So the ant colony has a very particular form of social organisation. The Queen is the only ant who reproduces. Her eggs will become the workforce of the colony. Each generation raises the next from egg to adult. This results in multiple generations working together for the good of the colony. And these attributes put our ants in a very special group of insects. The truly social or eusocial insects. Now eusocial insects are phenomenally successful. Whereas they only make up less than 5% of all insect species, they account for the majority of the insect biomass on Earth.

Termite Mounds
Termite Mounds

Apart from ants, the major groups of eusocial insects are termites, wasps and bees. Together, these insects outnumber all the others on Earth combined. Being eusocial is one of the most important, evolutionary developments in the animal kingdom.

It’s such a significant step, that scientists are trying to discover when it first occurred. And what it is about being eusocial that gives these insects such an advantage.

Dr David Grimaldi is the curator of fossil insects at the American Museum of Natural History.

Dr David Grimaldi
Dr David Grimaldi

He’s spent 25 years researching specimens of ants and other insects millions of years old. This sample is from the Cretaceous era.

David Grimaldi “We know that these ants were wandering around at the time of the dinosaurs. Dinosaurs, of course, died out but the ants went on to become astonishingly abundant.”

Cretaceous Sample
Cretaceous Sample

These ants are a window into prehistory. The sap of ancient trees trapped them as they foraged and then hardened into amber, preserving them for millions of years. Now these remarkable specimens are helping scientists discover more about the origins of eusocial insects.

Eusocial Insects

David Grimaldi “There is one remarkable piece from the Cretaceous, probably the most important piece, a chunk of 100-million-year old amber, that contains ten individuals, almost certainly workers. The ants are so rare in Cretaceous amber, so the probability that you would get 10 individuals preserved in one piece just based on chance alone is astronomically improbable. Unless of course, they were social. “

Honeypot Ant
Honeypot Ant

Other ancient samples revealed that being social didn’t just affect the ants’ behaviour, it also changed their anatomy. These ants have pouches to share food with their sisters. This feature of ant anatomy is most clearly seen today in the Australian honeypot ant. These ants are so full of food, they can hardly move. They’re like living larders, feeding their sisters.

David Grimaldi “When you have many, many individuals that specialise in foraging and protection and nursing of the larva and in defence of the nest, you can be much, much more effective. So, being social is a tremendous adaptation, perhaps one of the most effective adaptations in the animal kingdom, because we can see that when ants become highly, highly social, they become a very dominant lifeform.”

Dominant Ants
Dominant Ants

The advantages brought by eusociality have allowed these insects to dominate the globe. Ants have been called ecosystem engineers, as they can change the environment around them. Nutrients released from their underground nests fertilise the surrounding soil, which in turn promotes the growth of plant life on the surface. With the more plants come more animals, and studies have shown that an ant colony can actually increase the diversity of animal life around it.

Social Organisation

Bee Flower Pollenator
Bee Flower Pollenator

Eusocial insects can even affect our lives. Without bees to pollinate our plants, we wouldn’t be able to grow enough food crops. You might say it’s eusociality that feeds our world.

It’s been 15 days since we began following the progress of our ant colony. In that time, they’d been far from idle. They are now well-established in a nest we built for them.

Ants Fungus
Ants Fungus

The most amazing change since the colony has really become established is the incredible growth of the fungus gardens. You can see the green leaf material where the fungus hasn’t quite grown yet, so it’s just becoming white from the base up. You can see at the very outside edge, you’ve got all the chewed, green material, the food for the fungus, and then the fungus moves up. It’s as fragile as anything. Incredibly it’s just a miracle of micro-engineering, this. And it’s one of the most beautiful things I’ve ever seen.

To see how much progress the ants have made in re-growing their fungal gardens, we’re going to open up a nest box again.

Look at that. There we go. They’re not happy about this. You can really see the structure of the fungus garden. They are really… That whole thing is hollow and there is a soldier in there. This is their very reason for being, isn’t it? That is the major resource.

Adam Hart “Yes, it’s not like a mushroom or a toadstool. It’s a very fragile structure. It’s more like a sponge. There’s a huge surface area in here, so there’s lots of little Chambers and cavities and places for them to feed. This is unbelievable.”

Ant Colony Defence

Feeding Ant
Feeding Ant

It’s these white tufts, produced by the fungus, that feed the colony. They contain just the right balance of nutrients to support the developing brood.

This fungus garden alone, grown since the ants arrived, will feed thousands of new ants. It’s the clearest indication yet that our leafcutter colony is thriving. Before we get completely inundated, I think we’re going to have to put this back down very gently.

Ah! One’s gone down my front . Ah-ah! Ouch!

Adam Hart “See, they’re incredibly good at defending. This is their colony. And we’ve broken into it. And that’s the result.”

I don’t think we’ll be doing that again. I’m beginning to regret it now.

