You've probably heard of leafcutter ants or have even seen them yourself.

Lines of ants marching with pieces of leaves in their mandibles.

But if you look closely, there's something else weird going on here.

On many of the leaves, smaller ants are hitching a ride.

At first glance, this looks like the ants could just be confused, getting tangled up in the commotion and getting accidentally carried away.

But this is no accident.

These ants are performing a very important job.

They're acting as sentinels, protectors of the leaf carrying ants, defending them against a devastating enemy, phorid flies.

These parasitic flies are trying to inject their eggs into the ant's body, where the larva will eat the ant from the inside out.

So these smaller ants known as minims will ride the leaves and ward off any attack.

This is the moment I realized the extent of the war being waged on the forest floor and realized that leafcutter ants are so much more than a mindless foliage carrying conveyor belt.

The highway of leaves is one small part of an incredibly intricate and efficient society.

And the size and morphology discrepancy between the different ants only gets more extreme.

Not only are there the minims, the tiny ones, and the mediae foragers, but there are also these absolute units called the majors and super majors, who are absolutely giant compared to the rest.

Clearly the ants in these colonies are extremely diverse and extremely specialized, but why?

Why is there so many different types of ants within one colony and so much division of labor?

Because a colony needs to harvest and process an enormous amount of plant material, and that takes a lot of teamwork.

Leafcutters are the most prolific herbivores in the neotropical rainforest, accounting for about 25% of all herbivory.

Mature Atta-Columbica colonies harvest between 85 kilograms and 470 kilograms total plant biomass per colony per year.

Collecting all this material and defending those who do it is complicated enough, but there's an even higher goal here beyond collecting leaves, one unifying purpose that is only possible with tons of cooperation and division of labor among millions of individuals, and that is farming.

These little guys are farmers.

This weird pile that the ants are tending to is their life-sustaining fungus crop.

The leaves they so dutifully carry back to the nest aren't their food.

They're the substrate for the fungus crop.

The fungus is their food.

Leafcutter ants are one of only a few animals that have mastered the art of deliberate and methodical agriculture, and they figured it out 60 million years before we did.

But how does an animal so small figure out something so complex?

And how do all of the ants know exactly what job they need to carry out?

And what about this delicate division of labor allows these ants to transcend what almost any other creature on Earth is capable of?

There are 55 species of fungus-growing leafcutter ants belonging to three genera, Atta, Acromyrmex, and Amoymyrmex, within the tribe Atini.

The Acromyrmex and Amoymyrmex have colonies that range in the thousands, but the Atta is the genera with colonies that number in the millions, up to 8 million workers in some species.

Each gigantic Atta colony usually consists of just one mother queen, who is the exclusive reproductive individual, and millions of non-reproductive workers of different shapes and sizes, which are all her daughters.

And there are some reproductive winged males who leave the colony.

How does one mother create such a diverse offspring?

For us, it would be like having one kid that's two feet tall, and another that's 18 feet tall.

One explanation might be revealed by looking at the queen before the colony is even born.

Each year, colonies produce young reproductive females and males, which both have wings.

Then on one autumn night, every single flying male and new queen from every single colony within a species in the region takes flight in unison.

Scientists think this synchronized flight takes place based on environmental cues like temperature, humidity, and daylight hours, along with pheromones the males emit once airborne.

Then once millions of males and females are in the air, Atta mating takes place high in the atmosphere.

It's so high and so elusive that it has still never been directly observed.

But from what scientists do know from genetic analysis, this is unlike the mating we're used to hearing about.

Each queen is inseminated by three to eight different males, meaning her offspring will have several different fathers in a process called obligate multiple mating.

This is kind of unusual to say the least.

So what's the reason for so many different baby daddies all at once?

Having offspring with multiple fathers gives the brood more genetic diversity, and more genetic diversity provides resistance to disease.

And in a colony of millions who live in close quarters, that is pretty crucial.

And getting that much sperm all at once might be the only way she can keep having babies for her 10 to 20 year lifespan.

Atta cephalotes queens are among the longest lived insects.

