(host) The Serengeti is one of the richest ecosystems on Earth, but surviving out here is not easy.

You got lions, cheetahs, leopards, hyenas, crocs, and that's just the stuff with sharp teeth.

You've also got giant elephants, rhinos, buffalo to worry about too.

The hippos-- man, you definitely don't want to run into hippos.

They kill more people every year than sharks, which is a fact that continues to blow my mind.

(Joe) And that's just the big stuff.

There are countless little bugs who'd just love the opportunity to take a big old belly full of your blood, or maybe lay a few parasite eggs under your skin.

(speaker) At least I know-- (speaker) Wha--what's going on?

Swarm of Tsetse flies inside the vehicle.

Nothing to worry about.

Out, out.

(Joe) Surviving out here is hardcore, but it's easier if you don't go it alone.

Today, we're going to look at some stories where species cooperate to help each other survive.

If you know what you're looking for, you can find these relationships everywhere.

And we're also going to look at what happens when these alliances get a little out of balance.

Teamwork makes the dream work, and it's a good way not to die.

[gentle music] You guys know what makes this show work?

Post-production team.

Yeah, I was going to say producer Amanda.

I'll acknowledge my jokes are a big part of it, guys, I know.

But it's teamwork, and what makes teamwork work?

Like, I can't just ask you guys to do stuff for me.

I have to do something for you in return.

You scratch my back.

I'll scratch yours, you know.

Not really comfortable with scratching your back, Trace.

Joe, it's an-- I'm not actually-- it's an expression.

Cooperation within species-- that's one thing, right?

But what about cooperation between species?

Insects pollinating flowers and that kind of thing?

(Joe) Totally.

I mean, that's a classic example, right?

The flower gives the insect, like, some pollen and nectar to eat, and the insect helps the flower make baby flowers.

The technical term for this kind of relationship actually is calledmutualism.

It's sort of any relationship where both species get a benefit.

And if you go looking for these mutualisms in just about any ecosystem on Earth, you can get some pretty weird examples.

Acacia trees are an iconic sight across the African savannah.

These almost fractal, river-like branches and broad, wide canopies with all their leaves clustered up top as if they're reaching for the sun.

Do you guys know how they get that shape?

The gardener does it.

One of those snippers that you can get at the hardware store, the really big ones.

There's just one person doing that all day long.

I want that job.

It's actually because of this.

Nature's hedge trimmers.

(Joe) Acacia trees look like that because that's how high a giraffe can reach.

Those are the only leaves left.

Once you get up close to an acacia, you really going to wonder why anything would go to the trouble of trying to eat it.

Out here, we are surrounded by acacias.

And the one thing I've noticed about them is, they are covered in thorns, but the giraffe are still eating on them.

Like, it's like, it's not a problem.

They just don't care.

(Jahawi) I mean, giraffes are so well adapted with that long tongue, and they sort of get that tongue through the thorns and can still get at the leaves.

In the Serengeti, everything is mean.

(Joe) And some acacias are a little meaner than others.

It's not a meal I would take.

No.

But we have another type of acacia over here.

Yeah.

I mean, it's still covered in thorns.

I mean, this one's really interesting.

This is called thewhistling thorn, and it's adapted a really interesting kind of relationship with ants.

(Joe) So these are--these are little ant houses?

(Jahawi) Yeah, basically.

These?

Yeah, you can see them all around.

(Joe) Ant houses built right into the tree.

This little acacia grows these nodules, especially when they're young like this, and inside of those nodules live ants, an entire colony of ants in this tree.

You got ants in your plants?

Dad joke, check.

You only have to change one word in "dad joke" to make it a bad joke.

One letter, even.

You know, young plants need all the protection they can get out here with all the--and the browsers.

And this is like a super evolution of protection.

And this is just swarming with ants.

And this would send a very clear signal if you're a giraffe trying to munch on this.

I have a feeling you'd, like, just pick a different plant.

You'd go pick a different breakfast.

I mean, the minute you kind of, you know, disturb the plant, all these ants are going to come rushing out.

(Joe) I mean, the thorns are a--a big message, but the ants are a double no for me.

(Emily) So let me make sure I've got this.

You said mutualism requires both species to get a benefit.

The plant gets protection from hungry stuff, and the ants get a treehouse.

(Joe) Exactly.

And there's even more going on here.

(Joe) That's incredible.

So the plant gives them a place to live-- (Jahawi) Yeah, and in return, it gets protection, but there's even more, because the plant-- if you kind of look, yeah, look down there, it actually provides little nodules that secrete sugar.

Just came in and took a little-- took a little drop of nectar off the base of this leaf.

