NOVA

NARRATOR: 60 miles west of Bangkok is the Khao Chong Phran cave, famous throughout all of Asia.

For centuries, a sanctuary for the faithful-- and now, the curious.

Scientists-- who come to learn from the most unusual of creatures... As the sun sets, three million bats begin to stir, preparing for one of nature's greatest spectacles.

Rocketing to the skies in a blizzard of flapping wings, they will pass the night gorging on insects.

This epic nocturnal excursion is a feast for the eyes.

But for science, bats are much more: a biological treasure.

EMMA TEELING: They are by far the most fascinating of all animals.

(squeaking) SHARON SWARTZ: They are remarkable and extraordinary creatures.

JARED HOLMES: As a biologist, it's my job to really tell people that we, we need the bats.

NARRATOR: There are more than 1,400 different species of bats, playing crucial roles in ecosystems all over the world.

But for many people, bats are the stuff of nightmares.

Bats have been demonized in the society.

KENNY BREUER: I thought bats were scary and creepy and a little bit kind of unpleasant.

(screams) NARRATOR: Already vilified in pop culture, recent news reports have been giving bats an especially dangerous rep.

The ancestor of the virus in humans had to be a bat virus.

(translated): There is a virus that is 96% similar to this new coronavirus in bats.

REPORTER: Early research suggests human picked up the coronavirus from animals, possibly bats.

NARRATOR: Though we still don't know the exact source of the virus that started the COVID pandemic, bats are a prime suspect.

But rather than fear these flying creatures, biologists are hailing them as potential saviors.

MATAE AHN: They can really get infection without getting sick.

LINFA WANG: Bats teach us lesson, not to suffer autoimmune disease, diabetes, arthritis.

SBASTIEN PUECHMAILLE (translated): Whether you capture a bat that is two years old or 15, or 20 years old, you don't see any difference.

GARY MCCRACKEN: For the body size of these animals, they are way off scale in terms of their longevity.

TEELING: Bats hold the cure.

They hold our treatment.

NARRATOR: Science is beginning to decipher their strange powers.

Could these much-maligned creatures hold precious secrets for our own health?

(bats squeaking) "Bat Superpowers."

Right now, on "NOVA."

NARRATOR: Many experts believe that the coronavirus that tore through the world's population in 2020 came from a bat.

Virologist Supaporn Wacharapluesadee is world-renowned for her ability to track viruses in the wild.

Today, her team has come to test the giant colony at Khao Chong Phran.

WACHARAPLUESADEE (translated): There are bats in the caves, and we put this on to be safe while we work.

It doesn't mean that there are deadly viruses in there, but we need to protect ourselves to do our work safely.

NARRATOR: Once fully suited up, the scientists descend deep into the cave.

Under the gaze of the Buddha statues, the team installs a net in the large chamber that local monks share year-round with its native residents.

WACHARAPLUESADEE (translated): We have been doing research work here for more than ten years.

Now, for safety reasons, we have come back to test if there is coronavirus, which could be dangerous for the people in the area.

NARRATOR: A second team waits at the exit of the cave to catch bats flying outside.

Tonight, about 70 bats will miss their nighttime excursion.

Instead, they will spend a few hours in a makeshift lab set up at the base of the hill.

Each bat is given a careful medical checkup.

Trying to limit stress to the animal, scientists take multiple samples from the skin, the mouth, and even the intestines-- all organs that are susceptible to containing viruses, known or unknown.

WACHARAPLUESADEE (translated): We have discovered hundreds of viruses in bats.

Actually, there are more than 60 viruses in bats that could eventually be transmitted to human beings.

NARRATOR: In addition to being a key transmitter of the deadly rabies virus, bats are suspected sources for numerous viral outbreaks around the world: the 1967 Marburg virus in Europe; two waves of Ebola in Africa; the Hendra virus in Australia; the Nipah virus in Malaysia.

Then a series of coronavirus outbreaks: SARS, that started in China; MERS in the Arabian Peninsula; and now the COVID-19 pandemic that engulfed the planet in just a few months.

For some scientists, it is a trend that will no doubt continue as human beings encroach more and more on the bat's natural habitat.

Supaporn is hoping to discover why viruses circulate so well within bat colonies and how they might transmit them to other animal species that in turn could pass them on to humans.

