Fungus Farming Ants

By Jude Coleman

When you think of agriculture, you probably don’t think of ants. But think again: the tiny insects have been farming for tens of millions of years, far longer than humans. And their crop of choice is mushrooms. 

Fungus-farming ants first began their agricultural venture when an asteroid struck the Earth around 66 million years ago, according to new research published October 3 in the journal Science. Researchers used genetic data from ants and fungi to look back in time and see when the two organisms became intertwined.  

“We often think of humans as being the only things that have domesticated their food, but that’s not the case,” said Bryn Dentinger, the Curator of Mycology at the Natural History Museum of Utah and an associate professor in the University of Utah’s Department of Biological Sciences. Dentinger provided many of the present-day fungi samples for the work. 

A lower-fungus-farming worker of the rare fungus-farming ant species Mycetophylax asper, collected in Santa Catarina, Brazil, in 2014, on its fungus garden. Photo by Don Parsons

The fungi that prehistoric ants farmed were ancestors of the mushrooms on Earth today. But their farming methods would have been essentially the same. First, ants bring organic matter back to their nest to feed the fungus. Leaf-cutter ants, for example, chomp off pieces of leaves and other plants for their fungi, while others bring back bits of decomposing organic material. The ants lack the digestive hardware to eat these offerings themselves. Instead, the fungus digests the plants, and the ants feast on the fungus. Fungus farms are so important to the ants that queens will take a little portion of fungus with them if they ever relocate, so that they can start a new crop. 

“Basically what's happened is the ants have subcontracted their digestion to the fungus,” said Dentinger. “The fungus is essentially their sole source of nutrition.” 

Scientists first discovered that ants were intentionally farming fungi 150 years ago. In 1874, English naturalist Thomas Belt reported that he found mushroom-growing and -eating ants in Central America. They were leaf-cutter ants, and prior to Belt’s discovery, scientists thought the ants were eating the leaves themselves. Now, scientists have found over 200 species of ants that farm fungi, which plow their tiny fields around the Americas and in the Caribbean. 

A coral-fungus-farming worker of the fungus-farming ant species Apterostigma collare, collected at La Selva Biological Station in Costa Rica in 2015, on its fungus garden. Photo by Alex Wild

To explore the history of that ant-fungus relationship, the team compiled hundreds of ant and fungus genomes — the genetic makeup of living things. They looked for changes and similarities between the genomes, combing through them to find relationships between the species and creating an evolutionary tree for ants and fungi. By comparing the genomes of cultivated fungi and wild fungi, the researchers could see when ants started farming, and what groups they selected to cultivate. 

Dentinger and colleagues' work corroborates previous studies that found ants adopted fungi as a food source four separate times throughout history. The work is more robust than previous datasets, allowing the team to narrow down the timeframe that fungus-farming started: roughly 66 million years ago. Researchers refer to this time as the K-Pg or Cretaceous–Paleogene boundary. An asteroid hurtled into the Earth at that time, sending massive amounts of debris into the atmosphere. That caused a sharp decline in sunlight, killing plants and leading to mass extinctions of animals such as dinosaurs. Organisms that didn’t need sunlight suddenly found a foothold.

“The things that could survive that were things that didn't rely on photosynthesis,” said Dentinger. “Those would be smaller organisms, things that might live underground, things like fungi and insects.”

After around 40 million years of ants farming mushrooms, one ancient fungus lineage became domesticated. That means it was reliant on the ants for food and survival. Not only are fungi dependent on their ant stewards, but now, the ants also can’t survive without the fungi. Their bodies have lost the ability to make their own arginine, a vital amino acid. Instead, they have to get it from their fungal crops. 

The relationship between ants and fungi is an iconic example of mutualistic symbiosis, said Dentinger, which is when two organisms mutually benefit from working together. By teasing apart the origins of their relationship, it sets the stage for understanding how that symbiosis can begin and evolve. It also mirrors how humans domesticated their crops — repeated selection and cultivation of an organism — but represents a different, independent origin of agriculture. 

One benefit of being able to study domestication and agriculture in ants is that researchers can broadly compare the process to that in our own history. In the case of fungi, scientists can compare ant-cultivated fungi with their wild relatives to look for genetic changes. That can show them what genes might have been required for or acquired by the domestication process. 

“That's something we're still trying to understand, even in our own crop plants,” Dentinger said. “Maybe there is some kind of universal set of changes required for domestication of anything.”

Fascinated by Fungi?

Researchers estimate that there are millions of species of Fungi worldwide (estimates vary between 2.2 to 12 million), but only 5-8% have been documented. With such an abundance, scientists don't have to go far to find new species to study. In fact, Dentinger and his students found plenty of interesting (and potentially new) specimens to examine by simply strolling through the supermarket. You can hear Dentinger's perspective on that study here.