♪ NARRATOR: Mozambique's Gorongosa National Park, an African wilderness once ravaged by war.

(nickers) In the three decades since the fighting ended, many species' numbers have rebounded.

(rumbling) DOMINIQUE GONÇALVES: It's fascinating how nature works.

Now that it's coming back, to see this difference, I have to say, it's amazing.

(birds twittering) NARRATOR: But things are not as idyllic as they look.

ROB PRINGLE: For me, as an ecologist, things just seemed off.

Everybody's on top of everybody else.

Gorongosa is just a mess.

NARRATOR: The question is: why?

♪ Scientists had a hunch that there were two vital elements missing from the park: large predators and the behavior-changing fear they induce.

PAOLA BOULEY: When you remove predators from the story, things start to unravel.

NARRATOR: So, in a bold plan to bring fear back to Gorongosa, rare African wild dogs are being set free in the park.

Will they survive the relocation?

And can the science of fear help turn the tide, transforming a damaged park into a healthy wilderness once again?

(nearby snarl) "Nature's Fear Factor."

Right now, on "NOVA."

♪ NARRATOR: Nature: beautiful.

Tranquil.

Peaceful.

(zebra yelping) Also violent.

Unforgiving.

(deer bleating) Deadly.

A world seemingly ruled by a simple law... (animal squeaking) Eat and be eaten.

♪ But recently, scientists are seeing that nature's predators add another dimension to that picture.

Something instinctive.

Pervasive.

(sniffing) That something... ...is fear.

(bird screeching) ♪ TONECAS PAULO (speaking Portuguese): ♪ NARRATOR: New evidence about the role fear plays is changing the way scientists look at the world.

LIANA ZANETTE: How fear functions in nature was completely unknown, and now we're beginning to understand that it is actually a powerful force.

NARRATOR: Just how powerful is being put to the test at one national park in Mozambique, where a bold experiment is underway: bring back top predators to see if the fear they spread can push the ecosystem towards a better balance and help rescue a wilderness fighting back from the brink.

♪ PRINGLE: There's urgency-- there's definitely urgency.

We want to really get under the hood of the ecosystem and figure out what's going on in a way that just wasn't possible before.

♪ (device beeping) NARRATOR: Cruising through the clouds high over the African continent, a charter plane is on course with some unusual passengers.

The smell is overpowering.

And the floor is carpeted with some of the most effective and prolific killers in the animal world-- 14 African wild dogs.

Heavily sedated, they're on a 500-mile journey from a reserve in South Africa across the border to Mozambique, and their new home: Gorongosa National Park, a protected wilderness on the rebound after a tumultuous past.

♪ Nice!

NARRATOR: Waiting to greet them are António "Tonecas" Paulo, the park veterinarian, and Paola Bouley, Gorongosa's resident carnivore expert.

(indistinct chatter) NARRATOR: By this time, the wild dogs have been sedated nearly 12 hours.

They're a tough bunch, but it's important to move quickly, and get them safely to their prepared enclosure.

African wild dogs haven't been seen in Gorongosa for at least 30 years.

But this park was once a familiar hunting ground.

BOULEY: Wild dogs, otherwise known as painted dogs, painted wolves, have evolved on this continent for about a million and a half years.

So they split off from dogs and wolves a long time ago.

There's nothing like them.

Okay?

NARRATOR: There are fewer than 7,000 African wild dogs left on the entire continent.

Tonecas, the park's top wildlife vet, is in charge of keeping the precious animals healthy.

PAULO (speaking Portuguese): He's heavy.

NARRATOR: The females and males in this group of 14 come from different packs.

When they wake up in this strange place, there's no telling how they will react to each other.

But knowing that African wild dogs rely heavily on their sense of smell, the team has adopted a clever way to help defuse potential conflict.

BOULEY: This is a bonding method.

By rubbing their scents on each other, the idea is that when they wake up, they're going to be less aggressive towards each other.

And it's been shown to really work well for wild dogs.

This is just to make this transition a little smoother for them.

♪ NARRATOR: The new arrivals will be kept in this enclosure-- called a boma-- for eight weeks.

Enough time, Paola hopes, for the strangers to team up and become Gorongosa wild dog pack number one.

BOULEY: Just to get these dogs here was an immense project.

We're going to be learning a lot in the next few months.