But above all, she wants to know why this animal, infected by so many dangerous viruses, seems totally impervious to their effects.

WACHARAPLUESADEE (translated): As far as I know from the research work overseas and my research work here, bats with viruses aren't getting sick.

The bat aren't getting sick while the viruses still live within them.

Because of the whole world is so desperately trying to deal with, with COVID-19 and its horrible effects, bats have come into the limelight, and they've come into the limelight as potential reservoirs for many, many viruses.

And the question is, why?

Why can they... Are bats really special?

Is there something unique about bats' biology, their physiology, the genetics, that allows them to tolerate these viruses?

What's the reason?

NARRATOR: Will studying bats allow us to avoid the next deadly virus outbreak?

Could their disease-defying biology help us to live longer and in better health?

Laboratories around the world are mobilizing to find the answers.

Because just how this stealthy, nocturnal animal functions remains largely a mystery.

New Yorkers may not realize that one of the most unique biological banks in the world is just next door: a huge collection of bat organs and tissues, stored at Stony Brook University.

A veritable treasure trove for scientists like Liliana Dvalos.

DVALOS: It's a piece of brain from Belize.

This is, um, liver, liver sample, and it's from Colombia.

This is from our last expedition.

Our collection has everything from the top of the head, the brain, the nose, the eyes, and every organ in the body.

NARRATOR: Mummified bats, cabinets stuffed with body parts-- the Dvalos Lab might feel like something out of a Frankenstein film.

(electricity buzzing, machine roaring) Not to worry-- it's not what you think.

And Liliana, rather than being frightened or repelled by bats, is in fact one of their biggest fans.

DVALOS: What have we got here?

Oh, this is so amazing.

This is a horseshoe bat.

This collection happened in 1934, December 27.

Somebody was out there, in Chengdu, in China, catching bats.

This is the horseshoe down here, you see it?

NARRATOR: The horseshoe bat is widespread throughout Asia and suspected to be at the origin of SARS-CoV-2, the virus that causes COVID-19.

With this specimen, Liliana will be able to study just how bats become infected.

Since COVID is a respiratory disease, the team concentrates their efforts on the animal's respiratory tract, especially its nose and nasal cavities.

Could it be that the inside of this strange-looking nose contains the key to how bat viruses also infect humans?

Thanks to Laurel Yohe, a researcher at nearby Yale University, the team has access to a 3D scanner.

It's the first time ever this technique will be used to study the inside of a bat.

(typing) (scanner beeping) YOHE: Here are the teeth.

You can see the neurons in the teeth.

As we move through, here is the tongue.

Here is the nasal cavity.

NARRATOR: The horseshoe bat's nose is of particular interest to Liliana and her spouse and research partner, Angelique Corthals.

An expert in human biology, Angelique studied the respiratory tracts of COVID victims at the height of the pandemic.

The bat is very similar to humans, because you can see actually the same structure of the nose.

CORTHALS: Bats that are known to harbor the closest relative to SARS-CoV-2 have a nasal cavity that is to, that is actually closely resembling that of human, which is very likely part of the reason why we can be infected so quickly with SARS-CoV-2, because all of a sudden, it's not completely strange territory for coronavirus to enter the nasal cavity of a human.

NARRATOR: But once it has arrived in the nose of a bat or a human, how does the virus infect the rest of the body?

Liliana and Angelique focus their research on the cells that line the nasal cavity.

CORTHALS: You see those hollow points in this layer?

Those are not holes, they are cells-- they are called the goblet cells, which are mucus-producing cells.

They are the first barrier against pathogens, against allergens, against any kind of foreign bodies that enters through the nose.

NARRATOR: Mucus produced by goblet cells usually traps viruses before they can enter the body.

But when it comes to COVID-19, goblet cells have a weakness: they are covered by a receptor that the coronavirus recognizes.

Like a key entering a lock, the virus attaches to the receptor, opens a passage, and injects its genetic material.

The cell then starts manufacturing the virus by the hundreds, starting a chain reaction that can spread throughout the whole organism.

The coronavirus can enter both bats and humans in the same way, through these goblet cells.

So how come humans can become so sick, while bats don't?

DVALOS: Our scientific understanding so far is that the viral loads are fairly low, meaning that these infections are circulating, but they do not have the same consequences in the bats that they have in people.

