♪ ♪ ♪ ♪ SCOTT BURNETT: It might look dull and smell disgusting, but when you know how to read it, poop is the oracle of knowledge.

Ecologists like me call it scat, and we're passionate about it.

Just imagine the size of the bum that this dropped out of.

The world is full of fabulous feces stories, like the mystery of the Australian wombat and its cube-shaped poop.

PATRICIA YANG: How would a square come out of a round hole?

That's the big question.

(whale moans) BURNETT: Or the tale told by DNA in blue whale scat.

And what's really cool is, we've kind of figured that out by digging through their poop.

BURNETT: Even in tiny krill, we're discovering the real power of poop.

SO KAWAGUCHI: For me, it's the most important animal on the planet.

BURNETT: There's always a new mystery to be solved.

"Secrets in the Scat."

Right now, on "NOVA."

♪ ♪ ♪ ♪ BURNETT: My name is Scott Burnett.

They call me Scat Man.

When I was young, I wanted to be a wildlife tracker, looking for secret traces that would lead me to extraordinary creatures in the vast Australian bush.

My dream came true: today, I'm an ecologist.

And I use scats to tell the stories of species.

I look for clues in poos, facts in feces.

I know my (bleep).

♪ ♪ Scat is a gold mine of information.

It illuminates the role of poop in the health of ecosystems and how animals communicate.

It holds the secrets of wildlife behavior and can even help to save species from extinction.

All of this we can learn without invading their privacy.

That's why scat science is real science.

♪ ♪ Join me on a journey where few people have dared to tread, looking for scats, unraveling their secrets, and meeting other scientists with a passion for poop.

♪ ♪ ♪ ♪ This scat tale begins in one of the oldest rain forests in the world.

I'm here to investigate a spectacular dropping.

It may hold clues to why these forests remain so diverse and resilient over time.

It comes from the bottom of an imposing bird.

The southern cassowary.

♪ ♪ The cassowary is flightless, like the emu and the ostrich.

Its wings may be useless for liftoff, but powerful legs can propel the bird into the air to reach overhead fruit.

This taste for the bounty of the rain forest is the beginning of a beautiful story.

A story that can only be revealed if we stop and take the time to read its scat.

Well, thanks so much.

Yeah.

Good luck, all right?

Cheers.

Ta.

♪ ♪ (voiceover): You can sometimes hear and feel the cassowary's deep, booming call before you see one.

(cassowary rumbling) The low frequency may help them communicate through these dense tropical forests.

♪ ♪ (grunts) You have to be careful; you have to really respect these birds.

You never know when you're going to run into one when you're following their tracks.

They do have the potential to inflict considerable harm.

Ah, here's his trail here.

Whoa.

You can see the cassowary's come through the creek here, and there are these beautiful footprints in the soft mud.

It's quite slippery.

And he's come through...

This one, they look just like little dinosaur footprints, don't they?

♪ ♪ (voiceover): The tracks should lead me to fresh scat.

But first, they deliver a rare encounter.

♪ ♪ The first time you come across a cassowary in the forest on foot is a really intimidating experience.

This animal is virtually looking at you eye-to-eye.

And it has these massive talons which it uses in defense.

Looking at a cassowary, you can see straight back into the dinosaur heritage of all our birds.

It's very hard to escape the feeling that you're interacting with something not too far removed from a velociraptor.

♪ ♪ So that male cassowary just dropped this scat before my eyes moments ago, so you can still feel the warmth coming off that, and it's chockers full of rain forest fruits.

And there's this beautiful big cassowary plum seed, and this is really significant.

(voiceover): Because cassowaries and cassowary plums have a mutually beneficial relationship.

The fruit is large and poisonous, which limits the number of species that can disperse its seeds.

The cassowary is the only animal in these forests that can swallow this fruit whole.

The fruit passes through the cassowary's gut so fast that only a small portion of the toxins are absorbed.

And when the intact seed comes out the other end, it will sprout in a matrix of ready-made fertilizer at a distance from the parent tree.

The plant gets its seeds dispersed and the cassowary gets to eat-- a fruitful relationship if ever there was one.

The cassowary disperses the seeds of around 238 plants.

But just how effective is the big bird to the health of these precious forests?

