SHANE CAMPBELL-STATON: Meet my colleagues, Nate and Zac.
They take running pretty seriously.
NATE SENNER: Why do I run?
I mean, I run because I enjoy pushing myself to my limits
and trying to figure out what those limits are.
ZAC CHEVIRON: For the past few years,
I've been doing ultra marathons.
It allows me to clear my head and think.
SHANE CAMPBELL-STATON: I mean, when
it comes to ultra running and long distance running,
I mean do your thing.
You know, it's not going to be me.
Now I'm not a runner, but Zac, Nate, and I
do share a passion for biology.
For the past three years, we've been
studying how a mouse of all animals
has evolved to become one of the most extreme endurance athletes
on the planet.
ZAC CHEVIRON: So the deer mouse, Peromyscus maniculatus,
has the widest elevational distribution
of any North American mammal.
Right.
So it occurs on the floor of Death Valley below sea level
all the way to the summit of the highest peaks
out west and everywhere in between.
SHANE CAMPBELL-STATON: If you think
about the tops of mountains, some
of the most extreme environments on the planet,
so you're talking about extremely cold, very
windy environments with very little oxygen.
ZAC CHEVIRON: Many mammals that live on mountaintops
will ride out the winter by hibernating.
Deer mice don't do that so they're active all winter long
buried by feet of ice and snow.
SHANE CAMPBELL-STATON: Staying warm and active
during cold winters requires a lot of energy.
So we wondered, how do the mice on mountaintops
fuel their activities?
The first step to answering these questions
is catching mice.
So we're out catching deer mice.
We use these traps.
They're called Sherman traps.
And you can see there's a little trapdoor that you put down.
And there's a tiny little catch inside.
Inside there, you put in a little treat.
And then as soon as a mouse gets a whiff of that
and they cross that threshold, it closes shut.
And then you wake up in the morning
and you have a little friend waiting for you.
During a night, you can put out hundreds of traps.
Depending on the site, sometimes you
may only catch like two or three mice.
ZAC CHEVIRON: Hey guys, looks like we got one here.
SHANE CAMPBELL-STATON: It's kind of like having
a little Christmas present when you wake up.
NATE SENNER: Oh yeah.
Cool.
ZAC CHEVIRON: So what's the total now?
SHANE CAMPBELL-STATON: We capture mice
from low elevation habitats, which
we call lowlands, and the tallest mountains,
which we call highlanders.
We then take them back to our lab
at the University of Montana to collect data
on their metabolism and genetics.
Lowlanders and highlanders live in very different environments
and both groups use energy to find food and mates,
avoid predators, and stay warm.
For the mice, like all animals, that energy
comes from the food they eat.
Food provides metabolic fuel that animals
transform into usable energy inside their cells.
ZAC CHEVIRON: In humans, we have a really good understanding
of the ways in which we use metabolic fuels
for different kinds of exercise.
We use two main metabolic fuels to power exercise.
Those are fats and carbohydrates.
When you're doing really high intensity burst
types of exercise, so think sprinting, there your muscles
should preferentially use carbohydrates or sugars
for fuel.
When you're doing things that are submaximal or not
your fastest speed, think running a marathon,
there it's more efficient to use fats to power that kind
of exercise.
SHANE CAMPBELL-STATON: The reason
that different fuels work best for powering
different kinds of activities has to do with a trade
off between how quickly those fuels can
be converted to usable energy versus how much energy they
contain.
We use sugars to power high intensity exercise
because they can be converted to energy quickly.
Fats take longer to convert into energy but each gram of fat
contains more energy than each gram of sugar.
And that's why fats are better for powering endurance type
activities.
ZAC CHEVIRON: So these trade-offs
about what the optimal fuel is for a given activity,
these hold across most mammal groups at least
at low elevation.
SHANE CAMPBELL-STATON: So knowing that there are two main
types of metabolic fuels--
carbs and fats-- that animals use for energy,
we wanted to ask, is there a difference
in how highland mice use these fuels compared to lowland mice.
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So we're here at University of Montana in the physiology lab,
and this is the space that we use
to simulate high altitude environments
at the tops of mountains.
So we have a small refrigerator here that we can
use to manipulate temperature.
We also have an oxygen tank that we
can use to simulate the amount of oxygen they would experience
at the top of a mountain.
Animals use oxygen to transform metabolic fuels,
like carbohydrates and fats, into usable energy,
or ATP, releasing carbon dioxide, water, and heat.
[MUSIC PLAYING]
ZAC CHEVIRON: We know that you need
different amounts of oxygen to burn a gram of fat
versus a gram of carbohydrate.
SHANE CAMPBELL-STATON: In fact, burning fats
requires a lot more oxygen than burning carbohydrates.
In this machine, what we're measuring
is how much oxygen these animals are using in order
to generate the body heat that they need to maintain function.
ZAC CHEVIRON: So what you're looking at here,
this black line is the amount of oxygen
that a mouse consumes during one of these cold trials,
and this is the amount of carbon dioxide that they produce.
By comparing the amount of oxygen
that's consumed to the amount of carbon dioxide that's produced,
we can figure out the metabolic fuels that they're burning.
[MUSIC PLAYING]
So when a mouse is burning carbohydrate,
we expect this ratio to be 1.
So for every molecule of oxygen they consume,
they produce a molecule of carbon dioxide.
But when they're burning fat, we expect
that ratio to be closer to 0.7.
And ratios in between 0.7 and 1 suggest a mixture
of carbohydrates and fats.
And what we found is that the highlanders
are fat-burning machines.
Under high altitude conditions that we simulate
in these trials, the highlanders are
able to burn fats at much higher rates than the lowlanders are.
SHANE CAMPBELL-STATON: Staying warm on these mountain tops
is like running a marathon all winter long.
And to accomplish this, highland deer mice have evolved
to burn fats at higher rates.
But burning fats requires a lot of oxygen.
And at high elevation in the mountains,
there just isn't that much oxygen available.
In an environment where oxygen is scarce, how do they do it?
It turns out that millennia of natural selection
in this low oxygen environment has
led to several adaptations that allow
these mice to get more oxygen out of the air
and into their cells.
One example of this is hemoglobin.
So this is the protein in our blood
that binds to oxygen. In highland mice,
hemoglobin is better able to bind oxygen in the lungs
so that their blood can transport more
oxygen to the tissues, and this is
due to differences in the genes that make that hemoglobin.
These genes show strong genetic differences
between highland and lowland mice,
and those differences are indicative of this history
of natural selection.
[MUSIC PLAYING]
I became a biologist because I think life is a puzzle.
The way that life finds solutions
to all sorts of crazy problems that the world
presents to it, that, to me, is just fascinating.
Species around the world have adapted
to survive in virtually every environment.
In the mountains of North America,
deer mice have evolved the ability
to take more oxygen out of the air
and use it in their cells to burn fat at a higher
rate than their lowland relatives
in order to stay warm and power their activities
in this harsh environment.
NATE SENNER: Understanding exactly what forces
have allowed mice to colonize and thrive
in these high elevation environments is really
fascinating as an athlete, because when you go up
to those places, it's painful.
There are these times when you're running,
you're like, why can't I just bring in more oxygen?
I know it's there.
I'm just not getting it.
And then I think about these mice,
and you know that they're able to do it,
but also that it's taken thousands, if not millions
of years for them to actually become
adapted for these environments.
And so that also makes me feel a little bit better
about myself because it's like, OK,
they didn't do it in just one day.
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