We don't understand yet fully why.

NARRATOR: Somehow, the virus is able to enter bats' noses the same way it does in humans, but the similarities end there.

In bats, the virus is present, but at a consistently low level.

The question is: how are bats keeping the virus under control once it has entered?

That's what scientists in Singapore are trying to find out at the Duke-N.U.S.

Medical School, where the bats' immune system has come under the microscope.

Professor Linfa Wang, known to colleagues as "Batman," thinks he has found the secret to bats' super-immunity.

WANG: My students, when they first work in my lab, they got it wrong.

They say bats has a more efficient immune system to clear the virus.

I say.

"No, bats have a more efficient immune system not to develop disease."

They are more efficient, really, to control the virus.

Otherwise, they will not be good reservoir, right?

NARRATOR: Matae Ahn wrote his thesis under Linfa Wang's direction.

When he joined the team in 2014, the lab did not yet have a living bat colony to work with.

AHN: In the past, we had to fly over to Australia to get all sample for our studies, and now, we have a local bat colony, right here.

And this allows us to get the fresh sample easily and study bats really closely.

NARRATOR: The cave nectar bat has a fox-like head and lives principally in Southeast Asia.

In the wild, these bats are carriers of many viruses, but don't get sick.

But in the lab, conditions are strictly controlled and the animals remain uncontaminated.

AHN: We are using the fresh bat samples to analyze their contents in details, starting from genes, mRNA, protein, cells, to even tissues.

And all of these component can be used and utilized to study bats and their immune system.

NARRATOR: Matae's experiment concentrates on proteins involved in the immune response, and on one molecule in particular: interferon alpha.

AHN: To be simple, interferon alpha is a key molecule that alerts the body to the intruder.

It tells the surrounding cells that an infection is occurring.

NARRATOR: When a cell detects a virus, it unleashes a barrage of interferon molecules which spread through the body, spurring immune cells into action.

Which, in turn, wipe out the intruding pathogens and get rid of the cells already infected.

AHN: So we want to examine and compare the level of interferon production between human and bat cells before any infection actually occurs.

(whirring) (device beeps) So look, look at this curve.

This curve is a human sample, it's flat.

It means that interferon alpha is almost undetectable.

In contrast, in our bat sample, we have a lot of interferon alpha detected, even though there is no infection occurring right there.

NARRATOR: In other words, bats have adopted a proactive strategy of defense.

(whirring) Thanks to interferon being permanently present, when a virus penetrates the bat's body, their immune system is already active.

But in humans, that reaction is much slower.

While our body's immune system is ramping up to produce interferon, the virus can be spreading.

The risk of getting sick is therefore much greater in us than in bats, where the virus remains under tighter control.

WANG: Human, for example, our defense system is switched off most of the time, until we see enemies, and then we switch on.

NARRATOR: Unlike us, the bats' defenses are always on high alert.

Their immune system can prevent damaging infection while letting some virus hang around.

That's good news for the bat, but it might be really bad news for humans.

WANG: One theory is that if the virus live inside a bat body, you know, you already have elevated defense systems.

So when they jump to a different host, like human, and that's, it's, like, you know, free playground for them and they just go and rampage in us.

So very efficient.

NARRATOR: A virus that battles for survival inside a bat's super-immune system becomes a formidable enemy.

When it jumps to a less defended species, like a human, it's much more dangerous.

But why did bats develop such a highly functioning immune system?

Why did nature bestow bats with this superpower while our own defense system has proven so weak in the face of multiple epidemics?

It's a question that zoologist and geneticist Emma Teeling has spent decades researching.

Nearby her lab at University College Dublin, Emma takes advantage of the last few days of fall to visit a local colony before the bats start their winter hibernation.

TEELING: Some people don't actually like them, and the question is why?

As primates, we primarily get the information from our environment through our eyes.

At night, we're a bit frightened-- we can't really see them.

People think, "Oh, they're gonna get caught in your hair"-- they, they don't.

What they do is, they're flying, feeding on insects that are trying to bite you.

(animal clicking) There you go, there's a bat.

More than likely, it's a, it's a... Oh, hello, you beauty.

More than likely, this is a soprano pipistrelle.

