[whimsical music]
I'm just going to come out and say it.
I think bats are super weird.
I mean that in an endearing way.
I actually like them a lot.
I think they're these like fluffy, flying mammals
with a lot to offer, like tequila.
They pollinate the agave plant.
The trouble is, when bats hit the media
it’s almost always over some deadly disease
like Ebola...
or COVID-19...
and even the recent Nipah virus outbreaks in India.
Bats are frequently taking the blame.
And yet we're not finding caves
full of bats that died from coronavirus or Ebola...
because bats don't just host deadly diseases.
They seem to tolerate them really well.
Researchers think it's because they evolved
with a unique ability.
They're the only mammals that can fly.
So there’s a number of unique features of bat immune systems
and their physiology that we think have co-evolved
with the evolution of flight.
Flying requires an incredible amount of energy.
A bat in flight elevates its basal metabolic rate up to 15 fold.
And to put that in context, a human at full speed running
is only doubling their metabolic rate.
And a rodent, perhaps, elevating it seven fold.
So this is more than twice the energetic expenditure...
of any form of terrestrial locomotion.
Their heart rates can skyrocket
to over a thousand beats per minute.
That's around 4 or 5 times more than their resting heart rate.
But there's even more.
It's honestly remarkable that when a bat takes flight
it doesn't just drop dead.
So a bat in flight will elevate its body temperature
up to 44 degrees centigrade.
And 45 degrees centigrade is kind of the lethal limit
of what mammals can tolerate.
And that holds true for bats as well.
So when in flight
they're really pushing the sort of physiological limits...
of thermal tolerance.
And while all of this is fascinating, you're probably wondering...
what does this have to do with viruses?
Well, stick with me because we are getting there.
So flight is essentially a huge stressor that resembles aging...
and because it's a part of bats’ innate physiology
they’ve evolved unique molecular pathways
to mitigate that damage at the site of production.
So essentially, bat physiology is resilient
to the process of aging.
You see, despite burning a ton of energy all of the time
bats are shockingly long lived.
Usually there are trade-offs between
metabolic output and lifespan
and rodents are a great example of that.
Typically, they only live up to a couple of years.
Bats, however, are about the same size
and have metabolic expenditure
that even exceeds that of rodents.
But they're extremely long lived.
The oldest known that lives up to 40 years in the wild.
So after actually correcting for body size
bats are the longest lived mammals on Earth.
So these special pathways that they've developed
in order to mitigate the intense effects of flying
have likely made them extremely tolerant to viruses...
or at least primed them to successfully fight them off.
[percussive, pensive music]
When a virus enters a human, two things happen.
Okay, more than two things happened.
But let's focus on two, because who has the time?
Think of these like two switches that need to be turned on
when a virus infects a cell...
and off when the body is clear of the virus.
So when a virus shows up
it enters a cell and your body raises an alarm like:
Hey, there's a guy here who's not even supposed to be here!
That flips the alarm switch...
which releases some molecules
that slow the virus from replicating.
And then the second switch flips
which launches an immune response to kill off the rest of the virus.
But there's a catch.
Once that immune response switches on
our bodies can sometimes start to experience some pretty severe symptoms...
like a fever, body aches, and essentially inflammation...
which, sure, fights the virus but also...
The immune response causes
widespread inflammation in the body
which is and of itself highly fatal.
But bats can't have their body triggering inflammation
every time they experience an extreme stressor...
because their entire existence as a flying mammal
is basically an extreme stressor.
And so their switch responses to an invader
work a bit differently than ours.
So in some species their alarm switch is perpetually on.
In other species, it's entirely in overdrive...
which means they're sounding an alarm
and releasing molecules to stop a virus from replicating
before it even enters their body.
And their switch that triggers inflammation
kind of has a permanent dimmer.
So they don't feel these strong side effects.
Bats are particularly adept at reducing viral loads
to very low levels and simply tolerating whatever is remaining.
Which might be part of the reason why the viruses they do host
are so much more dangerous to humans.
The longer a virus stays in a host, the more chances it has
to replicate, mutate, jump to a new bat or animal
and ultimately becomes something more deadly.
But before you start thinking about bats as these
horrible, disease ridden flying creeps...
it's worth noting there is a lot we can learn from them...
including how to better treat disease in humans.
At least one group of researchers in Dublin
recently received a grant to study just that.
And some of our methods
already work to mimic the switches in a bat...
including how we treat many severe COVID cases.
We first treat patients with Remdesivir
which is an antiviral drug that tries to reduce the viral load...
but then later on in infections we’ll treat with dexamethasone
which is a steroid...
that's meant to actually blunt that immunopathology.
So it's anti-inflammatory.
And so we turn the alarm on and we dim the other switch.
Of course, bats aren't invincible.
And recent studies have shown that bats under extreme stress...
caused by things like rising global temperatures
habitat loss and human interference...
are more likely to shed and spread viruses.
So I feel strongly that there is a win-win argument
for bat conservation and human public health...
because a healthy bat is simply less of a zoonotic threat
to humans than as a sick bat.
While that does pose zoonotic risk
they also offer a really valuable...
research source for new approaches in medicine.
Hey everybody, thanks for watching.
I'm Kim, the producer of this episode, and there is just so much
we did not get to cover in this.
Including how a broad diversity of species of bats
might also contribute to why diseases they carry are so deadly...
or even just the things that do affect bats...
like white nose fungus.
There's a long list of topics to explore
and this piece is really just meant to be a primer.
But hopefully it makes you want to learn more.
I've included a bunch of links in the description
to help you get started.
Before you go, I do have a request.
We choose stories for Science, Explained
to help foster curiosity in the world around us
and maybe offer you a lens you haven't looked through before...
without a paywall.
To help keep our journalism free
free and support more of our work
you can go to vox.com/give-now.
As a special thank you to those who make reoccurring contributions
you'll get links to video tutorials, newsletters, and Q&As
with Vox journalists across our entire newsroom.
Thanks again for your continued support.