If you visit the tropical forests of Central and South America, you might be lucky enough
to spot a parade of leaf-cutter ants, tirelessly carrying food back to their nests.
But did you know that those ants aren’t taking those leaf-cuttings home to eat?
Because I did not know that
The fact is, they physically can’t survive on plant material.
Instead, the ants use those leaves to feed the gardens of fungi that they grow in their
colonies, which are their primary source of food.
And both the age and the sophistication of these little farms put our own farms to shame.
While we’ve been farming for around 10,000 to 12,000 years, the ancestors of these ants
have been doing it for around 60 million years.
And over that time, they’ve been through many of their own agricultural revolutions
that have changed not just how they farm, but also the very biology of their fungi that
they grow, and the biology of the ants themselves.
So when, and how, and why did ants start … farming?
It looks like the relationship between ants and fungi may have emerged from the ashes
of a famously devastating extinction event.
Now these ants didn’t start out as fungus-farmers.
For much of their history, they did what most ants today still do: they go out into the world
and eat the things they find.
Which also is how I go through life.
But everything changed for them when they started farming.
And this transition happened only once - in the subgroup known as the Attini.
because they're teeny ants, that's not why they call them that actually
There are more than 250 species of these ants, all of them in the western hemisphere and
all of them fungus-farmers.
But not all of these Attine ants farm the same way.
Most practice what some researchers have called 'lower agriculture' - the system with the
least complex farms, which is thought to have been the first to evolve.
In this system, the fungi aren’t fully dependent on the ants.
They’re what are known as facultative symbionts - they can survive without the ants and interbreed
with other, wild fungi.
So they’re not really fully ‘domesticated’, and if they end up disappearing from the colony
somehow, the ants can just replace them with wild versions of the cultivated fungi species.
But, other ants – including the leaf-cutters – practice higher agriculture.
In these colonies, the fungi are obligate symbionts - they’re completely dependent
on their ant farmers, they can’t survive in the wild, and they can’t interbreed with
free-living fungi.
They’ve been fully domesticated.
By ants.
And, just like with us, farming is even a part of the ants’ culture – if you can
call it that.
Like, when young queens of these ant species leave their families to start new colonies,
they take a piece of fungus with them to start their own gardens, passing these strains down
from queen to queen across generations.
So how did this relationship between ants and fungus actually start?
And how did it evolve over time into the different agricultural systems we see today?
Well, the fossil record has only given us some pieces of the story.
Mainly because, while we do have lots of fossils of ants - many trapped in amber - they can’t
easily tell us how those ants actually lived.
But there is some extremely rare fossil evidence of fungus-farming in ancient Attine ants,
in the form of nests that contain traces of fungus filaments.
But these are from relatively recently, like around 5 to 10 million years ago.
This means that, to dig deeper into the origins of fungus-farming in ants, we’ve had to
find another way to study them.
Enter the field of phylogenomics - or, using genomic data to reconstruct an organism’s
evolutionary history.
In a paper published in 2017, researchers at the Smithsonian tried to trace the history
of fungus-farming in ants across deep time, using their genomes.
They compared the DNA of more than a hundred different ant species from around the Western
Hemisphere - some that practiced higher agriculture, some that practiced lower agriculture, and
some that didn't farm at all.
By comparing their genetic relatedness, and how long ago the different groups diverged
from one another, based on the mutations that each lineage had accumulated, they could reconstruct
their evolutionary family tree.
And this revealed some intriguing clues about where, when, and why each agricultural system
first evolved.
The tree seemed to suggest that the Attini, which contains all fungus-farming ant species,
originally emerged in the rainforests of South America around 66 million years ago.
And almost immediately, they diversified really quickly - radiating into many different groups.
By 61 million years ago, they had established themselves as lower agriculturalists - full-time
fungus farmers.
And if this time period rings a bell, there’s a good reason for that.
This was the direct aftermath of the K-Pg mass extinction - with the asteroid and the
volcanism and the end of the reign of the dinosaurs.
- the non-avian dinosaurs anyway.
And the fact that the origin of fungus-farming in ants lines up with a period of global ecological
chaos is probably not a coincidence.
After all, the impact must’ve caused all kinds of environmental mayhem, including a
sort of nuclear winter in which clouds of dust and ash blocked out sunlight.
This would have been catastrophic for photosynthesizers like plants at the base of the food chain
and for species that relied on them.