Because you can hear, its peak frequency is about 45 kilohertz.

Do you see that little bat fly across?

This bat detector is picking up the sound that's been emitted from the bat's mouth.

And what's happening is, the bat emits its call and it listens to the echoes, and it uses this to be able to orient in complete darkness.

I have a head torch on right now.

Right now, this is dusk-- you can't see anything, but the bats have woken up and they are flying around, feeding on the insects, and are more than likely flying up and down this small stream here.

(bird squawks, bats clicking) Hear?

Bang-bang-bang-bang-bang?

(bats clicking) NARRATOR: Aided by their unique capabilities, bats thrive on every continent except Antarctica.

It's a story of extraordinary adaptation, the secrets of which are inscribed in their DNA.

A wing flap away is Emma's center of operations, a laboratory of mammalian molecular evolution.

Equipped with the latest tech, it's affectionately called the Batlab.

Here, Emma co-pilots one of the largest studies of bats in the world.

The project Bat1K connects over a hundred scientists around the globe in a joint effort to sequence the genomes of the approximately 1,400 bat species.

TEELING: We wanted to sequence the entire DNA code that's in every single cell of a particular species, but we wanted to do it to the quality of the genomes that we have for humans or mice, so that we could now use this to investigate the likes of, what have bats evolved to allow them live with coronaviruses and not die?

NARRATOR: Bat1K's approach is to compare the billions of letters that make up bats' genetic code with the DNA of other mammals.

In theory, finding out what is different will lead researchers to those parts of the bat genome responsible for its robust health.

TEELING: Darwinian selection-- did natural selection act on a particular part of the genome in bats that make it very different at the same region in bats and everything else?

And this may indicate that this is the region that's driving their unique adaptations.

NARRATOR: Bat1K has already fully decoded the genomes of six bat species: the velvety free-tailed bat, the greater horseshoe bat, the Egyptian fruit bat, the pale spear-nosed bat, the greater mouse-eared bat, and Kuhl's pipistrelle.

A meticulous comparison of their DNA with that of land-based mammals revealed something totally unexpected.

When the bat's ancestor developed wings and evolved the ability to fly-- at least 55 million years ago-- the genes controlling their immune system also evolved, mutating significantly.

It's as if their evolution as flyers somehow provoked or required a similar evolution in their immune system.

TEELING: They can fly.

They're able to tolerate all their, these, these unique viruses-- is there a connection?

What's the connection?

And this is something I've been working on for a very long time.

I have written research grants.

I've gotten slammed, I've gotten abuse left, right, and center.

It's caused such scientific controversy, and it still does.

So the idea is, evolving... Could evolving a new form of locomotion drive an immunological and a genetic response?

A physiological response?

So I'm going to argue that yes.

NARRATOR: For Emma Teeling, bats' extraordinary resistance to viruses seems to have evolved hand-in-hand with their other superpower: their supreme prowess in the air.

But how could flight protect this tiny mammal from sickness?

What is the link between the two?

As the only mammals known to have evolved true flight, bats' flying technique is totally unique in the animal kingdom.

Every year at the Frio Cave, about 70 miles west of San Antonio, Texas, newborn bat pups will take to the skies for the very first time.

(chuckling) Millions of female Mexican free-tailed bats migrate here in the spring, and it's the perfect opportunity for biologist Gary McCracken to observe the animals in action.

MCCRACKEN: This is the time of year when mothers are beginning to give birth to their pups.

We can't go very deep into the cave with everybody, lights or cameras, because it's just too disruptive at this time of year for, for the bats, so we're respectful for that, yeah.

There you go!

NARRATOR: Gary goes just inside the mouth of the cave so he won't disturb the pups.

MCCRACKEN: I well remember the first time that I went into a Mexican free-tailed bat cave.

I thought I was on the surface of the moon.

I mean, really, the dust covering the rocks, you, you walk and your footprints stay there, and then they get reworked by the beetles.

The atmosphere is heavy with simple compounds of carbon and nitrogen, methane and ammonia.

I mean, it really does seem like you're on another planet.

(bats squeaking) When I first saw the babies, the dense concentrations of babies, it was just amazing.

Soon, you've got 4,000 to 5,000 babies in an area of about a square meter-- 4,000 to 5,000 babies.