It's incredible.

So not only does this acacia give the ants a place to live, it actually feeds them, too, in return for protection.

(Trace) What's really crazy to me is neither of these organisms is conscious that they're in this kind of mutualistic relationship.

The plants-- they don't even have brains.

(Joe) I know.

Like, they just evolved this way, adapting and trying to survive, and you get this-- it's crazy.

I love natural selection.

Should get that on a T-shirt.

So anything that tries to take a bite, these ants are going to say, "No, thanks, not at our house."

This is a great example of mutualism.

(Jahawi) Mutualism, exactly.

(Joe) Species cooperating, and both getting a benefit.

(Jahawi) Yes.

(Joe) And when there's so many other things to eat, like, why--why would you waste your time-- (Jahawi) Yeah, I mean the last thing you'd want is ants on your tongue.

We'll leave them alone.

We're not hungry.

Go about your ant business.

But why is it called a--a whistling thorn?

Excellent question.

So it turns out, in addition to being an awesome photographer and filmmaker and safari guide, Jahawi is also a really talented musician, too.

The interesting thing is that when wind-- when you get really good wind coming through here, through these little nodules, it acts like a whistle.

So all of a sudden, the whole tree starts to whistle.

All those little holes that the ants are coming in and out of are like--like a little flute.

It creates, like, a flute.

That's why it's called the whistling thorn.

[whistling] I'm fooled.

Guys, okay.

It wasn't very windy.

I'm sorry.

I don't control the weather.

But it is totally true.

I would never lie to my--my teammates.

Mm.

(Joe) So the Serengeti is also home to one of the most iconic and awesome mutualisms in the animal kingdom.

There are these birds.

They're calledhoneyguides.

And I guess their name kind of tells you what they do.

The bird locates a beehive, but it can't get to the honey on account of all the angry bees.

So it calls to the honey badger in...however honey badgers and honeyguides communicate.

I don't know.

I don't speak bird.

Come on over, I've got some food.

(huskily) I'll be right there.

And the honey badger follows the honeyguide over to the hive.

Somehow, both species know that there are these delicious treats at the end of this trip.

So the honey badger breaks open the hive, eats its fill of delicious honey.

Then the bird comes in, gets its own meal of wax and larvae and honey all because the honey badger did the hard work.

Sure, the honey badger don't give a you-know-what, but it can lend a helping hand.

A helping paw.

Classic mutualism.

Each species is getting a benefit.

Okay, I've got one more.

And this one gets at a big unanswered question underlying all of these mutualistic relationships.

So you guys ever wonder how a giraffe scratches its neck?

(Emily) Wait, that is your big unanswered question?

(Joe) Okay, actually, no, no, that is not the question, but I'm asking this one before I get to the important one.

How do they scratch their neck?

They don't.

They--they actually have these little birds do it for them.

These cute little flappy flaps--flappy flaps?

I don't think we call birds flappy flaps, do we?

Flappy flaps.

Like it.

(Joe) These cute little feathery friends are called oxpeckers, and their name also kind of tells you what they do, but they don't just live on oxen.

You'll pretty much find them on the backs of any four-legged, plant-eating animal on the savannah.

So they sit on the backs of grazing animals, and they eat ticks and worms and even gross stuff like scabs and earwax, and those little boogers you get in the corner of your eye.

I mean, that happens to animals too.

Yes, fun facts, they actually will roost in the nether regions of giraffes too.

That is a very fun fact.

So the oxpecker gets their food and their yummies, and the host gets cleaned off and stays healthy.

And sometimes the oxpeckers will even warn their hosts that predators are nearby.

"Hey, there's--there's a lion."

But in, like, bird speak.

This is an obvious win-win mutualism, right?

Well, it's actually not always that simple.

And this is the big unanswered question.

What keeps these mutualisms going?

(Emily) Yeah, I mean, now that you mention it, it seems like it would be pretty easy for one half of the relationship to become a freeloader, right?

Getting a benefit without having to do any of the work.

Yeah, I was thinking the same thing.

Like if a butterfly decides to get the nectar, but you know, it doesn't do any pollinating, or if the ants, they live rent-free inside the acacia, but they stopped protecting the plant from hungry animals somehow.

Right, mutualisms require a balance.

But it might sometimes be easier to cheat.

Actually, when scientists study these oxpeckers, they noticed when there wasn't enough bugs and worms and nasty stuff to eat, they would peck new wounds in the backs of their hosts and drink their blood.

(Emily) What?

Like vampire birds?

We're talking about "Nos-bird-atu"?

(laughs) But there is a way to keep this in balance.

