How Ants Became the World’s Best Fungus Farmers

Ancient climate change may have spurred a revolution in ant agriculture, Smithsonian researchers find

Ingenious leafcutter ants have developed a successful symbiotic relationship with the fungi they farm. New genetic analysis helps pinpoint when, and why. (Giovanni Giuseppe Bellani / Alamy)
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When humans began farming some 12,000 years ago, they altered the future of our species forever. Our ancestors were ecological pioneers, discovering and cultivating the most valuable crops, scaling them up to feed entire communities and transforming wild crops so fundamentally that they became dependent on humans for their survival. Farming, in the words of National Geographic’s Genographic Project, “sowed the seeds for the modern age.”

But humans were late to the game. By the time our ancestors had launched their world-changing Neolithic Revolution, ants had already been farming fungi in South American rainforests for 60 million years. These wee agricultural wizards used sophisticated techniques that rival our own—including domesticaing crops that today are unknown in nature and are also unable to survive without their cultivators.

Now Smithsonian researchers have pinpointed when—and perhaps why—ants developed these remarkable farming techniques. By creating an evolutionary tree of fungus-farming ants, they report that the revolution in ant agriculture may have been spurred by a dramatic shift in climate some 30 million years ago. Moreover, the farming systems that emerged from that revolution may yet hold a few lessons for humans today.

Today, about 240 species of attine ants—the leafcutters among them—are known to farm fungus in the Americas and the Caribbean. Their underground crops fuel complex, agriculturally-based societies that are not only sustainable and efficient, but also resistant to diseases and pests. These diminutive farmers are united by a common strategy: They forage for bits of vegetation, but don't eat it. Instead, they use it to nourish their precious fungi, which they grow on an industrial scale.

In these cases, fungi are completely isolated in underground gardens, often located in dry, inhospitable habitats where their wild relatives can't survive. Nor can they escape, meaning wild and domestic fungi can’t get together and swap genes. As a result of this isolation, the domesticated fungi have evolved in complete codependency with their ant farmers. For their part, the ants rely so heavily on their crop that when a queen's daughter founds a new colony, she takes with her a piece of her mother's fungal garden to begin her own.

“The fungi that they grow are never found in the wild, they are now totally dependent on the ants,” explains entomologist Ted Schultz, curator of ants at the Smithsonian National Museum of Natural History. “That's like a lot of our crops. We cultivate things that are so highly modified that they exist in forms no longer found in the wild.”

In a study published April 12 in the journal Proceedings of the Royal Society B, Schultz and his colleagues used new genomic tools to uncover the roots of this unusual arrangement. Schultz and his team created an evolutionary family tree of fungus-farming ants, tapping stores of genetic data for 78 fungus-farming ant species and 41 species of non-farming ants. Most were collected by Schultz himself during decades in the field.

The researchers used this genetic record—which included the DNA sequences of over 1,500 genome sites for each species—to reason backwards from living species and identify the common ancestors of today's ant lineages. They substantiated this genetic data with a few key ant fossils, which were used to help calibrate dates for the changes they found in their DNA analysis.

With this data, Schultz was able to unravel when these ant species made the key evolutionary advance to more advanced agriculture—as well as come up with a theory for why.

Ted Schultz, curator of ants with the Smithsonian's National Museum of Natural History, holds a lab nest of a lower fungus-growing ant while standing next to a lab nest of higher fungus-growing leaf-cutting ant. (JamesDiLoreto / Smithsonian)

The DNA data suggests that this leap coincided with dramatic changes in ancient climate. Ants appear to have developed their advanced farming systems sometime after a global cooling event began lowering temperatures worldwide around 35 million years ago. The resulting shift from the wet rain forests of their hunter-gatherer ancestors to dryer environments, the researchers write, may have sparked agricultural innovation as ants maintained the controlled conditions to keep their fungal gardens growing. 

“It looks like whatever the ancestor of the higher, fungus-growing ants was it was living in a dry or a seasonally dry habitat,” Schultz says. “So if ants are growing wet habitat-loving fungi, and remove them to a dry habitat, that's sort of like humans taking one of their domesticates out of its native range.”

“Generally when we domesticate things we isolate them in plots and harvest the seeds from the ones we like the best, and keep planting those seeds,” he continues. “If you have a fungus whose relatives all live in a wet forest, and you take it to a dry habitat, it can't escape anymore ... Over time, isolated for hundreds of thousands or millions of years, that's a pretty good opportunity for domestication.”

But fungi weren't the only ones going through a transformation. At the same time, the ant lineage began to diversify significantly. Their genomes shifted during the move from hunting-gathering to agriculture, and again when higher forms of fungi farming were adopted. Schultz and colleagues noted in previous research that ants likely lost the ability to make a key amino acid, arginine, because they had a ready source in the fungi—and have now become dependent on that source.

Mycologists studying the same system may well view it as one in which the fungi used the ants, rather than the other way round. “It may sound kind of bad for the fungi but it's to their benefit as well. All their needs are being tended to,” says Diana Six, a University of Montana entomologist. “I think the fungi really do manipulate the situation as well.”

Six, who wasn't involved in the study, adds that Schultz and colleagues were able to tease apart a complex evolutionary story that didn't support many previous assumptions—namely, that the evolution of moisture-loving fungi would have been driven by pressures in moist rainforests where they lived.“The idea that with these symbioses there has to be something that enforces that specificity, and that isolation has led to this extreme dependence … It really makes a lot of sense,” says Six. “But it takes people to think a little outside the box to find those kinds of answers.” 

Advanced ant agriculture, as you might expect, differs from human efforts in a few obvious ways (fewer tractors, for one). Yet Schultz believes that we can learn a thing or two from observing how one of nature's few other farming species—including termites, beetles and bees—curate their crops.

For instance: Like some industrial farmers, fungus-farming ants grow a single type of crop. However, they manage to do so without succumbing to foes like disease or pests that threaten human crops when they lose genetic diversity. Ants achieve this remarkable feat by keeping their underground garden rooms spotless to limit the possibility of disease, and by producing a sort of natural antibiotic that acts as a pesticide, battling a parasitic fungus that threatens their food source.

These strategies effectively keep pathogens in check but don’t obliterate them as humans tend to do, sometimes without meaning to. Instead, ants have achieved a sustainable balance that humans would do well to observe, says Schultz.

“They grow a monoculture, but there's all kinds of bacteria and other microbes that might be benign or even beneficial,” Schultz says. “It's like a little ecosystem that they are cultivating.” Similarly, in human farming, “when we grow a crop we're not just growing something like corn,” he adds. “We're also growing all these microbes in the soil, and there's probably an optimal ecological blend of microbes that's the best for healthy soil and healthy corn.”

The ant colony's place in the larger local ecosystem may also hold a few lessons for human farmers, Schultz notes. Think of a leafcutter colony as a single large grazing vertebrate: A colony's combined weight is similar to that of say a cow, and it can consume similar amounts of local vegetation over a similar period of time.

“Why don't they just wipe out all the vegetation in an area and have to move?” he asks. One reason is that local vegetation has also evolved in sync with the colonies. A tree that's being grazed to death by ants may begin to express a toxin that makes its leaves unpalatable to the ants' fungi, causing them to move on so that the tree can regenerate.

“They're not doing it deliberately; it's not like they are consciously choosing not to decimate a tree," he adds. "But an entire local ecosystem and all the organisms in it have co-evolved into a sort of stable state, which produces this kind of sustainable agriculture." From the smallest among us, it seems, larger lessons can emerge.

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