NARRATOR: After about a month clinging to the walls, the young pups will attempt their very first flight.

The slightest error could be fatal.

MCCRACKEN: It's really awesome to imagine what it must be like to take that first flight.

Looking down below... (chuckling) Thinking about, what happens if I don't make it?

And, and if you don't make it, you're not going to get back.

You're going to, you're going to, you're going to land in the guano and, and be eaten by dermestid beetles.

And, you know, the amazing thing is that it seems that the vast majority of them do make it work.

NARRATOR: Once mature, the Mexican free-tailed bat develops into an extraordinarily powerful flying machine, and it's their outstanding performance in the air that Gary has come here to measure.

Helping him is local biologist Jared Holmes.

HOLMES: Yesterday, they started flying about 7:30.

MCCRACKEN: Uh-huh, yeah.

So we'll be ready by 7:30, for sure.

Okay.

Yeah, we'll have the plane ready to go.

So I'll tell you when we're taking off, and you get the bat ready and stick the radio on it.

Oh, all right!

These bats weigh a half an ounce, 12 grams.

They are too small, too light with current technology to carry GPS trackers.

But they can carry these little radios that are basically location locators.

HOLMES: And we're still looking for a female bat of, of average size... Average size.

Not too pregnant.

Gotcha.

And, uh, and obviously, good health.

A nice plump one.

Yup, yeah.

Yeah, just a nice bat.

Okay.

NARRATOR: The next day, on the tarmac at Garner Field airport, not far from Frio Cave, Gary adjusts the settings of his radio telemetry receiver.

This device will use radio signals to follow the bat that Jared is about to capture and equip with the transmitter.

(plane door shuts) MCCRACKEN: With the airplane, it is possible to triangulate the location of the bat.

And by carefully listening to the signal from the transmitter, we're able to pinpoint the location with some precision.

(radio chirps) HOLMES: Gary, the flight has started.

Are you in the air?

MCCRACKEN (over radio): Jared, we're just taking off right now.

We should be there in 15 minutes.

HOLMES (over radio): Okay, roger that.

I'm gonna go ahead and try to catch a bat.

MCCRACKEN (over radio): Be sure to get a nice, young, fluffy-looking one.

(radio chirps) HOLMES (over radio): I got a couple in the net, one looks good.

I'm gonna go ahead and tag it, gonna get it released.

Good deal.

This is working really well right now.

(laughs) MCCRACKEN (over radio): Okay, Jared, we're coming in, we're approaching the zone, we're approaching the zone.

I've got the signal.

We're right overhead.

(over radio): Okay, you can release!

(radio chirps) HOLMES: Releasing her now.

HOLMES (over radio): Gary, I see the plane, I hope the bat's coming with you.

(radio chirps) MCCRACKEN: Okay... okay...

Okay, okay-- got it!

Got it, good.

NARRATOR: When the bat flies just underneath the plane, the radio signal gets stronger and the pursuit begins.

As soon as the bat veers off, the signal weakens, allowing Gary to guide the pilot to stay on the bat's course.

(device beeping at low pitch) Can you speed up just a little bit?

We're losing her, we're losing her.

A little bit, a little bit.

(device beeping at high pitch) Right on top, got it!

NARRATOR: The plane is able to follow the bat for three hours as it circles the area, hunting flying insects.

Now she's heading back north, heading back in the direction of the cave.

I think she, our bat went home.

This is so cool-- wow.

NARRATOR: When radio telemetry was used a few years ago, it allowed scientists to track the Mexican free-tailed bat for the first time in mid-flight with jaw-dropping results.

MCCRACKEN: We knew the bats were flying long distances.

We knew that this particular type of bat can fly really, really fast.

But we, we didn't expect to see this, this sort of performance.

We think we've seen a bat going 100 miles an hour.

NARRATOR: After studying the data, initial field observations were confirmed: the Mexican free-tailed bat got up to speeds of about 100 miles per hour, the fastest horizontal flight of any animal ever recorded.

But even if bats have proved to be the fastest flyers, how would that help them to resist diseases?

Back on terra firma, scientists at Brown University are studying the possible connections between bat flight and bat health.

Kenny Breuer is an aeronautical engineer, and for the past 15 years, he has been creating mechanical wings that imitate the bat's anatomy.