This is historic, and it could be so important for the ecology of this park.

(chittering) NARRATOR: If all goes well, Gorongosa may provide a toehold for this endangered species.

That, in itself, would be a victory for conservation.

(quietly): How cool is that?

NARRATOR: But these wild dogs have been hand-picked for an even larger purpose: to see if they can help correct a growing imbalance in the park's ecosystem-- one caused by its tragic history.

♪ Gorongosa is still trying to recover from a brutal war that broke out in the 1970s.

For over 15 years, fighting between government troops and opposition armies raged across Mozambique, devastating the country.

The human death toll has been estimated as high as one million.

Much of Gorongosa's 1,400- square-mile wilderness fell under rebel control and was the scene of fierce fighting.

(helicopter blades whirring) Once one of the most celebrated and species-rich national parks in Africa, by the time the smoke cleared over 20 years ago, less than ten percent of Gorongosa's large mammals had survived the violence and poaching.

Aerial surveys returned some startling estimates: the elephant population dropped from around 2,500 to about 250.

Hippo: 3,500 to less than 100.

Lions: 200 to perhaps ten.

Other animals fared even worse: 3,500 zebra gone.

6,500 wildebeest gone.

14,000 buffalo nowhere in sight.

The devastation was so complete, many believed Gorongosa was finished as a wilderness.

♪ But there was one reason for hope.

The extraordinary landscape was still intact.

BOULEY: What makes Gorongosa really special is the diversity of habitats.

You can go from savanna woodlands to miombo woodlands, to open floodplain to lake to riverine, to gorge and mountain rainforest.

And you have that mosaic, that complexity of habitats, that really fosters diversity.

NARRATOR: A public-private partnership formed the Gorongosa Restoration Project to try and salvage the park, recruiting an international team of scientists to lead the effort.

PRINGLE: When the Gorongosa Project started, it was basically starting from scratch.

The first thing was the recovery of the species that managed to survive, but in very low numbers.

♪ NARRATOR: Animals that survived the fighting were finally free to return to their wild ways, while some species decimated by the war, like eland, were brought in from elsewhere.

(dogs chittering) The 14 African wild dogs getting to know each other in the boma are the latest-- and in some ways, most challenging-- of the reintroductions.

PRINGLE: How many collars do you have?

We have 13... NARRATOR: Princeton ecologist Rob Pringle is one of the scientists working with Paola and Tonecas to make the park a healthy wilderness once again.

Yeah.

Actually, I was saying to Tonecas, so it would be great to get some scats while they're still in the boma.

NARRATOR: Rob and other ecologists are carefully tracking the behavior of Gorongosa's plant-eaters... (shoots dart) ...hoping to better understand their role in the complex web of animal interaction.

(indistinct chatter) PRINGLE: It's a living laboratory where we can bring our science out into the field and try to figure out the rules that ecosystems work by.

(rotor blades whirring) NARRATOR: One thing is clear about Gorongosa's herbivores: peace is giving them a chance to recover.

In fact, their total number is approaching pre-war levels.

To the untrained eye, it looks like paradise.

But for Rob, there have been signs that the recovery may be veering out of balance.

PRINGLE: For me as an ecologist, the first time I came to Gorongosa, things just seemed off.

The wildlife is recovering, but the abundance of different species is vastly different than it was before.

NARRATOR: Decades of research reveal the most stable ecosystems are those with a rich diversity of animal life, each species finding its place in relative balance with the others around it.

(warthog snorting) But in Gorongosa, that's not exactly what's happening.

Some species are recovering far more quickly than others.

The numbers of one species in particular, waterbuck, are absolutely exploding, with 60,000 roaming the floodplain and beyond-- more than ten times their number before the war.

The ecologists are trying to figure out what's driving this imbalance.

Three, 20, 12.

PRINGLE: One of the things that I'm most interested in is trying to understand what we call species interaction.

This question of how species coexist has fascinated ecologists for decades, and the answers aren't easy to figure out.

NARRATOR: Understanding an ecosystem this complex demands exhaustive studies comparing the sizes, numbers, and behaviors of wild animals, analyzing what they eat and even what they leave behind.

We actually have nice, fully formed pellets.

NARRATOR: Each new detail adding to the picture.

One species' strange behavior has caught the particular interest of the team.

(radio static buzzing) PRINGLE: You got it?