♪ ♪ After the rainy season, I'll return to the Wet Tropics to take part in an experiment designed to find out.

It involves a much closer look at cassowary scat, which is right up my alley.

♪ ♪ Scats are splendid things, filled with fabulous facts.

From them, we can identify species... (animal shrieking) ...and their most recent meals.

And once in a while, a scat comes along that elevates poop science to another level.

Introducing the cube, the only six-sided scat in nature, an exceptional excrement, direct from the bottom of the common wombat.

Wombats are Australian marsupials.

They graze continuously.

It can take up to two weeks for them to digest their food.

The process squeezes out every last drop of water, a good strategy in Australia's harsh climate.

But that doesn't explain how, or why, the end result is a cube.

♪ ♪ The first serious research into this mysterious scat happened independently in the U.S. and here in Tasmania around the same time.

It seemed the scats had aligned.

I'm here to see the result of that research.

My scat investigations usually begin once the (bleep) hits the ground.

♪ ♪ In this case, I need to know what goes on behind the scenes.

♪ ♪ Like many scientific breakthroughs, the investigation into this mystery started accidentally, here, inside the laboratory of Scott Carver.

G'day, Scott, how ya goin'?

G'day.

Good to see ya, come on in.

BURNETT: There she is.

CARVER: Yeah, she is.

Oh, look at her.

Yeah, she's a female adult, would have been hit by a car.

Just pop her down here.

CARVER (voiceover): I study infectious diseases of wildlife, and as part of that work, we've been dissecting some wombats that are either, had disease and died or had been hit by a car and died.

And we were doing some of these dissections.

And to our surprise, for the first time, we actually discovered that they have these cubes formed inside of their body.

CARVER: So it was just a situation just like this, where we discovered that the feces were formed into cubes in the last part of the intestine.

And so before your work on this topic, there were a few competing hypotheses about where that cubed shape of the wombat scats comes from.

Yeah, one of them was that they had a square-shaped anus.

So that bum doesn't look too square-shaped, does it?

No, it's not square-shaped at all.

BURNETT (voiceover): Further confirmation came when they performed the first ever CT scan on a live wombat.

That's not a square.

CARVER: So we're just gonna start here below the rib cage and just cut a line down and finish straight down here and reveal the intestines.

Now if you peel this back here a little bit, we can actually see some of these cubes starting to form.

Yeah, it's amazing.

Yeah, it certainly is.

Yeah, you can really see the cubes here.

Mm, they're really well-defined, aren't they?

Yeah, really clearly defined.

Really sharp edges along those corners... ♪ ♪ CARVER (voiceover): So we started collecting some of the tissues from these intestines to look at the thickness.

At the same time, we had some colleagues in the U.S. contact us, and they study the fluid mechanics of many things, including how feces are formed.

And, and they were interested in this when somebody pointed out that wombats can produce cubes during a conference.

BURNETT: One of the scientists on the U.S. team was Patricia Yang, based at Georgia Tech, an expert on the physics of feces.

She thought she knew everything about poop, until she was faced with the cube.

So I did some online search, and a lot of images or photos come up, and those are... For, for my eyes, it looks very artificial.

And that increased the doubt even more, like, is that even a real thing comes out from animal body?

But I just couldn't believe it, because that is way too weird from my knowledge of feces.

And that triggered my curiosity.

I was thinking, if, for normal defecation process for most mammals, including humans, the feces come out like a toothpaste coming out.

So when you squeeze it, the shape of feces is the same as the, the orifice, and this sort of soft matter comes out slowly but smoothly, as a sausage.

But how would wombat have this cubicle shape?

♪ ♪ Our hypothesis is that it must be the intestine that molded this shape.

There might be something that's critical but invisible to our eyes, and we're going to figure this out.

CARVER: The logical next thing to do for us was to examine microscopically what the tissues looked like.

We found thicker and thinner muscle around that circumference, and along the length of the intestine, these sort of thicker and thinner regions formed bands.

YANG: So we have never seen this before in any kind of animals.

We are wondering if different thickness, the thicker and thinner bands, how would that affect the stretchiness of the wombat intestine?

So we put a wombat intestine into this tensile test.

Tensile test is the way to test how stretchy this material is.

♪ ♪ And we discovered that the wombat intestines stretch, but they are not stretched in even way.