His prototypes have improved, but nothing comes close to the real thing.

(mechanical whirring) They have, however, helped him understand the physical effort required for bats to navigate the skies.

BREUER: Flying is an expensive operation in terms of energy-- it takes a lot of energy to get into the air and to propel yourself.

And you have to not only generate your own thrust, but you have to overcome the drag that is, that is experienced by your body and by your wings.

NARRATOR: Scientists estimate that the physical effort expended by a bat in flight is about three times more than a terrestrial mammal of the same size running at full speed.

The heartbeat of certain flying bats can reach 1,066 beats per minute.

(heart beating rapidly) Could this level of activity, unrivaled by any other mammal, somehow explain bats' super-immunity?

SWARTZ: A few measurements have suggested that body temperature in bats might be unusually high.

This has led some scientists to suggest that bats' body temperatures might be so high, that it's as if they continually operate at fever-like temperatures during their nightly flights.

NARRATOR: Fever is well-known as a means of fighting infection.

High temperatures slow down the replication of the virus and boost the foot soldiers of the immune system to devour intruders.

A feverish body is a hostile environment for a virus.

So could the extreme energy spent during nightly hunting forays cause a spike in body temperature that would protect bats from viruses?

To know for sure, scientists must collect data in perfectly controlled conditions.

This is where the Egyptian fruit bat comes in.

With its two-foot wingspan, it is a remarkable flying machine.

Equipped with expertly placed mini-thermometers, the animal takes flight under the team's watchful eye.

ANDREA RUMMEL: Oh, my God, that's not bad!

I'm very impressed.

- Great spread!

RUMMEL: Yeah.

NARRATOR: The experiment was performed on four different bats, and the result was exactly the same for each one.

RUMMEL: We got these temperature traces for three muscles along the bat wing.

So the red is a muscle that's in the core, the pectoralis muscle, which is really important for flight.

And then we have the biceps and the muscle in the forearm of the bat.

So closest to the core, and then the blue curve is furthest from the core.

And as time proceeds, the red and the green muscle stay pretty close to the high body temperature that it started with.

But as we move through time, the blue muscle, the forearm muscle that's further away from the core, gets really cold and stays cold.

As they're flying, they're flapping their wings.

And so heat is going to be wicked away from, from the bat wings, just by virtue of their movement.

And so bats are really effective at dumping heat, even if they're generating a lot, and their body temperatures stay fairly normal.

NARRATOR: In other words, the naked wings of bats act as an ultra-efficient cooling system that keeps their temperatures from rising.

There's no fever-like temperatures that could explain their super-immunity.

But some researchers are still convinced that flight must have somehow helped shape their immune system.

It sounds, like, very promising... NARRATOR: One believer is Linfa Wang, and he thinks he's found out how.

(indistinct chatter) WANG: Especially in the very ancient bats, when they just acquired flight capability, the number-one challenge they have to deal with is this high metabolism.

NARRATOR: The high metabolism required for flight should lead to inflammation: when animals' muscles work really hard, the intense physical activity creates toxic by-products, and these usually trigger inflammation.

Inflammation intrigues Linfa, because it is also caused by viral infections, and in humans, too much inflammation can have devastating effects.

WANG: For other mammals, human included, when the coordination goes, you know, out a window, and then when you over-defense, that actually cause the pathology.

You know, now you get disease.

So we have a clich in our field to say, "Very few virus kills us, we kill ourself."

NARRATOR: This is what happened in some of the most severe cases of COVID-19, when patients' immune systems raged out of control with so-called cytokine storms.

Cytokines, like interferons, are molecules manufactured by the body to regulate an immune response in case of an attack.

Sometimes, the system goes berserk and produces too many cytokines.

The resulting inflammation doesn't just hurt the virus, but everything in its path, including organs like the lungs, heart, and even the brain.

(EKG flatlining) But bats don't seem to experience these symptoms.

So, have bats figured out a way to control the inflammation associated with both high metabolism and infection?

To find out, Wang's team is mixing bat immune cells with toxic molecules that, in humans, would trigger inflammation.

AHN: So actually, we have isolated bat immune cells and treated them with the toxic substances that are produced by the body when the metabolism is high.

NARRATOR: In most animals, like humans, these toxins trigger the production of a protein called NLRP3, which in turn ramps up the immune response and inflammation.