JUSTINE ATKINS: Do you hear it?

It's very faint.

NARRATOR: Rob and Justine Atkins are on the hunt for a famously skittish antelope called a bushbuck.

Its tracking collar emits radio signals picked up by a hand-held antenna.

Okay.

It's really strong now.

Maybe, like, 1:00.

Okay.

♪ It keeps coming in and out, so I wonder if he's, like, on the mound or something.

He's probably behind something.

(insects chirping) Actually, maybe straight ahead.

They give you the slip.

They, like, sit on top of a termite mound, and when you approach from one direction, they basically slide down off the back side.

Yeah.

It's super-annoying.

(insects chirping) Sounds like he's back behind us.

Behind us again?

Yeah.

Okay, it's very strong right now.

Yup, there he goes, there he goes, there he goes!

Let's see... Oh, yeah!

There he is!

And right up to a termite mound.

Onto a termite mound, oh, nice.

Oh, brilliant.

Okay, that, that's excellent.

(insects chirping) NARRATOR: This is more than just a game of hide and seek.

PRINGLE: Oh, yeah, that'll work.

NARRATOR: Rob and Justine are rewarded with a fresh morsel of evidence for one of their Gorongosa investigations-- some precious bushbuck scat.

This elusive individual is living up to its reputation-- hiding in the bush.

But the team has been surprised to see other bushbuck behaving much differently-- throwing caution to the wind, venturing far from their forest habitat onto the open floodplain.

PRINGLE: That struck us as very strange to see bushbuck out in the middle of the wide-open plains.

We started calling them "plainsbuck" because they were no longer in the bush.

So when we see animals acting differently than we expect them to, the first thing you have to do is try to figure out why.

What is this telling me about how the rules of the ecosystem have changed, and what does that mean about how we manage the ecosystem?

♪ NARRATOR: Rob had a hunch as to why bushbuck were leaving the forest, and thought it might connect with a much deeper issue facing Gorongosa: its relative lack of large predators.

(snorting and growling) In Africa, lions, hyenas, cheetahs, leopards, and wild dogs make up what ecologists call "the large predator guild."

Gorongosa once featured nearly the full complement, but the years of war spared only a handful of lions.

♪ Hyenas, wild dogs, and leopards were either killed or forced out by lack of prey.

Gorongosa was a wilderness with no sharp teeth.

PRINGLE: A bunch of things that we were seeing in Gorongosa seemed like signatures of missing predators and just animals behaving in ways that indicated that they had lost their fear.

NARRATOR: The fear Rob refers to is the fear all animals have of becoming someone else's dinner-- perhaps the most basic instinct in the animal world.

Until recently, fear's role in nature has been largely underappreciated.

♪ It was thought a predator's impact could be easily measured: simply count the number of animals killed and subtract it from the overall population.

ZANETTE: That's the traditional view of how predators can affect prey.

They kill them, they eat them, they eat their offspring.

But focusing only on killing, as we have been doing in the past, we are greatly underestimating the total impact that predators might be having out there in nature.

♪ NARRATOR: The successful re-introduction of grey wolves to Yellowstone National Park 25 years ago led more ecologists to attribute another importance to predators: the power to shape the behavior of many while ending the existence of a few.

Scientists now classify some healthy ecosystems as "landscapes of fear"-- when the wariness predators inspire in prey has an impact beyond what they kill and consume.

(wolves howling) OSWALD SCHMITZ: The landscape of fear is a disarmingly simple idea that seems to be applying broadly.

NARRATOR: Oswald Schmitz of Yale University studies fear in controlled settings, using smaller and much less exotic species than wild dogs.

His breakthrough experiments with spiders and grasshoppers were among the first to reveal the surprising effects predators can have on ecosystems.

So, there was this assumption that the predators don't kill enough to actually have a huge impact on the populations of these animals.

NARRATOR: Schmitz set up several boxes to compare grasshopper behavior-- with spiders and without.

Those confined together for a few weeks seemed to prove the old assumptions were true.

Even though the grasshoppers were eaten by spiders, their overall numbers initially stayed the same as new ones were born.

But, Schmitz found, spiders did affect the grasshoppers' behavior in significant ways-- where the insects grazed and what they ate.

And the interesting thing that you find is that these grasshoppers change their diet in the presence of the predators.