She only gets inseminated this one time, and then stores the sperm cells in her spermatheca, her internal sperm bank.

But the final reason could be the explanation for how she has so many different types of offspring.

Having lots of different dads gives a much bigger range of phenotypic differences, which could lead to the big range of different sizes.

But how the queen ant can ensure that she mates with the right males to give some gigantic babies, some tiny babies, and everything in between, we still don't know.

So this might partially explain how such morphological differences are possible in a single colony.

But what's the evolutionary purpose of creating such wildly different babies?

And what role do each of these morphs play?

To find out, let's go back to the new queen to see how her incredible queendom fully develops.

Before she departed on her mating flight, the new Atta queen packed a small amount of fungus in her mouth.

This will be essential in starting her new colony.

Once she is inseminated to her satisfaction, the queen lands on the ground and sheds her wings.

Here, she will excavate a nest chamber in the soil.

Once inside, she spits out the fungus she's been storing and feeds it with the first few eggs she lays.

Within three days, the mycelia have begun to grow.

For now, the queen cultivates the fungus garden herself.

But soon, she will need help.

And for this reason, the first batch of offspring she will have are the minims, the smallest ants of the colony.

These smallest of ants take over, tending to the fungus.

Their small size allows them to maneuver easily within the delicate fungal structures, removing waste and secreting bodily fluids to keep it healthy.

Soon, slightly larger ants hatch, and they are just big enough to leave the nest entrance and begin to forage in the immediate vicinity.

They collect bits of leaves and add them to the fungus culture.

And on top of foraging, all of these worker ants so far are also the ideal size for tending to the brood.

At this point, the queen stops doing any fungal tending and just becomes an egg-laying machine.

And now, the colony can take care of all of the essential tasks, egg-laying, brood tending, foraging, and fungal maintenance.

But this isn't enough to ensure the colony's survival, because predators are lurking nearby.

Army ants are known to raid leafcutter ant nests and prey on leafcutter brood young.

And larger animals like birds, bats, and spiders are known to feed on atta queens.

Without adequate protection, leafcutter ant colonies would be pillaged.

So at this point in the colony's life cycle, a new cast of ant emerges.

The soldier cast.

These extremely large major ants don't have any formic acid.

They don't have a sting.

But what they do have are extremely sharp mandibles powered by massive adductor muscles.

Their heads can measure five millimeters wide and they can weigh around 50 milligrams.

And they are especially good at defending against large enemies.

Large soldiers can even pierce human skin.

And this bite force is around 2.5 times greater than scientists thought it should be.

You might expect that a soldier ant that is 30 times as heavy as a smaller worker would have a bite force that is 30 times as strong.

But it's actually around 80 times as strong.

Scientists analyzed the heads of these massive soldiers and found the reason to be an increased amount of muscle volume compared to the smaller ants.

And with these absolutely yoked bodyguards, along with some of the smaller workers also taking on defense tasks like against the parasitic flies, the leafcutter colony can grow their numbers and even expand their territory.

But with increasing numbers, the colony needs increasing food.

Therefore, the colony needs the fungal garden to grow larger.

And for this, they need much more leaf mass.

And the minims can no longer keep up with foraging duties.

Enter the mediae workers.

These medium-sized ants are built to foray far into the world to gather resources in the form of tons and tons of leaves.

Unlike the minima who can carry out a number of different tasks like brood and fungus tending and minor foraging, the mediae workers do little else except cut leaves and bring them home.

This intermediate size is optimized for exactly this job.

With all of these different sizes of ant, there's one obvious question.

How do the different worker casts appear in the right order?

And how does the colony ensure it has the right number of each of them?

It all comes back to the queen.

The queen lays eggs with varying nutritional content, which contributes to size differences in the developing larvae.

And the size of a developing ant larva determines which cast it will become.

This, along with the different baby daddies, gives rise to all the different phenotypes.

How the queen knows when it's time to lay the right eggs and inhibit the growth of some versus others probably comes down to environmental cues.

But with all of her workers now hatched into the world, the colony can truly blossom.

