I have been writing about bats for a while and I had also stopped reading the book on ants for a week. I picked it up again this week and started reading the Economics of Ant Society, and came across a fascinating phenomenon in the very first section: ants that grow plants for food.

They started doing it about 50 million years before humans, and it is not a trivial phenomenon either. As the "input" for their agricultural system, a group of these ants use up about 15% of the total leaves produced in the tropical forests in which they live.

So this post is about a group of more than 200 species of ants that cultivate fungus in their nests and depend on these crops for food. These ants are collectively called “the attines”, and they live in the rainforests of South America. Among the attines are two different types of ants: the “lower” attines cultivate the fungus in decayed matter in the nest and the “higher” attines collect leaves, flowers, and grass that they bring back to the nest and use as substrate for cultivating the fungus, which is then harvested for food. The “higher” attines are also known as “leafcutter ants.”

Leafcutter ants are seen here transporting pieces of leaves to their nests. They do not eat the leaves, but use them as substrate for growing fungus, which is their food.
Image created by Bandwagonman; accessed on Wikimedia Commons.

Green house inside the ant nest

An interesting fact about the fungus cultivated by the higher attines is that it cannot survive outside the ant nest. So the ants make sure that a comfortable garden is maintained inside the nest. The fungus lives inside a climate-controlled chamber that is kept dry since it cannot live in the wet climate of the tropical rainforest. In a way, ants maintain a greenhouse that human beings use to regulate climate for their own plants. This is done if we are in a cold place and need to grow crops that grow in warmer temperatures, for example.

The ants are careful to maintain the right growing conditions: they will bring back some water inside the nest if the air is getting a little too dry for the fungus. They also carry bacteria that acts as antibiotic to kill off harmful microbes that may infect the fungus. The seeds are carefully guarded, and if a daughter queen leaves the colony to form a new one, she will carry the fungus with her to start a farm in her new nest.

It’s a mutualism

The ants also cannot survive without the fungus, which makes this association a mutualism. Mutualism is an association in which two species co-exist such that they both have beneficial results from the association. The association of human beings with domesticated animals and plants can also be called a mutualism since most of these organisms have become dependent on humans for their survival.

How it all began

The fact that the fungus grows in the dry ant garden underneath a tropical rainforest probably sounds a little puzzling. The question here is how these plants survived before the ants took them into the comfortable gardens. Or did the ants carry them into the rainforest from some other drier place?

Several theories have been proposed to explain the origin of this association between ants and the fungus. One widely accepted theory states that the evolution occurred around 30 million years ago when the earth was getting cooler and drier. There are also theories that the ants might have started it after a widespread disaster in which many species perished, such that dead and decaying matter became the major source of food for the ants, and since fungus thrives in decaying matter, the association developed from there.

There are also at least seven theories that attempt to explain exactly how the ants came into contact with the fungus and then started using it as food. The most popular theory appears to be the one that says the fungus was encountered on the walls of an ant nest. In human terms, it would be something like finding a field of wild rice growing near your cave and learning to keep the seeds and cultivate it year after year. And the ideas of using fertilizers and pesticides and climate-controlled environments follow naturally.

The pesticide and the antibiotics

The “pesticides” that these ants use in their farms have been of great interest for research on antibiotics.

Antibiotics are drugs that are used to treat bacterial infections by either killing the bacteria or by at least stopping their growth. Human beings have been using a lot of antibiotics during the past 90 years, and millions of lives have been saved because of their use. However, bacteria also evolve, and since they are being attacked by the antibiotics, some random bacteria will have a mutation that makes it resistant to the antibiotic. Obviously, this strain of the bacteria will survive and reproduce, and in the process, it will cause a huge problem to human beings.

That is exactly what has been happening. Antibiotic resistance has been called a serious, public health threat by the World Health Organization. It says that bacteria responsible for diseases such as pneumonia and tuberculosis have become much more dangerous and life-threatening because of the rise of strains that are resistant to antibiotics. Even grimmer facts state that several infections may become untreatable in the near future making surgeries and childbirth deadly again. It has been called the coming antibiotic apocalypse.

The first antibiotic was discovered and used by humans less than 100 years ago, and if we are having such a problem, what about the ants and their pesticide?

A view of the interior of a leafcutter ant nest. The large queen is visible at the center, and their food, the white fungal structure, can be seen on the nest floor.

The secret of the ants

Attine ants do not have a mutualistic relationship only with the fungus; they also carry (on their cuticles) a special bacteria that produces an antibiotic to protect the fungus from infections. Since the ants have been cultivating the fungus for more than 50 million years, human experience would show that they should have come across some form of antibiotic resistance by now. But these fungus farms are found be remarkably healthy. In order to understand how ants manage to do this, scientists in Britain have been studying the ants, their fungus, and their bacteria . Results from these studies could potentially tell us more about how we can create smarter antibiotics.

The domesticator and the domesticated

I recently started watching the documentary series Cosmos (from 2014), and in the second episode, Neil deGrasse Tyson demonstrates the process of evolution by artificial selection by talking about the domestication of wolves that were the ancestors of our dogs. For this, he makes us imagine humans around a fire in the forest and wolves walking in the forest.

Here is a paraphrased version of the story: Most wolves are afraid to go near the humans, but one solitary wolf, with a lower level of stress hormones (and thus less fear), goes ahead and walks to the human camp. It finds discarded bones and other food. It learns that food is plentiful around the human beings, and that if it is timid around the humans, they don’t mind. It gradually moves closer and closer and learns that if it is tame and obedient and if it keeps the other wolves away from the camp, the humans will keep it with them and feed it. And that is how the wolves learned to domesticate the humans.

We will probably not accept the fact that wolves tamed us to get their food from us, but what about the ants and the fungus? Did the ants cultivate the fungus for their food? Or did the fungus find the ants that would feed it in a safe, climate-controlled home and even provide treatment against infections?

The super-superorganism

I have previously talked about the concept of the ant colony as a “superorganism.’ In a superorganism, several organisms behave in such a way that the overall colony can survive and succeed, and the individuals are more like “organs” of a body. That applies easily to ants that have a division of labor and a system that prioritizes the survival of the colony over that of the individual (as in the case of sterile workers, for example). The addition of other species into the colony makes it a little more complicated, but it could still be considered a superorganism with multiple species. Or a super-superorganism with ants and fungus and bacteria.

Considered with this idea in mind, aren’t human beings a part of a giant superorganism too?

And I found a fascinating article

While doing my research for this article, I came across a blog post by Sadeer el-Showk in which he compares the agricultural practices of humans and ants. Ant agriculture is described from the human perspective, and human agriculture is described from the perspective of the ants.

One of the many interesting things ants have to say about human agriculture reads: “In addition to the difficulties of communication, other biological limitations of humans may serve to explain some of the shortcomings of their agricultural practices.”

You can find the article here.

Links to references and image sources are provided in the text.