See, the host never quite lets the oxpecker get comfortable enough to do its job.

They're kind of always swatting and shaking and, like, flicking their ear.

So the bird never gets totally comfortable unless it's doing its good grooming behavior.

So the host keeps the relationship as a mutualism and doesn't let the bird become a parasite.

Right, and we can totally see these organisms working together, but not all of the mutualisms that are out there are easy to find.

Some of them are a little more complex.

Easy ones are like trees, right?

They grow fruit so animals will eat them and distribute seeds.

Yes, something that you can't see, but there's, like, plants give off oxygen and animals give off carbon dioxide.

(Trace) Exactly-- all of these relationships are pretty easy to find, easy to understand, but some mutualisms aren't so easy to see, and they can be way more intricate and complex.

Just off the 101 highway, around San Francisco, lives this butterfly.

It's beautiful.

It's shimmery.

Flits around flower to flower, living its little butterfly life.

I didn't know something so pretty can be so tiny.

It is a cute little butterfly.

(Trace) Yes, it's called theMission Blue.

And if you see one in the wild, you're super lucky.

But what I want to show you today doesn't really involve looking for the adult butterfly.

It's about the caterpillar.

To find these caterpillars, we had to go to one of only three places in the world where these butterflies live.

Not actually that hill.

It's--it's this other hill over here, San Bruno Mountain.

We're talking about a seriously rare habitat, and you couldn't just go there any time.

Mission Blue caterpillars-- they're only feeding for a few weeks of their year-long life cycle.

So we had to come to this specific mountain on this specific day, and we would check in daily to see if volunteers and park rangers had spotted any Mission Blue caterpillars on the three species of plant that they live on for their entire lives.

This is like the definition of a microhabitat.

Yes, and everything had to come together perfectly.

So one day, we got this call from our guide, Kirra.

She found caterpillars on the lupines-- that's the type of plant-- and the next day, we're on San Bruno Mountain on a caterpillar hunt.

(Kirra) I like to be this close to the ground.

I don't think I've ever spent this much time looking at one plant.

(Kirra) Welcome to my world.

I have no idea what I'm looking for, but I feel like I should at least try to look.

We spent the next half-day down in the dirt, looking for a very well- camouflaged caterpillar that was, like, the size of my pinky nail.

And, luckily for us, we found some.

These creatures spend almost their entire existence on a single plant.

So when you see an adult Mission Blue flying around, that part of its life is maybe, like, two weeks max.

It's been alive for a year.

It's been alive for a year on the same plant.

So its entire life is wrapped up on what's going on in this one plant.

(Trace) Whoa.

Talk about small world.

(Trace) So these caterpillars are on these lupines, but they weren't alone.

They have this mutualistic secret weapon.

These guys.

These are native California formicine ants.

And if you watch closely, you can see them tapping their little antennae on the caterpillar's back; that's like a little massage, and it stimulates the caterpillar to squeeze out some sweet honeydew.

It's a nutritious amino acid cocktail that the ants-- they eat it to survive.

Feel like honeydew is a very generous word for what that looks like.

They're squeezing sugar juice out of their butts.

Some sweet, sweet honeydew.

Talk about "pour some sugar on me."

Def Leppard was--caterpillars-- okay, but that is not all that is going on here.

The caterpillars really only have one job.

(Kirra) Which is to eat, and get fat and poop, like babies do.

(Joe) As a dad, I'm very familiar with the concept.

(Trace) These big, fat, juicy caterpillars are delicious for predators, not just as a snack, but also for caterpillar-shaped incubators for parasitic wasps.

The wasps will actually lay eggs inside the caterpillar, and when they hatch, the wasp larvae eat their way out.

Amazing.

The caterpillars actually keep the ants around and give them the honeydew because the ants will fight off the parasitic wasps, thus protecting the caterpillars and their honeydew goodness.

Okay, so let me get this straight.

You've got this caterpillar lives on this one hillside on this very specific plant that is friends with ants that are fighting off parasitic wasp that just want to lay eggs in its big, chunky body?

Yes, that's it, yep.

And the caterpillar then grows up big and strong to become a tiny butterfly and start the whole cycle again, thanks to the ants.

But it gets even weirder, because if you think about it, the caterpillar is a parasite on the lupine because it's eating the lupine's leaves.

Once the caterpillar pupates and becomes a butterfly, it then switches its relationship to mutualistic, drinking the nectar and pollinating the plants.

It's super complex and cool.

And if any one thread of this little web disappears, everything falls apart.

Well, that is amazing.

What, Trace, how did you even get this footage?

(Trace) We have a friend up in the Bay Area who--this is all he does.

It's incredible stuff.