NARRATOR: The grasshoppers living predator-free feasted on nutritious grass.

But when spiders were brought in, the hoppers moved up into some goldenrod, which provided them with better cover but less nutritious food.

Fear acted as a kind of force field, pushing players around the landscape, changing the game.

SCHMITZ: When I study grasshoppers and spiders and what they're doing, I'm not just studying grasshoppers and spiders.

I'm really trying to understand the tradeoff game that they're playing: the tradeoff between feeding so that you survive and reproduce well and avoiding predators.

NARRATOR: That predator/prey dynamic observed in small-scale, tightly controlled experiments is now being put to the test in wild settings with large mammals.

PRINGLE: Some of the best studies of the landscape of fear are with relatively small animals that you can keep in laboratory conditions.

It's been much more of a challenge to try to scale that understanding up from the laboratory to the real world at the very largest scales.

And that's one of the things we're trying to do in Gorongosa.

♪ NARRATOR: In preparation for the release of the wild dogs, Justine Atkins wants to know whether the prey species' fear responses are still active after so many generations with few predators around.

(insects chirping) She sets up an experiment using the sounds of another top carnivore displaced by the war: leopard.

ATKINS: What I'm doing right now is going to set up a speaker system out on the floodplain, and what that's going to do is play the sounds of leopards, actually.

And what we're interested in seeing is whether or not the bushbuck respond, and whether or not they change the habitat that they're using.

NARRATOR: Experiments with sound are a common tool in landscape-of-fear studies.

Scientists use them to test how prey will react to predators without needing the predators to actually participate.

As a control to see whether the antelope are simply bothered by strange sounds, some nights, the speakers are programmed to play nothing but static.

(static playing) But on other nights, the leopard calls sound intermittently through the darkness... (leopards growling) Using GPS signals from collared bushbuck, Justine is able to remotely witness their reactions over the course of 48 hours.

(mouse clicking) The movements of one individual clearly reveal her overall findings.

When the static was broadcast, the bushbuck seemed unfazed by the speaker.

But when the leopard sounds played, it was a different story.

The speaker was in the center of the home range, right?

ATKINS: Yeah, and these blue points are showing where the individual was before I deployed the speakers.

And then what's showing in the red are the locations where the bushbuck was while the sounds were playing, while the leopard calls were playing, at night.

Wow, that's really dramatic.

So, it's completely avoiding this whole area in the vicinity of the, of the leopard noises.

Yeah, it's pretty striking, I think.

Yeah, and this is what we saw with all our bushbuck.

When we, when we played the leopard calls, they moved farther away from where the speaker was, and basically just avoided using that at all.

(bird calling repeatedly on recording) NARRATOR: Other ecologists studying fear in the wild have run similar experiments.

Liana Zanette has used audio cues and camera traps to test the reactions of prey species to predator sounds in a South African reserve.

(bird calling continues) While bird calls are ignored... (lion growling on recording) ...lion growls not so much.

(growling continues) The odd chirpings of wild dogs set off similar alarms.

(dogs whining on recording) If just the sounds of a predator can change behavior, the team is optimistic about what the real wild dogs will do when they are unleashed in the park.

As the weeks pass, lead veterinarian Tonecas Paulo monitors the wild dogs' behavior in the boma 'round the clock, and treats them to regular helpings of the local cuisine.

They don't have to hunt for their food yet, but carnivore director Paola Bouley sees clear signs that the newcomers are organizing themselves into a pack.

And that an alpha male and female are emerging as pack leaders.

Oh, look, mating.

Or, at least, they're trying.

But you can see something is starting there.

PAULO: Yeah.

NARRATOR: After seven weeks, it becomes clear that the alpha couple has been doing more than just trying.

The alpha female, named Beira, shows signs of being pregnant.

If Beira delivers her pups in the boma, the pack won't leave for months.

The time has come to set them free, with hopes Gorongosa will provide a good home and the wild dogs will inject a healthy dose of fear back into the landscape.

Are you ready?

Okay?

Yes.

NARRATOR: This last free meal will be the one to lure them out the gate.

(speaks softly) Yeah.

Let's go!

Let's go!

Go!

Go!

BOULEY: At this point we feel a little stressed out, but also excitement.

Go!

Oh, wait!

Wait!

Wait!

Wait!