For the thicker part, they stretch less than the soft part.

We were surprised that the stretchiness were four times different among these two bands.

With this measurement, we fitted this information to a mathematical model, and that it contracts.

And over several cycles' contraction, we see a shape with four corners.

CARVER: I've got a really simple way of showing pretty much how they form these cubes using this rubber band here.

And so we can just kind of pinch the sides of the rubber band and kind of simulate those, those more stiff regions of the intestine.

And if you just hold it like this, you can just go like that, and it begins to form a cube.

So when the contractions happen, it is basically squeezing the feces into this shape, and it does this rhythmically over time, and so they eventually get molded into that.

And as the moisture is absorbed out of the feces, it starts to fragment along its length and form the distinctive actual cubes that come out at the end.

♪ ♪ So after three years of collaborating and work with a big team, we finally figured out why wombat cube have six faces.

(audience cheering and applauding) BURNETT: For their discovery, the international team won a coveted Ig Nobel Prize for science that makes you laugh and then think.

Thank you.

Wombats are solitary herbivorous marsupials.

They produce cube-shaped scats.

When feces dry, it contracts and forms cracks.

Wombats are scientific proof that you can squeeze a square peg into a round hole.

(audience laughing, applauding) BURNETT: The time we were able to spend in the lab gave me a really great understanding of how wombat scats get formed.

But the question still remains: what role does it serve in the wombat's ecology?

♪ ♪ The most popular theory about why wombat scat is cubed is that the shape makes communication easier.

But first we have to ask if wombats even use their poop to message one another, whatever its shape.

In the next two days, we'll conduct a pilot experiment to help find out.

♪ ♪ Maria Island in Tasmania is the perfect natural laboratory.

There are between 2,000 and 3,000 wombats here.

♪ ♪ Wombat scats are not all obviously cube-shaped like we saw in the lab.

Variations in physiology, diet, and moisture cause shape changes.

It can even depend on how you look at them.

But they do all have six sides, which is what makes them special.

Wash your hands.

There are thousands of wombat scats aggregated in clusters across the island.

These are latrine sites.

Many animals use latrines for social communication.

They sniff out the scent of who's around and leave their own scat messages.

But are wombats using their latrines for this purpose?

It's never been directly investigated, until now.

Our pilot experiment is simple.

We'll drop the poop of a stranger wombat into a latrine and watch what happens.

If the wombats are communicating with their scat, they should react to the new poop.

The home ranges of wombats overlap and vary in size.

To ensure the poop is foreign, I pick fresh scat from one side of the island and take it to the other, where I've found a very impressive latrine.

That's a lotta (bleep).

And here's a selection of fresh, fresh pellets.

(voiceover): If these latrines are being used for social communication, you'd expect that where we dropped foreign wombat pellets in, there'd be some sort of response from the resident wombats.

I set up camera traps on ten latrines across the island.

I add stranger poop to five of the sites and leave the other five unchanged.

These are our control sites.

We'll use them to gauge what normally goes on around wombat latrines so we can quantify any effects from adding scats.

I'll return to check the camera traps in 24 hours.

♪ ♪ Scott Carver has left the lab to join me on Maria Island.

He's going to perform an elegant experiment designed to test another element of the wombat communication theory: that the cubes give the wombat an evolutionary advantage.

♪ ♪ Here's the thing: wombats have bad eyesight, but they have a keen sense of smell.

If they are communicating through their scat, they're well equipped to sniff it out.

So it shouldn't be surprising to discover that the process of evolution has found a way to amplify the scent of wombat scat.

There's a little wombat poo tower over here.

(voiceover): Meet the wombat scat stack.

Does the unique shape of wombat scat have any bearing on poop stacking?

Tonight, we're going to find out.

But first, we need some essential data.

♪ ♪ CARVER: So I reckon we might be able to get a measurement on how high the, the bum is from the ground here, Scott.

This one seems pretty habituated.

Yep, looks like about 15 centimeters.

BURNETT (voiceover): This is science.

This is what we do.

What would happen if a wombat had round-shaped scat?

Would it stack?

We've made cubes and spheres out of modeling dough.

We'll drop them from the height of a wombat's bottom and compare how they land.

The hypothesis is that the cubes will aggregate and the round ones will roll away.

