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When the world gets super chilly, humans like us can just throw on some extra layers
to stay warm because our bodies make tons of heat.
But it does basically no good to put a sweater on a turtle,
because they rely on external heat sources instead.
You might think that would restrict them to warm environments,
but so-called “cold-blooded” animals live everywhere—even Antarctica!
And that’s because they’ve come up with some
brilliant ways to avoid freezing to death.
We human beings like to keep our body temperature very stable,
so we think of being cold as uncomfortable.
But cold isn’t so much the issue for critters like reptiles and bugs — ice is.
Cells contain lots of water, and water expands
to form the crystal structure we call ice.
So, if the water inside of cells freezes, it can literally bust cells open.
And, in case it’s not obvious, that’s bad news for a cell.
So when temperatures are below freezing,
species that don’t make or trap a lot of heat inside their bodies
need some way of ensuring their cells don’t get busted to bits.
One way they do this is by hiding out in warm refuges—
like ponds that are frozen on top.
The ice cap acts like a layer of insulation, keeping the rest of the water from freezing.
Painted turtles, for instance, can spend an entire winter in a frozen pond.
And that’s because they’ve actually figured out how to breathe water! Kind of.
They can absorb oxygen in the pond water through their skin.
The only real problem is that at the top of a pond is also where gases like
oxygen and carbon dioxide are exchanged between the water and the air.
So when you put a barrier there, bacteria and other organisms
in the pond end up depleting the oxygen in the water.
Plus, underwater plants don’t get much sunlight to make more.
And that means any animal taking refuge under ice
must survive weeks or months with very little oxygen.
But painted turtles have a solution there, too.
They basically put their body in slow-mo. Since there’s less heat, they slow their
metabolism to a sluggish 1% of normal. And if things get really desperate,
they can break down stored sugars in an oxygen-free way.
Of course, when they do this, they generate compounds which can mess up the
acidity of their cells. And, turns out, cells need oxygen to get rid of these.
Luckily, the turtles have one more nifty trick: they can store these compounds
in their shells until they have access to oxygen again.
Evolution really thought of everything!
Now, not everyone can snuggle in a pond all winter—
some animals have to tough out sub-freezing temperatures.
Wood frogs do this by… freezing.
These frogs live in northern climates like in Canada and Alaska,
where things get pretty cold.
Sure, they hunker down under leaves and snow on the forest floor
where the temperature stays above -7°C. But they still spend much of the winter
with up to two-thirds of their body water frozen.
In fact, they can survive at temperatures as low as -2°C for nearly 200 days.
And during this time, there are no signs of life: no heartbeat, no breathing.
The frog is fine, though — it hops away when thawed.
This miraculous survival is all in the preparation.
As winter approaches, the cold temperatures activate enzymes
which pump out sugars from a stored form called glycogen.
This sugar gets shuttled through the bloodstream
and packed into the frog’s critical organs.
For example, levels in the heart jump as high as 10 times normal!
And it acts as a natural antifreeze; the sugar literally gets in the water’s way,
making it harder for water molecules to align into ice crystals.
Ultimately, that means its presence lowers the freezing point of liquid in the organs.
Wood frogs also hold onto their pee so that
their urea levels soar as high as 50 times normal.
This, too, acts as antifreeze, and maybe helps slow their metabolisms down.
The key here is that the frog’s essential organs don’t freeze,
even though much of their body water does.
Scientists are still working out how they survive that part.
But it seems like as long as those organs don’t end up frost damaged,
the frog can bounce back from being frozen!
Lest you think frogs have perfected becoming a popsicle,
let me introduce you to Antarctic nematode worms.
They’re the only animal we know of that can survive a full-body freeze.
These nematodes live in Antarctic soils
that get saturated with water which freezes and thaws repeatedly.
And they actually have two ways of surviving this.
If they’re cooled slowly, water from inside their cells
is gradually drawn into surrounding ice. So, their cells dehydrate rather than freeze.
But, if cooling happens quickly or to temperatures lower than -1°C,
their cells actually freeze.
In lab tests, more than 80% of these critters survived
being frozen for 24 hours at -1°C. And to be totally honest,
scientists are somewhat mystified by this feat,
especially since, at the molecular level,
it looks like frozen nematode cells carry on in spite of the ice inside them.
Studies have found that dozens of genes get activated after they hit -10°C —
including weird things like antiviral proteins.
And after they spend a day frozen,
most of the molecular action is by enzymes called proteases.
These chop proteins into smaller molecules,
a process that releases energy, which can be used in a pinch.
That might help explain how they survive?
But clearly, we have a lot left to learn —
like, literally how their cells don’t get busted open by the ice.
This kind of intel could prove super useful for cryogenics.
For instance, it could help us keep organs for transplant alive longer,
or figure out how to freeze whole people
so we can survive a really long space journey.
Plus, I don’t know about you, but my abilities to survive winter
under pond ice or frozen in Antarctica are pretty much nil.
So, I think we owe a round of applause to these cryogenic superheroes!
Thanks for watching this episode of SciShow!
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