(speaking Portuguese): ♪ BOULEY: It's all about the perception of risk.

It's, like, "I really want that food, but, you know, do I trust the situation?"

BOULEY: One's out.

Uh-huh, here we go.

BOULEY: Dogs don't exist solo.

They need each other, so the pack is essential.

Once you open that gate, can things fall apart?

Go!

BOULEY: You know?

Will the pack just, like, disintegrate and disperse and break into smaller units?

That can happen.

BOULEY: Okay... PAULO: Okay!

BOULEY: Go, go, go, go, go!

(The Who's "I'm Free" playing) ♪ ♪ I'm free ♪ I'm free ♪ And I'm waiting for you to follow me ♪ ♪ NARRATOR: Beira, the pregnant alpha female, is the last to join the party.

BOULEY: She's out!

All right!

(laughing): That's amazing.

♪ ♪ I'm free ♪ Oh, I'm free ♪ And I'm waiting for you ♪ To follow me ♪ NARRATOR: Now the experiment truly begins.

The hunters are free to roam in a living laboratory as big as the great outdoors.

Several of the wild dogs have radio collars, but the pack is essentially on its own.

And no one really knows what will happen.

BOULEY: What we do know about Gorongosa is that we have a lot of prey.

But the question is, as a pack-- 'cause they hunt together-- can they successfully hunt?

Can they successfully breed, have pups, and raise those pups, and hunt for those pups?

Those were the milestones that we were monitoring.

Then the other dark thought that crosses your mind-- and this is so common in other places-- is, dogs tend to stray into communities, and we are unfenced.

So, you can imagine when wild dogs stray across the river and into communities, what kind of chaos that can cause, right?

NARRATOR: It's not just a hypothetical problem.

The rural areas surrounding the unfenced park are home to some 200,000 people, many of whom depend on farming for survival.

But their hard-earned crops also prove irresistible to one large and headstrong species from the park.

(rumbling and trumpeting softly) In the southern part of our park, in our border, is just a river, which is not really a limit or border for elephants.

NARRATOR: Dominique Gonçalves heads up Gorongosa's elephant ecology program.

Oh, there he is!

NARRATOR: She's in charge of helping Gorongosa's elephant population recover and thrive.

GONÇALVES: He does not have a collar.

NARRATOR: Since the war, their numbers have multiplied to more than 650, a promising rebound.

GONÇALVES: Very relaxed.

I'm not sure he's aware of our presence.

Where is the wind going?

NARRATOR: But keeping the elephants within the unfenced park is a constant battle.

This way.

NARRATOR: Elephants have plenty to eat in Gorongosa, but some-- especially males looking to bulk up-- go after the tastier, calorie-rich crops grown around the nearby villages.

For the farmers who depend on their fields for survival, elephants can be a terrifying threat to their lives and livelihoods.

(yelling in world language): MAN: NARRATOR: When the giants approach, alarms sound.

Rapid response teams come to defend the crops.

MAN (speaking world language): NARRATOR: Gorongosa ranger units assist the locals, supplying them with tools-- such as fireworks-- to help frighten the elephants out of the fields.

(fireworks popping) (elephants trumpeting in distance) (fireworks boom, elephant trumpets loudly) But how to keep the bulls from eating and trampling the crops in the first place?

With Gorongosa's help, some farmers began building fences at key crossing points along the river, fences specially designed to spread fear among the giant intruders.

It's fascinating how nature works.

As people and elephants are afraid of each other, elephants also afraid of other things-- small thing, which is bee.

(buzzing) NARRATOR: To make the crops across the river from the park more trouble than they're worth, the fences are booby-trapped with beehives.

GONÇALVES: There's a wire connecting them all, and as the elephant try to cross, that shakes all the bees, because they're all connected.

And then they come out and attack the elephants.

African honeybees are very aggressive.

Their sting is really painful.

The elephants also have sensitive parts on the body; the eyes, the trunk, back of the ears, so just that pain and also the noise, it's a natural reaction that they have to avoid bees.

(bees buzzing) Using camera traps, we could see the elephants trying to cross, shaking, and the beehives coming, and they're just turning back to the park.

It's working with nature and just understanding the system better to stop them, to put more fear in elephants so they don't go to places where people are.