But!
Fungi are decomposers and don’t rely on sunlight for growth.
So they might actually have thrived in these apocalyptic conditions.
The dark, humid, post-K-Pg world, filled with dead and decaying organisms, would’ve been
a paradise for fungi.
The earliest Attine ants, on the other hand, would have had a much harder time.
Suddenly, foraging for food would have been a real challenge, at least above ground.
So it’s easy to imagine why Attine ants might’ve begun harnessing fungi as their
sole food source.
It’s a stable and reliable crop they could grow underground, perfectly suited to
the apocalypse!
And this changed not just their lifestyle, but elements of their biology too.
Research has shown that Attine ants lost the ability to produce an important amino acid
called arginine very early on in their evolution.
And ever since then, they’ve been totally reliant on fungi to get that amino acid!
This may explain why we’ve yet to find a single example of an Attine ant species that’s
stopped farming.
They literally can’t survive without their crops.
So, while the evidence is kind of circumstantial, it’s at least plausible that the devastation of the
K-Pg was the catalyst for the first agricultural revolution in Attine ants - transitioning
from foragers to farmers.
But it wasn't until 30 million years later that one group of ants experienced their second
agricultural revolution - transitioning from lower to higher agriculture.
The researchers found the genetic signal of this transition at somewhere between 27 million to
31 million years ago, in the early Oligocene epoch.
This is when the group of ants that practice higher agriculture seems to have emerged and
branched off from the other Attines.
And just like the ants’ first agricultural revolution, the timing of this one matched
up with a period of environmental upheaval.
This was the aftermath of the Terminal Eocene Event that took place at the end of the Eocene
Epoch, around 34 million years ago.
For reasons we still don’t fully understand, the planet went through a period of global
cooling at this time, which allowed drier, less humid habitats to expand.
And some Attine ants left the rainforests of South America for drier habitats and brought
their fungi with them, which may have spurred a radical shift in their relationship.
After all, dry habitats are pretty inhospitable to fungi.
So researchers think that, in these new environments, the fungi became completely dependent on the
ants for survival.
They couldn’t live outside of the well-tended, humid, underground gardens of the ant colony.
And, over time, they became reproductively isolated from their free-living relatives,
which had largely remained in the rainforests.
This marked the switch from facultative to obligate symbiosis in the fungi, and from
lower to higher agriculture in the ants.
These fungi are now found only in higher Attine ant colonies, and they’ve developed some
specific adaptations in this fully domesticated existence.
For example, they have nutrient-rich swellings called gongylidia that can be efficiently
harvested and eaten by the ants.
These structures aren't seen in wild fungi or in lineages grown by ants that practice
lower agriculture.
And this relationship seems to have reached new heights in the leaf-cutter ants.
This group is represented by around 50 species, which are widespread in South America, Central
America, Mexico, and some southern parts of the US.
And the researchers’ analysis suggests that the leaf-cutters were the most recent group
of higher agriculturalists to evolve, emerging around 18 to 19 million years ago.
They have incredibly complex colonies, with big fungal gardens, and even sophisticated
hygiene practices to protect their fungal crops from disease.
Like, there are certain areas of their nests where they leave their waste, to keep it away
from their gardens.
And some ants even have bacteria on them that produce antimicrobial compounds, which help
protect the fungus from pathogens! I kinda wish I had that
And unlike other Attines, which usually just collect whatever dead plant and animal material
they can find to feed their fungi, leaf-cutter ants source only the finest, freshest biomass
- cutting it straight off the plant.
*chef's kiss*
In fact, leaf-cutters get through so much plant material they’re considered the dominant
herbivore of the neotropics - harvesting more total plant material than any other animal
group.
So in a way, the leaf-cutter ants have kind of brought fungus-farming full-circle.
What may have started as a way to survive without plants after the K-Pg mass extinction
has now come all the way back around to depending on plants for the system to keep going.
If the story of the Attine ants tells us anything, it’s never to underestimate their ability
to innovate over time.
Through 60 million years of evolution, facing challenge after challenge in the form of global
environmental change, the ants and their fungi adapted, adjusted, and thrived.
If this got you itching for more ant content, then check out our episode, “The Reign of
the Hell Ants”.
Gi/ant/ thanks this month’s /fun/gi Eontologists!
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What did the big flower say to the small flower?
What’s up bud! oh that's cute!
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