This microscope reveals the presence of the protein in the form of a red dot.

AHN: We are comparing the inflammatory response between human and bat cells.

NARRATOR: In the human cells, the red dot shows that the protein is being produced, meaning the immune response has begun.

But over on the bat side, there are no red dots, meaning no protein and no immune response.

Their cells seem to have tolerated the attack of the added toxins without any immune reaction.

AHN: So, the bat have naturally tempered NLRP3 protein, so that the stress-related and the viral-induced inflammation always stay under control.

NARRATOR: To become successful flyers, bats had to tamp down their response to the toxins produced during flight and prevent inflammation.

Linfa Wang thinks this same anti-inflammatory chemistry is what's preventing bats from over-reacting to viruses.

WANG: Bats are very good virus reservoirs.

You know, we believe is that their adaptation to flight.

So that created a very different immune system.

Of course, that was evolved not to host virus, per se.

That's evolved adaptation to flight.

So their ability to host virus is almost like a by-product, in my view.

NARRATOR: For the team in Singapore, this unique adaptation isn't just an evolutionary curiosity-- it could pave the way to revolutionary new therapies for all sorts of human diseases that involve inflammation.

AHN: In COVID-19 infections and many age-related chronic diseases, such as Alzheimer's or stroke, coronary artery disease, diabetes-- in all these diseases, inflammation is over-activated.

That cause a lot of problem.

WANG: I'm really excited, from a basic scientist's point of view, is that we are studying a very important mammal as a model for living, you know, a health, you know-- I mean, to health living and longevity, yeah.

NARRATOR: This is the paradox of the bat.

Held responsible for a pandemic, could the bat also be the source of potential new cures?

Not just to fight disease, but also old age?

Could the bat, maligned and misunderstood as it is, also teach us the secret to growing old healthier?

This is Beganne, a village in Brittany, France, whose bell tower is a well-known refuge for bats.

Every summer, dozens of female greater mouse-eared bats roost in its rafters, giving birth to their pups-- a species whose exceptional, long lifespans fascinate scientists like Eric Petit.

But he must wait for nightfall to spot the newborns and their mothers.

PETIT (translated): With the greater mouse-eared bat, you have to be patient.

They don't come out very early, so we've often got to wait a long time.

(speaking French) (translated): We're in front of the exit.

In this colony, there's about 90 adults.

We're hearing something over there.

I think they're just behind the drainpipe.

NARRATOR: They're difficult to see, discreetly slipping out from behind the drainpipe.

But a thermal camera reveals the frenzied nocturnal ballet taking place around the church.

In the surrounding underbrush, this nocturnal acrobat shows the full range of its agility.

PETIT (translated): The greater mouse-eared bat is known for hunting between 12 and 24 inches from the ground.

They listen for beetles making noise walking through the underbrush.

(speaking French) (translated): As soon as they hear one, they jump on top, grab hold of it, and fly off.

NARRATOR: But it's not their agile flight or unusual hunting methods that have caught the attention of scientists the world over.

It's their amazing longevity, which seems to defy the laws of nature.

PETIT (translated): There is a general rule in biology.

Smaller animals don't live very long, while larger animals live much longer.

Mice live for a couple of years, while elephants can live dozens of years.

NARRATOR: The oldest greater mouse-eared bat ever recorded was 37 years old.

But the record for longest life is actually held by a cousin of the greater mouse-eared bat.

The Brandt's bat weighs less than a quarter of an ounce, yet researchers captured a specimen that was at least 41 years old-- a lifespan ten times longer than theoretically expected.

PUECHMAILLE (translated): What's really fascinating with bats is that if you capture an individual that is two years old, or one that is 15 or 20 years old, you can't see any difference between the two.

With humans, dogs, and most other species, you would see an individual that has aged.

NARRATOR: Sbastien Puechmaille studies aging at the Institute of Evolutionary Science in Montpellier, France.

(Puechmaille speaking French) (translated): When we study aging, one of the first things we look at is the central part of the cell, which is shown here, the nucleus.

Inside the nucleus, you see these kinds of small Xs.

These are the chromosomes.

I've zoomed in on the most important part of the chromosome here, its extremities, which we see in red.

These are what we call telomeres.

