[♪ INTRO].

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! And a special thanks to all our  channel members here on YouTube.

The money you pledge each  month as a channel member helps us keep making educational  science videos like this one. And we think keeping science education free  and accessible to all is really important! So, we really appreciate the support.

And if you’re not a member yet  and want to get in on the action, you can click on that Join button to learn more. [♪ OUTRO].