(wildlife chittering) NARRATOR: Back on the floodplain, Rob thinks the brazen bushbuck might have something in common with the crop-raiding elephants: that they are also leaving their comfort zones in pursuit of more nourishing food.

But to test the idea, Rob needs to know what these "plainsbuck" are eating.

PRINGLE: Some impala pellets here.

(voiceover): Surprisingly, we don't know a lot about what these animals actually eat in the wild.

How do you figure it out?

You can try to do it by just watching the animals.

That's more problematic than it might sound, because identifying plants is really tricky.

Just in the floodplain behind me, you know, there are hundreds of plant species, and some of them are coexisting side by side.

Even for somebody with a PhD in botany, it can be very difficult to figure out what is this plant species' name, much less for, you know, an ecologist who mainly studies antelope.

NARRATOR: It turns out the surest way to know what goes into an animal is to sort through what comes out.

MATTHEW HUTCHINSON: Okay, Gorongosa... NARRATOR: Over the years, more than 3,000 fecal samples have been collected in the park.

The droppings produced in Gorongosa end up 8,000 miles away, in Princeton, New Jersey.

Here, they are put through a process called DNA meta-barcoding.

PRINGLE: When the herbivore eats food, digests it, it doesn't digest everything.

It leaves behind quite a lot of DNA of the food that it ate.

NARRATOR: This deep dive into Gorongosa dung is a key step in understanding the eating habits of the park's herbivores.

HUTCHINSON: So right there in this little drop of liquid is all of the DNA that we've extracted from our fecal sample.

In that DNA is encompassed everything that the bushbuck had been eating for the last couple of days.

NARRATOR: Once the DNA molecules are extracted, they're sequenced and compared with a database to decipher exactly which plants the animal has ingested.

And so, we're able to sequence that DNA and identify the plant species so that we have an idea of how much of a given food that animal had been eating.

NARRATOR: The Pringle lab data reveal that the bushbuck on the floodplain are eating better than their counterparts in the forest.

PRINGLE: The bolder individuals who were more willing to take risks were reaping this huge reward of being out in the floodplain, growing large, and presumably having lots of babies.

NARRATOR: With few predators around, what's to stop them?

The more Matt Hutchinson studies the data, the more he sees that bushbuck aren't the only ones in Gorongosa behaving strangely.

Matt isolates the eating habits of the park's main herbivore species.

Then he compares them with data from a similar but more stable ecosystem in Kenya.

The Kenyan herbivores have distinct and separate eating habits.

Each species has established a specialized diet and is sticking to it.

HUTCHINSON: In Kenya, there's a very clear separation between who eats grass and who eats trees and shrubs, which is more or less what we would expect in a mature ecosystem with separation of diet.

Each species got a very characteristic diet that doesn't really overlap that much with the other ones.

NARRATOR: But Matt's data from Gorongosa reveal a different story.

This Gorongosa pattern is really exceptional, because their diets are running together, they're overlapping, and everybody's out of their lane and colliding with each other.

Gorongosa is just a mess.

Everybody's on top of everybody else.

NARRATOR: Gorongosa's overlapping eating patterns are more than just a curiosity.

Several herbivore species pursuing the same plants sets up a situation of all competing against all: an ecosystem out of balance.

PRINGLE: The big glaring difference between Gorongosa and all these other places is that Gorongosa hasn't had big carnivores until just recently.

When there's no landscape of fear, that is enabling the prey to basically go everywhere in the landscape and eat everything, and the conventional lanes and boundaries that we see between species in other ecosystems have broken down.

(animal shrieking) NARRATOR: And that's where the African wild dogs come in, to help re-establish order among species that have been feeding and breeding without restraint.

But for that, they're going to need reinforcements.

Paola and Tonecas keep daily tabs on Beira's GPS collar data as she travels through the park.

They know she's searching for a place to establish a den, where she can give birth to her litter of pups.

It isn't long before they get a clue.

Beira's GPS collar stops transmitting for a few days, then several data points pop up all at once and close together.

BOULEY: The collar disappears, so it stops pinging, because underground, it just can't connect with the satellite.

But as soon as she emerges, the data downloads, and we see a cluster.

So, you see that she's concentrating in one area, and that's a strong sign of a den.

NARRATOR: But within days, Beira's GPS signal indicates trouble.

She seems to have abandoned the den.

At some point, she left and never went back, and it's, like, okay, what happened here?