So it's super-simple little experiment.

Wombats often drop them on a rock or a log that's at a similar angle to this.

And here we can just test with the circular ones how well they go relative to the cube-shaped ones.

If the hypothesis is correct, the cube-shaped poo, the yellow ones, shouldn't spread out as far as the pink ones, correct?

Yeah, they shouldn't, they should end up a little bit more closer together and...

It's looking good so far, looks right...

It's off the table!

Hey!

Pretty striking, eh?

(laughs) You, you couldn't hope for a better result.

That might be our best trial so far.

Yeah.

So, not surprisingly, balls roll further than boxes.

But, as scientists, we have to actually do the experiment, hey?

BURNETT (voiceover): We have so much fun that we replicate the experiment into the wee hours of the morning.

Time and again, the cubed shapes aggregate and the sphere shapes dissipate.

It looks like pooping cubes would have an evolutionary advantage for a near-sighted wombat that needs to sniff out a latrine.

It's a fine theory, but we still have no scientific proof that they do communicate through their scat.

Scott Carver's on his way back to the lab, and I'm about to find out what's happened overnight at our pilot communication experiment.

This is really exciting.

There's a new pile of fresh scats right here beside the stranger's scats I dropped in yesterday afternoon.

So a wombat's definitely been here, and, you know, pooed overnight.

Let's have a look at the camera and see if we can see any sort of interaction with this new pile that we've dropped in.

That's very exciting.

(inhales) (exhaling): Let's see.

Ah, here comes a wombat.

Ah, it's sniffing the new pile.

Oh, my God.

And then it's taking a poo right in front of the camera.

(laughs): It's got its backside facing directly towards the trial camera.

So there we go.

That's our first bit of evidence that these latrines do trigger some sort of scent-marking behavior, or that the strange scent here has, has some influence on the behavior of these wombats.

At the five control sites where I didn't place scat, the wombats showed no reaction.

But at three of the five manipulated sites, wombats responded by sniffing the foreign scat and then pooping.

What could the wombats be talking about?

Because they're mostly solitary animals, it would be really important to know reproductive status of females or to stay clear of dominant animals.

That all still remains quite speculative, but I think at this point, we can say that the latrine sites are used in some sort of social communication way.

♪ ♪ While we need a lot more information before we can say for certain what's going on here, the latrines of some mammals have been well researched, including one animal under constant threat from poachers.

Across the ocean, the wondrous latrines of Rhinoceros unicornis await me.

They may hold the key to survival for this vulnerable species.

♪ ♪ The greater one-horned rhinoceros is best viewed from a distance.

It's the second-largest of all rhinoceros species, as heavy as an S.U.V.

and able to charge at the speed of a galloping horse.

The beauty of scat science is, we don't have to catch one to learn about its life.

So this is an amazing spot here.

We've got rhinos grazing out on this side, maybe a couple of hundred meters, and on the other side, rice paddies and villages.

There's just over 100 rhinos in this area of just under 40 square kilometers.

♪ ♪ (voiceover): Which makes the Pobitora area a great place to find a rhinoceros latrine.

♪ ♪ Look at the size of this scat.

This is one that was dropped last night.

It's really fresh, sort of beautiful stuff.

If this had come out of the bottom of my compost heap at home, I'd be really pleased with how well it's gone.

There's all these really finely masticated and digested plant material.

And just by coincidence, there's a cowpat here in front of the rhino heap.

So you can really get a sense of how massive these animals are, and they're just roaming in and out of the village here, and everyone seems to be getting along.

Just imagine the size of the bum that this dropped out of.

♪ ♪ (voiceover): These are just single scats.

I'm after the motherlode.

I've made a pair of tongs out of bamboo to handle the scat hygienically.

And there we go.

(voiceover): Coming across fresh scat in the wild is a golden opportunity for a scatologist like me, even if it's not for my research.

Collecting samples for other scientists can help their studies.

Some of this rhino poo may even be used to help save the greater one-horned rhinoceros.

So this large pile of scat here is a latrine site used by a few different rhinos.

They communicate to the other animals in their population via the chemicals in their scats.

They communicate social status and reproductive status and who's in the neighborhood.

It's like a Fecesbook for rhinos.

♪ ♪ (voiceover): But this social platform also attracts ruthless killers.