During cutting, a worker usually anchors her hind legs on the leaf edge and slowly pivots around the body axis, pushing the cutting mandible through the leaf tissue.

This is why their cuttings are usually semi-circles.

As they cut, one side of the mandible inches across the leaf and the other drags as it cuts.

The head of a leaf cutter has strong muscles attached to its mandibles, and the mandible itself is made of chitin nanofibers and proteins and coated in a heavy element biomaterial, a mixture of proteins and zinc, making them as durable as a stainless steel knife.

And when you compare these media mandibles to the minima mandibles, you can see a clear difference in form, with the media mandibles much more robust and specialized for cutting.

This sharp and durable anatomy allows leaf cutter ants to cut thousands of leaf fragments in its lifetime.

But there's one more thing that helps a leaf cutter ant cut leaves that is kind of unexpected.

And that is sound.

If you listen closely to leaf cutter ants cutting leaves, you hear some clicks.

That is the sound of the mandibles cutting the leaf, and you'll hear a background sort of humming or chirping.

This is a high-frequency vibration called a stridulation, and it's produced by the stridulatory organ.

This organ is located on the gaster and is made up of a file and scraper.

When the ant rubs the file against the scraper, an audible vibration is made.

And this vibration stiffens the soft leaf, making it easier for the ant to cut it.

And these stridulations serve another incredible purpose.

Ants can use them to communicate.

If you've seen video clips like this, leaf cutter ants have the tendency to completely demolish certain leaves, while leaving other leaves almost completely untouched.

The leaves that get demolished seem to be more desirable.

Maybe they are more tender or have more sugar quantity.

And in experiments, scientists found that significantly more ants stridulated when cutting tender leaves than tough leaves.

And similarly, when the scientists coated the leaves in sugar, almost all of the workers stridulated.

The vibrations are affected by the quality of the leaves, and the stridulations act as a way to communicate to nearby ants to come and get some.

This is one of those moments where it's fun to wonder which came first.

Did the stridulations evolve to facilitate leaf cutting and the communication about those leaves came as a result?

This is what I would have guessed.

But scientists think it's actually the opposite, that facilitation in cutting leaves is more likely an auxiliary benefit emerging from the communication process.

Scientists think this partly because this stridulation is used in other contexts for communication as well.

When an ant accidentally gets buried, for example, they stridulate to call for help.

And the vibrations are used to coordinate things like nest building and excavation.

But these vibrations are only a small part of the communication of leaf cutter ants.

Their chemical communication is one of the most robust in the animal kingdom.

And much of it occurs on harvesting roots.

If you ever wander into leaf cutter ant territory, you might notice a parting of the leaves on the ground, like Aunt Moses himself had been there.

But this was no Aunt Moses.

It is a leaf cutter trail, cleared fastidiously by thousands of workers.

Of all of the animals in the world that construct trails, like elephants, cattle, or voles, Atta ants have the most complex trail construction behavior.

Workers remove vast amounts of vegetation, cut passes through large obstacles, and level the soil to make a smooth surface.

On average, colonies clear nearly three kilometers of trail per year.

And these routes lead masses of foragers to and from harvesting sites.

And the way the ants begin these trails and continue to follow them is not through vision, but through scent.

The ants chemically mark the trail with secretions from their poison gland sacs with two goals in mind.

One, to recruit other ants to join the trail, and two, to create a long lasting orientation cue.

To recruit other colony members, the ants release a particularly volatile chemical signal, which makes sense.

So the signal travels farther and can attract ants from farther distances.

And to leave a lasting chemical signal that delineates the trail, the ants release a chemical that is much less volatile, which also makes sense as volatile signals don't last very long.

And the volatile compound in some Atta species is mind-blowingly effective.

Scientists found that workers of one Atta species follow trails with minuscule amounts of their volatile ant trail pheromone.

Scientists artificially laid a trail for the ants to follow and found that for a distance of one meter, only 0.4 picogram was needed to induce trail following behavior.

A picogram, for reference, is one trillionth of a gram.

By this math, just one milligram of this pheromone would be enough to lay a trail 60 times around the planet with the ants still following it.