And to be able to see this interaction and to get it on camera-- it's pretty darn rare.

While we were putting this piece together, I kept asking experts and local, you know, volunteers and guides about footage and clips.

And it's not really something that people film.

This story that we're telling, these images of that ant tending its caterpillar teammate, this might be some of the best footage of this interaction in existence.

And it's an important record of this relationship because these butterflies are actually at risk.

I don't want to touch it or disturb it too much 'cause they are endangered.

We have to watch where we step.

We have to watch where all of our body parts are because we don't want to crush them.

One of the things that actually helped make them endangered: not just human activity in terms of development, but also trampling is one of the ways that you can kill them.

I gotta be really careful.

I mean, that legitimately gave me chills.

Did I forget to say that part?

Yes, it's endangered too.

And not only does the Mission Blue have this super complex web of relationships that all hinge on each other in this one place on the whole planet, but MB, too, has been on the endangered species list since 1976.

And it's hanging on by a thread.

In 1998, this unknown fungus attacked the lupines, the one plant that it lives on, and almost wiped out the whole Mission Blue species.

It's been super tough.

The Mission Blue was actually named for San Francisco's Mission district, now one of the most popular neighborhoods in the city, and we humans have sort of stranded it on the outskirts of human developments and these little islands of nature.

And now, to keep them in existence, we have to step in and take some responsibility for them and their mutualistic web of partners.

Today, park rangers with the San Mateo Parks Department, the Golden Gate National Recreation Area, the National Park Service, and volunteers all over the place are trying to save the Mission Blue.

Every year, they plant lupines, and in that tiny window of time where the adults are flying around but haven't laid eggs yet, the rangers capture them and release them in other parts of the Bay Area to spread them around.

It is complicated because, you know, you're out, you are chasing around butterflies with a butterfly net, and then it's a matter of, you know, very, very carefully containing them, putting them on ice, like in coolers, and then driving them, moving them from one island of wild space in the Bay Area to another because they can't cross these swathes of city.

They need our help.

So one of the things we've been focusing on in terms of conservation is getting more of these different species of lupines planted and growing so that if and when a meltdown like that happens, that there's an alternative food source available and that the butterflies don't plummet.

The butterflies track right along with what the plants are doing.

So there's some variation throughout the site, and that's what you want, is more variability, more diversity.

That builds for, sort of like, a stronger, more resilient butterfly population.

(Emily) So, by protecting the butterflies, they're protecting the lupines, the ants.

It's everything.

Yeah, choosing which species to save from extinction isn't just about the species.

It's also about these relationships.

Without the butterflies, the lupines suffer, the ants suffer, and you could argue the wasps might suffer too.

Their larvae--they need to eat.

They're all connected.

(Emily) When I watch this, I'm thinking, what is this footage going to be in the future?

Is it a time capsule of a future extinct species, or is this capturing the efforts that saved the Mission Blue?

It's when people start to develop a more intimate connection with the landscape around them, with these wild spaces, it really reduces any sense of loneliness because if you can look around and you say, "Oh, I know that that's a lupine.

"and I know that the butterflies live here, "and I know that that's a California poppy, "or I know that that's, you know, this golden violet, and there's another rare butterfly that utilizes that," then all of a sudden, you're not just out here walking along the trail, you're out here with thousands of other things, doing their thing.

And it's-- it's a much more exciting place to be.

It's, like-- it's an alive place.

Mutualism and symbiosis is really the lifeblood of billions of individual life forms all over our planet.

And I could even argue that you, Emily, and you, Joe--you are walking examples of mutualism.

Your gut bacteria needs a nice place to live and some food, and in exchange, you get help digesting your lunch.

(Joe) These relationships-- they give me so much more respect for the power of natural selection and evolution.

I mean, there's no direction or intent, right?

I mean, these ants and these caterpillars-- they're not conscious that they're helping each other, yet nature is still able to weave these beautiful and intricate webs.

Absolutely, and one of the reasons that we made the series is that we wanted to look at nature more holistically, right?

It's not just individual animals or specific places, but relationships.

Even conservationists have forgotten this in the past, sometimes looking at things species by species.

And we can't forget these links.

It's not just lions.

It's not just bison.

It's not just humans.

We are all connected.

When you look at a small thing and you're like, "Well, that must be simple."

But in reality, they're both kind of equally complicated.

'Cause it requires the entire mountain for them to exist.

So they're big.

They're the size of a mountain.

[gentle music] And next time, we'll be bringing you another story of these intricate webs in nature, one that connects a pile of elephant dung in Africa to a football stadium in California.

[making skittering noises] There's bugs.

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