A fresh poop pile is a message to poachers that rhinos visit here.

They're killed for their horns, which are smuggled out of the country and used for luxury goods or in traditional medicine in parts of Asia.

(people talking indistinctly) Scientists have figured out a way to protect the rhinos by using their scat for forensics.

They collect scat in the wild and extract DNA, which goes into a database.

When a dead rhino is found or a rhino horn confiscated from criminals, authorities can match its DNA with DNA in the database to locate where it came from.

The DNA evidence can be used to convict poachers and to know where to concentrate police efforts.

In the 1900s, they faced extinction.

Now there are over 2,600 greater one-horned rhinoceros in Assam.

Perhaps this scat will go towards saving one more.

Just using my GPS here to get a geolocation for this sample.

(voiceover): Latrines helps rhinos communicate, and now they may also protect these superb pachyderms from extinction.

That's a good poop story.

♪ ♪ Rhinoceros scat is easy to collect.

But some scat is more elusive, especially that of the most prodigious pooper on the planet.

♪ ♪ I am about to leave my comfort zone, turn my back on the terrestrial realm-- where animals leave their scats in forests and fields-- and head to sea with a whale biologist who shares my passion for poop.

I'm known as the whale poo girl, which I embrace wholeheartedly, and it goes back to where this whole blue whale project in Sri Lanka began.

WOMAN: Blow!

DE VOS: I was off the southeast coast of Sri Lanka doing some research on sperm whales on a whale research vessel, and had this amazing encounter with an aggregation of blue whales and a floating pile of whale poop.

Everything that I had been taught in my undergrad and in school about blue whales was that they did these long-range migrations between cold feeding areas and warm breeding and calving areas.

BURNETT: Asha de Vos discovered that the blue whales in these waters have a different lifestyle from most others.

They don't migrate and they have some unique behavior.

♪ ♪ What was going on?

Turns out the answer could be in their scat, and today, I hope to see some of it while Asha continues her research.

Hi, Scott, good morning.

Hey, Asha.

Welcome.

Come on board.

It's good to see you.

Great to meet you.

Come on in.

Thank you.

♪ ♪ DE VOS: As soon as you see a blow-- so it's a very tall, powerful spout that you'll see in the distance-- do shout it out.

We've got the team ready to take down the data.

Yep.

You'll keep your hand out so that we can make sure we don't lose the animal.

We'll try to get a distance and sort of a bearing to the animal and start to approach to, within a reasonable distance so that we can start to get all the other information that we need.

So, do you think you're ready for the task?

Cool, let's go do it.

All right, let's go.

♪ ♪ BURNETT (voiceover): We're waiting for a whale to come up to breathe and, hopefully, to leave a gift-- one of the largest scats in the world.

♪ ♪ We don't know where, when, or even if a whale will appear.

Neither do the hundreds of whale watchers that are out on the water today.

But I... and they just... Yeah, and these whales were here just waiting.

These whales have been here for centuries.

Yeah, yeah.

And that's the thing.

And most people in Sri Lanka didn't even know these whales were in our waters.

And then I started getting, like, mail from people saying, "We didn't know there were whales in our waters."

So is this when the tourism industry all sprung up, too?

Just in the last decade?

WOMAN: Blow!

Yeah-- sorry.

(people exclaiming) (people speaking non-English language) (people calling out): Blow!

(camera clicking) (De Vos speaking non-English language) Blow!

DE VOS: Hurry up.

(camera clicking) WOMAN: Blow!

BURNETT (voiceover): The encounter only lasts a few minutes.

BURNETT: Blow.

(people exclaiming) Blow.

BURNETT (voiceover): We can follow their breaths, but we can't follow them down.

DE VOS: It's gonna go, it's gonna go, it's gonna go, it's gonna go, it's gonna go.

WOMAN: Whoo!

(people talking in background) Dive angle?

Dive angle 110.

WOMAN: One-one-zero (inaudible).

Time: 10:07.

DE VOS: And you were so excited when you saw the blue whale, and then when you saw, you know, it skimming under the surface, and you saw that beautiful, um... That blue color.

..blue color.

Turquoise, mm-hmm.

But wait till you see the poo.

I cannot wait to see your face, I bet you.

Mm.