'Cause it's too soon, you know?

They spend months on a den.

And we began searching for evidence as to why.

NARRATOR: A camera trap placed near the den entrance captures Paola as she discovers the cause of the problem.

I wanted to go check out what was going on.

But I was thinking, maybe we could fish out a little bit more evidence to suggest that the pups were in fact in that den.

And when I stick my head in there, I kind of hear something drop out of the trees on top of me.

(gasps) (voiceover): And boom, I'm face to face with this big snake that just goes straight into the den.

NARRATOR: What had briefly been the wild dogs' den was now home to an African rock python.

BOULEY: And unfortunately, all evidence suggests the python took the pups.

Yeah.

Dogs have no defense against a snake like that.

NARRATOR: Fortunately, Beira, the alpha female, survived.

It's a disappointing setback in the effort to re-establish wild dogs in Gorongosa.

But out here, death comes with the territory.

BOULEY: That's natural.

In nature, lions, snake, fire gonna impact reproduction for dogs.

As long as it's natural, we let things run its course.

It's the human factor that we want to remove from the, from the situation.

NARRATOR: As they explore their new environment, the wild dogs encounter an even bigger threat than snakes: the one large carnivore species that barely scraped through the war.

(lion growling softly) In this case, the wild dogs' fear response kicks in.

(lions growling loudly) Certain parts of the park are better avoided.

BOULEY: Dogs don't mix with lions.

I mean, lions will kill dogs, and so they tend to avoid lions.

(lion growls) NARRATOR: As fearsome as they are, lions have not been a strong presence in Gorongosa until recently.

When Paola first came here eight years ago, the estimated lion population was somewhere around 40.

Thanks to a massive recovery effort, that number has risen to about 150.

(lion sniffing) But even with the lion's amazing rebound, the fear they spread is limited to areas well-suited for their ambush-style of hunting.

BOULEY: So, this is your classic Gorongosa lion habitat.

This is where lions like to make their kills, in this thick, tall grassland that sits at the margin between the floodplain and the woodlands.

It's a perfect area to hide and ambush prey.

So, a warthog or a waterbuck making its way from the open floodplain into the woodlands has to pass through this tall grass.

And this is where they get snatched by lions.

And so, it's easy prey.

They don't have to go out into the open and chase their prey down for kilometers.

It's all about lying in wait, quietly in the grass, until their prey walk on by, and they snatch it.

I wouldn't walk through that grass.

You'd definitely have a high chance of meeting a lion in that tall grass.

NARRATOR: Gorongosa will eventually need several members of the predator guild, targeting different territory and prey, to get its ecosystem back into balance.

BOULEY: Each predator functions differently.

That's the beauty of it.

And so, we're missing some of those pieces, and those are the pieces we're bringing back.

♪ NARRATOR: The wild dogs don't take long to make their presence felt.

As they expand their range, chases and kills become a common sight around the park.

On average, a pack of African wild dogs will take down at least two antelope a day.

(dogs chittering) Measuring the impact of added fear is much trickier, but ecologists are finding ways.

Studies covering the entire range of the animal world, from spiders to sharks to house cats, indicate that fear's influence can be multifaceted and include a surprising indirect effect-- a lower birthrate among prey.

ZANETTE: Because animals prioritize survival over anything else, they're going to be looking for predators if they think that there are predators in the environment.

Having your head up is highly beneficial to keep you alive another day, to avoid a predator attack, but at the same time, it carries a cost in terms of not being able to eat as much as you might if the predator wasn't there, which could then mean that your condition suffers, so you can't produce as many offspring as you could if a predator wasn't there.

NARRATOR: Oswald Schmitz discovered this in his early studies.

Over time, the goldenrod-eating grasshoppers living with spiders had fewer offspring.

SCHMITZ: Grasshoppers eating grass tended to be robust, producing really good-quality eggs, and then the grasshoppers feeding on the goldenrod weren't as robust.

And so, the eggs that they produced were poorer in quality than the eggs of the grasshoppers feeding on grass.

So, there's the cost, in terms of reproduction, to these grasshoppers.

NARRATOR: The African wild dogs in Gorongosa are certainly eating well.

The pack has its pick of prey to feast on.

And their healthy diet pays off.

♪ Ten months after Beira's first litter of pups was lost to a python, camera traps record the dawn of a new era.