Adam Hart “We’ve had a good look at the fungus garden, and we’ve seen a great response, but yeah, perhaps not again.”

Away from the nest, there are more signs of progress. The ants are constantly monitoring their long foraging trails. If any blockages occur, workers swiftly clear them. What our colony is showing is organisation on a massive scale. And that begs an important question.

How did they organise all this? How did they know what to do?

Resolving a Conundrum

Humans wouldn’t be able to do this without some kind of hierarchy, without somebody taking responsibility, giving instructions. But, as we’ve seen, this is not the case with ants. There is no hierarchy. No central command or control from any individual or group of ants. Not even the Queen.

So how do the ants do it?

To help answer that question, Adam’s going to put them to the test.

And, in turn, add her own pheromone to the route. When this is applied to hundreds and thousands of ants, very strong trails are produced that link the nest directly to food sources in the environment. And what that means is that the branch that’s got food at the end of it is much more concentrated in terms of pheromone than the branch that doesn’t. So that when ants come to that fork and they have to make a decision, they follow the trail head that has the most amount of pheromone.

Adam Hart “I’ve set the ants a problem. I’ve given them a Y-shaped trail. At one end of that Y is food and at the other end is nothing. And I’ve connected that trail up to the main trail, so they are pouring down out of the nest, coming on to this trial and are being faced with a choice. Do they go left or right? Our ants have a clear 50-50 choice between right and left. But after just 20 minutes, virtually all of them are heading down a path that leads to the food.

Prying on the Ant Chatter

So, how do they know where to go? At this distance, they can’t see the food. Their eyesight isn’t good enough. Instead, it’s all down to the ingenious way the ants share information with each other, using their acute sense of smell. The ants moving down here are laying behind them a chemical pheromone trail that marks the way for the other ants. They can detect tiny amounts of these pheromones using their antennae.

When an ant goes out foraging, she leaves the pheromone trail on the ground behind her that her sisters are able to follow. If she finds food, she will then lay down even more pheromone on her way back to the nest, making the original trail even stronger. If she doesn’t find food, she won’t lay any more pheromone and the trail simply evaporates away. The stronger the pheromone trail, the more likely an ant is to follow it.

So they’re much more likely to go right than they are to go left. That means these ants can organise themselves. The Queen’s not in the colony going, ‘turn right, turn left, take the third exit.’ They follow the trail of pheromone to the food. So each individual ant is dealing with simple signals, simple rules. But collectively this system achieves complex results. It enables the colony to find new food sources, exploit them efficiently and react swiftly when they are depleted. This is what underpins the entire leaf cutting operation. But pheromones aren’t the only way leafcutters communicate. They are constantly exchanging information, and with the right technology, we can even listen in.”

Eusocial Insects Hearing

Is it possible to actually hear?

Adam Hart “Yes, luckily, I’m festooned with gadgets, so we can actually… we can actually mic these up. We can get some sound out of these.”

They’re very small animals and it must be a very faint noise.

Adam Hart “Yes, it’s a very small noise and it’s quite high-frequency, but if we just press that onto there…”

To human ears, the ants’ world seems silent, but amplified by the microphones, the leaf comes alive with noise. And there is one particular sound weird listening for . in amongst the sound of leaves being cut and ant footsteps as a high-pitched chirrup. This is stridulation, a sound the ants make by rubbing two sections of their abdomen together.

Adam Hart “So they’re making this sound but it’s part of a group of sounds that they make, these sounds.”

Group Sound

That little chirruping noise is a recruitment signal. The more nutritious the leaf is, the more they make this noise, sending a cascade of vibrations through the plant. And this draws other ants to the tastiest part of the plant, which means the ants will tend to take the best leaves first. But there’s more to stridulation than simply leaf cutting. It can make the difference between life and death.

Ant Colony
Ant Colony

As they build their underground network of tunnels and chambers, our eusocial insects, just like human miners, face an ever-present risk. A roof collapse could bury them alive. To discover how they respond, we’re going to simulate this catastrophe. And time for another gadget.

Adam Hart “This is a plate microphone, so this is recording directly what’s on the surface.”

However, we’re going to put an ant on the surface of this microphone and bury it with soil, just like a roof collapse in the nest. And then, we listen.

Adam Hart “So this is going to be the sound of ant fear. This is an ant that has been trapped under the soil. It’s calling its nest mates. There’s a bit of hiss there, that you can hear… that’s the noise they are making by moving their abdomen backwards and forwards.”

Moreover, that’s very obvious, isn’t it?

Adam Hart “It’s a very clear signal that causes a very specific behaviour. Come over here, dig me out.”

But, what we are hearing is the ant’s alarm call. She is appealing to her nest mates for help. With only a loose covering of soil, this ant isn’t in any real danger. As she digs her way to the surface the noise of panic stops and she emerges.

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EUSOCIAL Insects are a unique form of social organisation