(people talking in background) BURNETT (voiceover): We get a distant glimpse of surface feeding.

There's a wealth of marine resources in these warm waters for the whales.

MAN: Whoo!

DE VOS: We've been seeing more feeding this year than any other year, really.

Mm.

Which is fantastic.

Mm.

But the real mystery is, where is all that feeding going?

What happens to it?

Why is it not coming out of the back?

Mm.

Where's our poo?

Right?

(chuckling) ♪ ♪ BURNETT (voiceover): The whale leaves no trace.

But Asha doesn't disappoint.

She's defrosted samples no longer needed for research.

Not only will I get a chance to see it up close, but to experience just how difficult this scat is to collect.

DE VOS: You have to be pretty much there when it happens.

BURNETT: Mm.

Quickly get the net out, grab as much as we can.

Um... BURNETT: It doesn't last long in the water.

It really is red gold, isn't it?

Gold of the ocean.

(De Vos murmuring) I'm complete as a scatologist.

♪ ♪ DE VOS: This is where the science happens, because when we start trudging through the data, that's when we start to see the trends, that's when we start to recognize individuals, and that's when we start to build the stories around these animals.

In Sri Lanka, our blue whales fluke up before a deep dive more often than anywhere else in the world, maybe, like, 60% of the time.

So this is lifting their flukes out of the water..

Exactly.

DE VOS: And for a long time, we tried to figure out what that was, what was the reason.

And what's really cool is, we've kind of figured that out by digging through their poop.

♪ ♪ BURNETT (voiceover): A piece of prey DNA from a scat sample was analyzed.

What they found showed just how unusual the Sri Lankan blue whales really are.

Everywhere else in the world, blue whales would feed on krill.

Mm.

Here, they're feeding on a particular kind of shrimp that is actually deeper in the water column.

And so they do have to get deeper.

It makes a lot of sense that you want to make yourself, turn yourself into a kind of a torpedo to descend.

(whale calling) And so, you know, the poo's sort of been this really big clue into not just understanding, you know, what are they feeding on, but how does that impact their life cycle, and their life history and their behaviors, which I think is really neat.

♪ ♪ BURNETT: After three days on the boat with Asha, I have my sea legs.

Now biologist and underwater cameraman Simon Enderby has invited me to see a blue whale beneath the water.

He won't be letting us jump in unless it's completely safe for the whale and for me.

But our encounter today is with something much bigger than a whale.

(ship horn blaring) There's been a flotilla of whale watching boats following this whale and her calf all morning this morning, really harassing it, and they've chased her out into the shipping lane, which is where we're sitting now.

It's dangerous out here for them.

Strikes from propellers of these big cargo ships kill whales.

I had hoped to get into the water, you know, sensitively, ahead of the whale, and let her swim past me, but I'm feeling quite distressed at what I've witnessed here this morning, and I don't really feel like adding to any of the stress that these whales are obviously feeling.

This reminds me, it really drives home why it's so useful and so, such a powerful tool to use scats to understand the ecology of animals-- whales or animals on the land.

It allows us to get into their lives without being in their faces or invading their personal space.

(voiceover): Scat helps us to understand whales, and it may also help fertilize the oceans.

Here's how we think it works.

Migrating blue whales head to Antarctica in summer to feast on iron-rich Antarctic krill, then they poop.

The iron in their poop may feed microscopic phytoplankton, which generates a large percentage of the planet's oxygen.

Phytoplankton are the favorite food of krill, which in turn are eaten by whales.

And on it goes.

It's unlikely that whale scat alone is sufficient to feed the world's phytoplankton.

But there's another, not so obvious poop in the loop: the scat of the Antarctic krill itself.

Could it be providing the iron that phytoplankton need?

♪ ♪ Unfortunately, that's not me on deck.

Scientists from the Australian Antarctic Division are investigating the biology of krill.

Doing research in the Southern Ocean is an enormous challenge, so they've figured out how to breed this little crustacean in captivity.

Krill ecologist So Kawaguchi has helped develop a world-class aquarium dedicated to krill research.

Today, he's invited me to watch the pooping of the krill.

They look like giant icy poles, don't they, in those plastic bags?

(laughing): Yes.

So these are the phytoplankton isolated from Antarctic Ocean, so we culture these for, as a food for krill.