(pups whining) With the addition of 11 newborns, the wild dog population is nearly doubled.

Then a surprising development.

A second female delivers a litter of her own, and her eight pups are welcomed into the pack by Beira and raised along with the others.

And pups in the park just keep on coming.

Four adults split off and form their own pack... (pups whining) ...and its alpha female delivers another eight pups.

(pups yipping) With these three litters, the wild dog population of Gorongosa has jumped from 14 to over 40.

(dogs howling softly) Within months, the youngsters are joining the hunt and the packs are carving out their place in the park's ecosystem.

BOULEY: They're making kills daily, sometimes twice daily.

And they're feeding on species that lions don't typically feed on.

So, they are really fulfilling a unique role as a predator in this system.

I think the most impressive thing about wild dogs, as a species, is the collectiveness of it.

♪ The pack is more than the individual sum of its parts.

It's like a superorganism.

♪ Everything they do, they do together, and a kill is like that, too.

One animal might take that prey down, but within seconds, the whole pack is on that kill, and it's gone, done.

It's a strike zone.

Everything is consumed and just disappears.

Even the vultures that follow them around hardly ever get anything.

NARRATOR: Head veterinarian Tonecas Paulo has witnessed clear changes in Gorongosa since the wild dogs' release.

(speaking Portuguese): NARRATOR: Tonecas' first-hand observations are backed up by the satellite data flowing into the Pringle lab.

ATKINS: So, we've kind of lined up where the wild dogs were during that time and where the bushbuck were during that time to try and see how much overlap there was between their spaces.

PRINGLE: They're covering this huge area.

You know, it's, it's, and, and they're hitting all of the major habitat types, from, you know, this really dense pocket of sand forest down here to the more sort of intermediate savanna, you know, woodland, all the way out into the, you know, wide open floodplain grasslands.

NARRATOR: Between the areas claimed by lions and the home ranges established by the wild dogs, predators are now covering over 1,000 square miles of Gorongosa.

DNA analysis of wild dog feces confirms that bushbuck are a favorite target.

What we're getting from the DNA and the dog scats, it looks like bushbuck are about half of the diet, at least, so...

So how is that going to affect what bushbuck eat?

HUTCHINSON: We think the first thing that happens is, the bushbuck retreat from the floodplain.

It's a tradeoff between nutrition and the risk of being eaten yourself, so we could expect that, yeah, the nutrition is going to take a hit in exchange for staying more safe.

(dogs yipping) NARRATOR: Large-scale field experiments like this are measured in years, but the early signs are encouraging.

PRINGLE: I really think that this is the future of environmental conservation.

If we want to have intact, healthy, functioning wild ecosystems, this is what we need to do.

BOULEY: I think what year one demonstrated is that the species can do very well here.

They exceeded our expectations, so that's a good sign.

It means the system is ready for them, and that's what we strive for.

We wouldn't be bringing in additional packs if we didn't feel like we had succeeded in year one.

Tough guys?

(men speaking in background) NARRATOR: A year after the first reintroduction, 15 more wild dogs arrive in Gorongosa, diversifying the gene pool and increasing the fear factor in the park.

PAULO (speaking Portuguese): NARRATOR: Down the line, the Gorongosa Project hopes other members of its lost large predator guild can return, continuing one of the most ambitious restorations of this kind ever attempted.

(insects chirping) Hyenas won't be welcomed back until the wild dog and lion populations are more stable and better prepared to defend themselves against their natural enemy.

But another iconic and essential predator may be finding its way here all on its own.

MAN: What is it?

Over there!

GUIDE: Shhhh!

NARRATOR: Recently, a tour group returning to camp spotted something just off the dirt road.

MAN (softly): A leopard!

Holy cow!

NARRATOR: A solitary male leopard roaming Gorongosa, perhaps in search of a mate.

BOULEY: That's the first confirmed sighting in more than a decade, so you can imagine the camp, uh, feeling at that point.

MAN: Just experience it.

BOULEY: But it was a very special moment.

I think it also validates that if we protect this place, these species come back.

But there are certain things you have to have in place before that can happen, and leopard was a sign of that.

MAN (whispers): Oh, my God!

BOULEY: If you give nature a chance, it comes back.

GUIDE: Amazing, eh?

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Or call 1-800-PLAY-PBS.

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♪ ♪