So this telomere is a long fragment that is in charge of protecting the chromosome's extremity.

On young cells, the telomere is very long, and over time, as the cell ages, the telomere gets shorter.

At some point, it will get so short that it will directly affect the integrity of the chromosome and the health of the cell.

NARRATOR: Scientists think that the shortening of telomeres over time is one of the key triggers of cell death, influencing the aging process and the lifespan of all mammalian species.

So what's the deal with bats?

(bell tolling) For the past ten years, the bat colony at the church in Beganne has been at the heart of a study to figure out the secret to bats' long lives.

Every summer, Sbastien Puechmaille meets up with Emma Teeling and her team to collect samples that allow them to follow individual bats and their aging process.

An implant gun is used to insert a magnetic identification chip the size of a grain of rice under the skin between the shoulder blades.

Dozens of juveniles had been tagged this summer.

(translated): When we say "tag," it means inserting these tiny microchips, like we do with dogs and cats at the vet.

This allows us to recognize the same individuals year after year and to follow their aging.

NARRATOR: The oldest tagged individuals are now ten years old.

These are our sample numbers, so what we take back to UCD.

Right.

So we know who's who, and there she is.

TEELING: Isn't that beautiful?

WOMAN: Gorgeous.

Is that a baby?

TEELING: We're gonna find out now in a minute!

PUECHMAILLE: It looks like an adult.

TEELING: Do you want to bet?

WOMAN: Yeah!

TEELING: You say a baby or an adult?

PUECHMAILLE: Adult.

TEELING: Okay, can we, will we check to see by shining?

It's an adult!

Now we are going to take the blood.

Quite dark, isn't it?

NARRATOR: Whether it's a drop of blood or a small skin fragment, the samples taken every year are conserved carefully in liquid nitrogen.

TEELING: Do you see how relaxed the bat is?

WOMAN: Yeah.

TEELING: It doesn't hurt them at all.

As long as they're in capable hands, with people who know how to hold them properly and correctly.

So there it is!

Secret of everlasting youth.

NARRATOR: Some of the precious samples taken in Beganne are stored in Sbastien's basement laboratory in Montpellier.

To see if the greater mouse-eared bat's longevity could be linked to the length of its telomeres, scientists have compared them with those of the common bent-winged bat, a species of bat that usually dies before it reaches 20.

(translate): What you see with the common bent-winged bat, which has a short lifespan, that the telomeres shorten with age.

You see that very clear progression.

On the other hand, the greater mouse-eared bat shows absolutely no shortening of the telomeres.

On the contrary, you can see clearly that they remain constant as the individual ages.

So an individual bat which is ten years old or one year old, the telomeres will be exactly the same length.

TEELING: What we found was extraordinary.

In the longest-lived genera of bats, the myotis bats, their telomeres do not shorten with age.

And this was very unique.

We didn't really see this in any other mammal.

Telomeres shorten in us, in badgers, in sea lions.

So this was extraordinary.

NARRATOR: Emma and Sbastien believe that the greater mouse-eared bat's extraordinarily long life is linked to the resilience of its telomeres.

But how does this genetic material withstand the passage of time?

To find out, scientists compared the genes of the greater mouse-eared bat with other mammals and uncovered some key differences.

TEELING: We found two or three genes that we think are evolving in a different way in bats, that we think are the genes that allow this thing called alternative telomere lengthening happen in bats.

So bats are able to use a different mechanism to maintain their telomeres with age.

NARRATOR: Are these genes the key to the bats' long and healthy lives?

And could they one day protect against the effects of aging in humans, as well?

Scientists aren't about to turn this discovery into an elixir of youth, but researchers like Emma are optimistic for the future.

Their adventure with bats has just begun.

(crow cawing) Echolocation that allows them to see in total darkness.

Flight speed that is unrivaled by any other animal.

They are impervious to most viruses, insensitive to aging, and the masters of a marvelously controlled immune system.

Not bad for an animal so long despised.

TEELING: Looking at bats, one of the most vilified and terrifying, potentially, of all mammals.

If we look at them in a slightly different light, we will be able to find ways to improve human existence.

NARRATOR: The product of millions of years of adaptation, bats are now emerging from the shadows as extraordinary creatures that could potentially light a path for longer, and healthier, human lives.