Are we going to have a chance to feed some krill here today?

Yeah, of course.

I'm looking forward to that.

I'm imagining sort of the water boiling like a pool full of piranhas.

(Kawaguchi laughs) KAWAGUCHI: You don't really want to pour it, you know, from the, the pot.

You just gently, you know, submerge it and then release it into the, the water.

Okay.

Yeah, that's it.

Oh, they're very reactive, aren't they?

Yes.

They're straightaway-- they're even going into the bucket.

This is exciting.

Yes.

They're beautiful little animals, although not even that little.

I mean, they're...

I'm surprised at the size of them.

KAWAGUCHI: They're really cute when they, you know, come up and see you, because they have really big black eyes, you know?

BURNETT: Mm.

And yeah, it's just really nice to watch.

Mm.

Yeah, you know.

♪ ♪ KAWAGUCHI: For me, it's the most important animal on the planet.

Because, you know, the amount of krill that we have on the planet is enormous.

Like, it weighs almost equivalent to the total weight of the human being on the planet.

♪ ♪ You know, when they poo, they poo a lot.

BURNETT: Researchers estimate there are over 400 million tons of Antarctic krill.

The Australian Antarctic Division recently recorded a super-swarm made up of hundreds of millions of them.

There are records of super-swarms several kilometers long.

Imagine all that scat!

30 minutes after feeding, So's krill begin to poop.

The delicate scat threads are easily fragmented by the constant motion of their tiny legs.

At a microscopic level, when the pellets break up, a cloud of nutrients is thought to be released.

What the nutrients are and how they are used are questions that researcher Abigail Smith wants to answer.

My field is trace metal chemistry, and that's a really important field in the Southern Ocean, where we have such low concentrations of iron.

So we're talking, like, nanomolar level.

Really, really low.

Sources of iron down there often come from things like melting sea ice.

When ice is melting in spring, you get this sudden input of iron, which stimulates this big phytoplankton bloom.

But once those sources have pretty much been exhausted, we need something that recycles and maintains the iron supply.

And that's where krill come in, and we believe that through their excretion and their defecation, they're actually putting iron back into the water.

BURNETT: And that iron may be fertilizing phytoplankton in a continuous cycle.

Krill ingest iron when they eat phytoplankton; it's then excreted in their scat.

Bacteria break down the scat, liberating the iron in a form that the phytoplankton can absorb.

But it's difficult to know how much iron is involved in the process.

SMITH: It's not something that we can measure very easily in the field, you know, going down to Antarctica.

You can try and measure the iron, but the iron's taken up so rapidly that, you know, you really have to be there on the spot to measure any influence.

BURNETT: As a step to finding out how much iron is available for the phytoplankton to take up, Abigail conducts a controlled experiment in So's lab.

She'll measure the time it takes a scat to travel from krill bottom to ocean bottom.

(inaudible) So you can see it coming down there now.

SMITH: The speed that it's dropping is gonna be really important for how much of the nutrients are able to leach out of the fecal pellet.

Mm-hmm.

As soon as it drops out of the upper ocean, it's sort of exported to depth where phytoplankton can't access it.

BURNETT (voiceover): After over 50 of these experiments, Abigail found the sinking rates are variable.

Some scats stay around, others sink fast.

Our pellet was one of the latter.

The bacteria and phytoplankton would need to act fast to take up the iron.

But all is not lost, because even a fast-sinking scat has a gift for the planet.

It can remove carbon from the atmosphere.

Krill ingest carbon from phytoplankton and excrete it in their scat.

The sinking scat transports millions of tons of carbon a year to the depths.

The Southern Ocean is critical for controlling how much CO2 is in the atmosphere.

The scat of this unassuming little crustacean may be one reason why.

Right now, below the surface, hundreds of millions of tons of krill are pooping out torrents of iron- and carbon-rich pellets.

That's the real power of poop.

♪ ♪ My journey has come full circle, back to the ancient rain forests of the Wet Tropics, home to the southern cassowary.

We want to find out how important the big bird's scat is to the future of those forests.

It will require patience.

We must wait for a cassowary to answer the call of nature.

♪ ♪ (birds chittering) I'm going to perform a cassowary pooping experiment with colleague Andrew Dennis.

