Leafcutter ants. It gets complicated. [Image Credit: Alex Wild]
The only way into Soberanía National Park is a long, crumbling road from Gamboa, Panama, a town originally built to accommodate workers dredging the Panama Canal. Park visitors must cross a single-lane bridge; when there’s congestion, the cars slow to a crawl, like a trail of marching ants.
The dense vegetation in Soberanía houses massive armies of actual ants: leafcutters. Their activity and close association with other species attracts curious minds like Jarrod Scott, who still vividly remembers visiting the park in 2003 as a young research technician at the University of Texas. That December, at the tail end of the rainy season, he spent his days toiling on slopes slick with mud and rain, shovel in hand, covered in ants. “All of these colonies have a soldier caste with large heads meant for one thing, which is biting,” says Scott. “We were trying to dig these colonies, and getting ravaged by thousands of these ants crawling on us.”
Scott, now a microbial ecologist at the Bigelow Institute for Ocean Sciences in Maine, was collecting just a few samples — a few spoonfuls — of rich biological material from subterranean chambers within the nests of Atta cephalotes leafcutters. It’s the same kind of material — a thronging mixture of leaves and fungi and microbes — that today is yielding surprising insights for researchers working to grow hardier crops, discover new antibiotics and make biofuel production more efficient. At a deeper level, meanwhile, scientists are developing a different way to think about ecology as they decode the ants’ intricate interactions with the fungi and bacteria they “garden” in tiny chambers within their nests.
These relationships give the ants a “wow factor” for many researchers. “There’s just something about the leafcutter ant system that’s really awesome,” says Lily Khadempour, a doctoral student at the University of Wisconsin who studies leafcutter ants that harvest grasses.
The traditional view has been that the leafcutters are agricultural masters of their microbial realm, cleverly cultivating their own supply of fungal food in those buried chambers, and even using bacteria to kill off fungal species that aren’t as appetizing. A new view, distinct yet not necessarily exclusive of the old one, suggests that the ants aren’t necessarily running the show, and instead are just one part of a complex system in which all of the components — from the ants all the way down to the smallest microbes — are effectively functioning as a single organism.
“When you look at a model system like these ants, it becomes clear how fundamentally co-evolved and how fundamentally dependent these organisms are on each other,” says Frank Aylward, an ecologist at the University of Hawai’i at Manoa who studied leafcutters as a doctoral student.
Leafcutters have been an object of scientific fascination since at least the 1870s. The English naturalist Thomas Belt commented on the mystery of the ants’ leaf-gathering: “I believe the real use they make of them is as a manure, on which grows a minute species of fungus, on which they feed;—that they are, in reality, mushroom growers and eaters.”
He was right. Not only are leafcutter ants growers and eaters of fungi, but they are sustained almost entirely by the fruits of their labor: nutrient-rich fungal swellings. Along with a few other closely related fungus-eaters, they form a group known as the attine ants.
Often described as farmers, attine workers forage for plant material, cutting leaves and bringing them back to the special fungus-growing chambers in their burrowed colonies. Inside the chambers, fungi break down the leaves with enzymes, producing food for the ants from otherwise indigestible plant clippings. The ants tend to these fungal chambers, also known as gardens, weeding out pests, and moving older, expended leaf material from the bottom of the garden to refuse sites.
Many of the attine ants cultivate a fungus of the genus Leucoagaricus in their gardens. If these fungi are the bountiful harvest, then Escovopsis fungi are the weeds. And Pseudonocardia bacteria, found on the bodies of many of the attines, is a sort of “weedkiller” that eliminates unwanted fungus.
Biotech companies are already borrowing from some leafcutter behaviors. For example, when creating a new garden chamber, the ants tend to begin with fungi transported from a successful garden. But the fungus itself is genetically identical in every leafcutter garden; what makes some chamber gardens more bountiful than others is the particular mixture of accompanying microbes.
Human farmers can do the same thing, by screening crops and transferring the most successful microbial communities to new seeds. A single strain of tomatoes, for example, could be grown with increasingly beneficial microbes over the course of generations. “I didn’t invent this,” says Ulrich Mueller, a professor of integrative biology at the University of Texas at Austin. “I observed the ants. That’s what they do.”
The concept was patented independently of Mueller’s research in 2014 by a biotech company. And while Mueller doubts its practicality for large-scale farming outdoors, “it may be doable in greenhouse agriculture of some specific crops, perhaps also in subsistence farming,” he wrote in an email.
Another area where scientists are looking to leafcutter ants for solutions to human problems is the search for new antibiotics. Some researchers think the antibiotic arsenal of microbes in attine systems can be harnessed to kill bacteria that cause human diseases too. It’s an urgent need: In the United States alone, the Centers for Disease Control and Prevention report 23,000 deaths associated with drug-resistant infections annually. “Resistance rates continue to rise and the antibiotic pipeline is inadequate to meet our needs,” says Brad Spellberg, an immunology and antibiotics researcher at the Keck School of Medicine at the University of Southern California. “The promise of the leafcutter ant approach is to use nature itself to guide us to entirely new classes of antibiotics.”
Useful molecules aren’t limited to antibiotics, though. Leafcutter fungal gardens harbor enzymes that break down plant material, that perhaps could be harnessed for some other purpose. For instance, it’s long been a goal to convert the inedible parts of crops, like corn stalks and leaves, into alternative fuels like ethanol. The challenge is to develop a cost-effective process for large-scale bioethanol production — perhaps by using the enzymes in leafcutter food chambers.
Researchers already know that enzymes in the chambers break down food waste, but the process isn’t perfect. There’s still plenty of waste leftover that has to be dumped outside. “A dump pile is just a big cow patty,” says Maine’s Scott, “All it is, is undigested material.” Still, an attine enzyme could one day be a key step in an elaborate reaction that produces affordable biofuel from plant matter.
The extremely close, interdependent relationship between the ants and the cultivated fungus raises philosophical questions. “Who cultivates who?” asks Scott, “Who is truly in command of the system?” Mueller, the Texas biologist, compares the question to the human domestication of cats. Or the feline domestication of people.
Perhaps the answer is that neither the gardeners nor the garden are in control. Khadempour uses a metaphor of the entire attine system as a single animal. She notes a similar relationship between microbes in other animals, including cows and humans. “All these different roles are split up within the colony,” she says, “and the fungal garden is serving as the gut of the organism.”
Whether or not leafcutter garden samples like the ones Scott collected in Panama end up contributing to the next big idea in ecology, or the next breakthrough in antibiotics, crop science or biofuels, Scott looks back fondly at the sometimes arduous fieldwork that risked tropical disease, venomous snakes and biting ants. “For me it’s entirely worth it,” he says, “that’s what I love most about being a biologist.”