the most impressive and large-scale farming operations are carried out by the Atta and Acromyrmex genera – the leafcutter ants. These particular ants transitioned from small subsistence farms to large-scale agricultural enterprises about 20 million years ago, thanks to the process of coevolution.
Coevolution is the term used to describe two (or more) species reciprocally affecting one another’s evolutionary strategy, and tends to occur when these species interact closely over long periods of time. The most well-known example of coevolution is driven by predator-prey interaction: a plant develops spikes to deter an herbivore, which in turn grows a pointier snout to get around those spikes, which then grow closer together. This kind of “arms race” is frequently cited to support the view of evolution as a perpetual competition – a “survival of the fittest” power struggle. However, competition is just one piece of the evolutionary puzzle. No complex system can function without the benefits of mutualistic coevolution between species, and leafcutter ants as we know them would not exist without it.
For these ants, the mutualistic leap that changed their agricultural game (and eventually their very biology) occurred 30 million years into the small-scale farming experiment. At this point, one particular lineage of cultivated fungi (Leucoagaricus gongylophorus) evolved gongylidia: nutrient-filled growths occurring on the tips of cultivated fungus. These gongylidia served as specialized food packages for the ants, allowing the colony to sustain a much larger population. They do not exist in any uncultivated fungal species, and would be functionally useless outside an ant colony. Within an ant colony, however, they provide a terrific boost to the fungi’s caretakers, leading to a larger fungal crop and wider distribution.
Developments in ant agriculture accelerated once gongylidia began to appear. After 10 million years of nutrient packets, the farmers had their second breakthrough. The ants switched from gathering low-nutrient detritus for their farms to living plant material. These leaves, previously impossible for ants to consume due to their toxins, could now be neutralized by enzymes produced by the fungus. And they were much higher in nutrients. Today, the largest farms are over 100,000 times bigger than unspecialized small-scale ventures, with each individual colony containing up to eight million ants. Leafcutter ants consume 17-20% of all leaf litter in the Neotropics, and are considered the ecosystem’s dominant herbivore despite not directly consuming any of the material themselves.
The magnitude of these mutualistic farms do come at a price. Both species are now obligate symbionts, meaning one cannot exist without the other. The ants themselves have actually lost an entire amino acid synthesis chain, outsourcing this task to the fungus. However, both sides do their part to make sure that the other is properly taken care of. The fungi gongylidia, in addition to providing ants with concentrated lipids and carbohydrates, produce enzymes that help ants break down the leaves that nourish fungal gardens. The ants provide the fertilizer that the fungi need as well as bacteria that protect them from disease.
Ants and fungi have become so intertwined that the whole enterprise might be described as a single immense superorganism, with each individual ant, fungal crop, and bacterial colony forming one part of a cohesive, mutually-beneficial whole. Mutual coevolution is a powerful resource in nature, which, when harnessed, can make all the components greater than the sum of their parts.
References
Henrik, H., Jacobus J. Boomsma, and Anders Tunlid. “Symbiotic adaptations in the fungal cultivar of leaf-cutting ants.” Nature communications 5 (2014).
Mueller, Ulrich G., et al. “The origin of the attine ant-fungus mutualism.” Quarterly Review of Biology (2001): 169-197.
Schultz, Ted R., and Seán G. Brady. “Major evolutionary transitions in ant agriculture.” Proceedings of the National Academy of Sciences105.14 (2008): 5435-5440.