To stay safe and let the bird get on with its business, we'll remain a captive audience for as long as it takes.

There's a lovely bowl of fruit there for you to eat.

Come on... BURNETT (voiceover): This is just one experiment Andrew performed for a comprehensive seed dispersal survey.

(murmuring) DENNIS (voiceover): We've done a large project over six years looking at the entire process.

We looked at 65 different vertebrate animal types and about one-and-a-half thousand plants that have fleshy fruit that are moved by those animals.

It became very clear that cassowaries provided a very unique service within the rain forest.

BURNETT: We want to find out how efficient the cassowary is at dispersing seeds.

To do that, we'll time how long it takes for a seed to travel from mouth to bottom.

We offer a selection of fruits, including the cassowary plum and the blue quandong.

(birds chittering) ♪ ♪ BURNETT: Four, five.

DENNIS: Yes.

Eight.

Nine.

A good feeding day.

Yeah, yeah.

(birds chittering) BURNETT: Yep, 14.

DENNIS: 14, yep.

♪ ♪ BURNETT (voiceover): We expect the digestion process to be relatively swift, because this giant bird has a very simple gastric system.

It has short intestines, a stomach that gently massages the pulp from the fruit, and a unique combination of enzymes that allow it to digest the fruit while leaving the seed intact.

(plopping) ♪ ♪ (birds squawking in distance) (plopping) Ooh.

Oh, cool.

There's one.

Yep, I got that.

Time?

Yep.

It's, uh...

Ten to 10:00.

BURNETT: Ten to 9:00, I mean.

DENNIS: 51.

Yep.

All right, well, looks like Airlie's done his job.

Time to get him back into his pen.

BURNETT: Mm, looks like...

I'll get these two scats.

Yes, I had 14.

Yep, and I got 18, so we got 32.

32.

So three-and-a-half hours ago, that bird ate 73 quandongs.

Often, in three-and-a-half hours, cassowaries will move between one and two kilometers.

Mm, so it seems quite a quick passage rate...

So... ...but still, just the mobility of those birds means that these seeds are getting... Can go quite a long way.

...kilometers away from the parent tree.

Yep.

BURNETT: The distance the cassowary travels is important.

Seeds have a better chance of survival when they're transported away from the parent tree.

If the fruits stay where they drop, they're often eaten by seed predators or infected by fungi.

Andrew's study found there are many successful seed dispersers in these forests, but no other animal can transport large seeds great distances.

♪ ♪ Thanks to the cassowary, hundreds of species of plants in these ancient rain forests remain diverse and abundant.

This marvelous scat grows forests.

Given the area of habitat that's available, the cassowary population is stable, but threats remain-- like the effects of climate change.

DENNIS: Cassowaries prefer lowland coastal rain forests.

There's modeling now that shows how much of that rain forest is going to be inundated by storm surges from cyclones or just plain sea level rise.

So what we're now expecting is that cassowaries will have to start finding new areas to live and disperse seeds.

The trouble they're facing is that the lowland areas in Australia have been fragmented.

A lot of the forest has been cleared for agriculture, and cassowaries can't move through.

♪ ♪ BURNETT: The only way cassowaries will have enough habitat is if we plant corridors of trees that connect with larger tracts of inland forest where they can find food.

(Dennis murmurs) BURNETT (voiceover): Now we are becoming the forest gardeners.

These are a massive bird (bleep), aren't they?

(chuckling): Yeah.

♪ ♪ BURNETT (voiceover): Volunteers have raised over 100,000 trees from the seeds in cassowary scat and planted them to create cassowary corridors.

♪ ♪ This seedling came from a cassowary scat.

We've taken their trees.

Now we're trying to give them back, hopefully in time to make a difference to all of our futures.

♪ ♪ Scat just keeps on giving.

♪ ♪ This is fun science, but that's not the main reason we do it.

Scats really do make the world go round.

And they unite the world, as well.

They unite us through our, our natural inquisitiveness and the sense of wonder that we have at nature and the natural processes that sustain this planet.

♪ ♪ (engine starting) ♪ ♪ ANNOUNCER: To order this program on DVD, visit ShopPBS or call 1-800-PLAY-PBS.

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"NOVA" is also available on Amazon Prime Video.

♪ ♪ ♪ ♪