ALEX: Maple and Willow were found five days after their mom was found dead on a highway here on the peninsula in California.

UMA THURMAN: Dr Alex Herman works at the Oakland Zoo Veterinary Hospital.

She's overseeing the arrival of a pair of orphaned mountain lions.

When they came in, they were tiny, about four to six weeks old.

They were very hungry, very dehydrated.

Willow very soon after that developed pneumonia just from the trauma of the whole situation.

She was really, really sick.

We were very concerned that we could lose her.

UMA: Thanks to the team's expertise, Maple and Willow have both survived.

ALEX: It's really nice to see them moving around, climbing, playing.

UMA: These kittens may be safe, but they're not the first the team has rescued.

ALEX: Here at the Oakland Zoo, we've treated 26 orphaned mountain lions since our program began in 2018.

Mountain lions bring hit by cars is a common, big problem.

UMA: Without their mother, the future for these youngsters is limited.

None of the orphaned mountain lions that we've seen so far here at the Oakland Zoo have been able to be rewilded.

They need to stay in captivity because they're just too young when they lose their mother.

So normally they'll hunt with their mom, be mentored, taught, protected by their mom until they're two years old.

Because their mom was killed, they've lost that opportunity.

♪ UMA: In the wild, mountain lions are found from Canada to southern Chile.

They're so widespread, they have different names - pumas, cougars, even Florida panthers.

But the more their ranges overlap with ours, the more they hit a dead end.

(HORN BLARES) JEFF: So the freeways in our study area are extremely busy, some of the busiest in the world.

The 101 Freeway sees over 350,000 vehicles a day.

Roads in our area are definitely a deathtrap for our local mountain lions.

UMA: Jeff Sikich and the National Park Service team have been studying mountain lions here in LA for over 20 years.

It's been a tough ride.

JEFF: Last year we had 15 get struck and killed by vehicles.

Some of our animals that we've been following, we mark as kittens at three weeks of age.

I have followed them their whole life.

And to see these animals, you know, struck by a vehicle is just awful.

UMA: LA may seem an unusual place to find North America's biggest cats, but they're smart, adaptable, and stealthy.

JEFF: We rarely get sightings of these animals, and it really speaks to their elusive nature.

In this environment we have here, they can be 20 meters in front of me and we won't see them.

So we do a lot of looking for signs.

So looking for tracks, um, scat.

And then also our greatest tool, we use our remote cameras in areas where we think we might get a mountain lion to walk by.

♪ And we have been studying them by capturing them, placing GPS radio collars on individuals.

UMA: By charting the mountain lions' movements, Jeff can see the limits of their territory.

JEFF: So we're just north of the 101 Freeway right at Liberty Canyon.

This is a natural pinch point for them, this natural habitat on either side leading up to the freeway right at Liberty Canyon.

UMA: This human barrier is having a devastating impact on the population.

JEFF: These mountain lions aren't crossing the freeways often, and this has led to very low genetic diversity in our population and also close inbreeding.

And recently we've started to see the physical effects of that low genetic diversity in our population.

We have documented mountain lions with these physical abnormalities, um, a distal tail kink.

And we've also seen some reproductive abnormalities as well.

UMA: The longer they stay trapped, the bleaker their future.

If inbreeding depression sets in to our Santa Monica mountain lion population, there's pretty much a 99% chance of extinction within 50 years.

♪ We get asked often, "Well, what if we lose mountain lions "from this area?

What will happen?"

And we like to say that's an experiment we don't want to conduct.

You know, the mountain lion being an umbrella species, that if mountain lions are doing good in an area, if that population is healthy, that equals healthy prey populations and a healthy ecosystem overall.

UMA: The more nature there is, the more effective the ecosystem is at drawing down carbon.

But remove the apex predator, and it can fail.

Mountain lions are facing a disastrous future.

They need a hero.

Ten years ago, conservationist Beth Pratt was inspired by the tale of a mountain lion living in LA.

BETH: I read about P-22.

A mountain lion shows up in Griffith Park, and I was like, "Is that true?!"

You know, "Is there really a mountain lion in LA?"

He had to cross two of the busiest freeways in the nation, the 101 and the 405, and then he ends up on a dead-end area of eight square miles, the smallest known home range ever recorded for a male mountain lion by science.

And he makes it.

Lives under the Hollywood sign.

In fact, he became a celebrity in a land of celebrities.

We had the P-22 Day Festival, a festival for a mountain lion.

15,000 people showed up to honor this cat.

And so I think P-22 has really evoked something magical in us that even in the second-largest city in the country, nature had not been fully banished.

If a mountain lion could live under the Hollywood sign in the middle of LA, what else was possible?

♪ UMA: Inspired by P-22, Beth is part of a campaign that's aiming to change the fate of all mountain lions.

BETH: It's launched a movement.

The science has always been there.

The public support is now there.

It really got people to reconsider, like, "OK, we can coexist with large predators," and indeed, it's not just that we can.

We have to.

We have to save these cats.

It involves people having the will to do something visionary.

All my work is about coexisting with wildlife, and there's lots of angles to that, but the biggest is infrastructure, right?

The biggest is, animals need to get across roads.

UMA: Determined to help them, Beth has spent a decade raising nearly $90 million for a seriously ambitious project.

BETH: This is where we're putting a wildlife crossing.

♪ UMA: Stretching 210 feet long and 165 feet wide, the Wallis Annenberg Wildlife Crossing will be the world's biggest bridge for animals.

BETH: This is science come to life.

This is hope.

I've stood here at 2am and I wouldn't even try to cross.

So you can imagine an animal just isn't going to make it.

You know, the Wallis Annenberg Wildlife Crossing is truly one of the most significant conservation projects of our time, not just because it's going to be the world's largest crossing, but because what it represents - that we are doing this.

We are making way for wildlife in one of the most populated areas in the country.

JEFF: The wildlife crossing right at Liberty Canyon couldn't have come at a better time for our population.

Once the wildlife crossing is built, we really only need one mountain lion every two years or so to come down and successfully breed to make a difference in our small population here.

ALEX: These are beautiful, iconic, important animals that are a part of California.

The wildlife corridor is being recognized as a key to coexistence, I think is the future that we should all work towards.

♪