ago. The kilometers-thick ice-sheets that  covered much of the earth started to recede,  

and the earth entered into the  Holocene, the current geological epoch.  

But the once mighty glaciers did not  disappear entirely, and the extremes  

of our earth - the north and south poles  - are the remnants of the last ice age.

These frozen tundras easily reach temperatures as  low as -40°C, and sometimes dip far below that.  

Darkness envelops them for entire months  at a time. They are places that seem wholly  

inhospitable to any living creature. Temperatures  are so low that hypothermia can quickly set in,  

and ice itself can form inside the body - its  crystals shredding tissue, ripping cells apart.

But for millions of years, evolution has forged  remarkable adaptations across the animal kingdom  

to ward off the icy grip of death. It's the  reason Arctic animals have specialized heat  

generating tissue, and the reason polar bears are  invisible in infrared vision. It's what has shaped  

our circulatory system, and why so many traits  appear in cold weather animals that appear nowhere  

else in the evolutionary tree. When the ice starts  to take hold, what hidden strategies, compounds,  

and dynamic physiologies of animals that live  in these frozen tundras allow them to survive?

Subzero temperatures are a  problem for all living things,  

because all living things are largely  made up of water, and when water freezes,  

it expands and becomes sharp - this is frostbite,  a death sentence for any cells it forms inside.  

Cells that make up vital organs also  have optimum operating temperatures,  

and when core body temperature falls, the  heart, nervous system and other organs can't  

work normally. This hypothermic state can lead to  complete organ failure, and eventually to death.

And so to survive an extremely cold climate,  

there are 2 things an animal can do: prevent  heat from leaving the body, or create more.

The animal kingdom is made up of endotherms and  ectotherms - the fancy words for warm-blooded,  

and cold-blooded animals. It's a misconception  that cold blooded animals don't create heat like  

warm blooded animals - they do. They just can't  retain it and regulate it like the endotherms can.  

Endotherms - primarily birds and mammals  - use metabolic heat to maintain a stable  

internal temperature, often one warmer than  their environment. Cells produce heat as they  

burn up energy, in exothermic chemical reactions,  and endotherms can use this to their advantage.

When it gets particularly cold outside,  endotherms can increase metabolic heat production,  

in what's called thermogenesis. One  of these methods is something we are  

all familiar with on a very cold day -  shivering. Skeletal muscles tighten and  

relax in rapid succession to produce  extra heat and keep the body warmer.

But some animals, especially hibernating  ones, have an extra heat generating power.

Using a specialized fat tissue, called  brown fat, or brown adipose tissue,  

animals can create heat without moving a muscle.  This is called non-shivering thermogenesis.  

Brown fat works very differently from white  fat, which is the fat we are more familiar with.  

White fat stores extra energy, and is what  builds up in obesity. But brown fat breaks  

down sugar and white fat to create heat -  burning up calories instead of storing them.

Brown fat generates three hundred times more  heat than any other tissue in the body. It is  

loaded with much more mitochondria than white fat,  which also act differently than most mitochondria.  

Rather than creating ATP - the energy  carrying molecule - like most mitochondria do,  

these mitochondria are designed to  turn fuel molecules directly into heat.

Small mammalian hibernators use brown fat  more effectively than any other creature.  

Before hibernation, small mammals undergo  a large increase in brown fat - not to  

get them through hibernation itself, but  rather, to help them to get out of it.  

Hibernation involves going into a state  of decreased physiological activity,  

usually paired with a reduced body temperature and  metabolic rate. This decreased state of activity  

is called hypothermic torpor. It enables animals  to survive periods of reduced food availability,  

like in harsh frozen winters. But when more  favourable conditions arise, these animals need  

to be able to raise their body temperature  to return to the world in search of food.  

And arousal from this state is no trivial task. It  takes about one hour, and involves violent shaking  

and muscle contractions, while brown fat works to  burn off energy stores to rapidly warm the body.

But creating heat like this comes at a great  cost, energetically speaking. As much as 75%  

of the total energy expended during a torpor  bout is used just for arousal. A huge amount  

of calories needs to be consumed for any amount  of thermogenesis. And this is why sometimes,  

it's better to not just create heat, but to  keep it from escaping in the first place.

The temperature of water in the Arctic  Ocean hovers around −1.8 °C. It can be  

at a below-freezing temperature due to its salt  content. Since water conducts heat 25× times more  

effectively than air, water this cold is capable  of sapping away large amounts of body heat. But  

what should be an inhospitable place for mammals  is instead full of them. And surprisingly, marine  

mammals have no special increased heat generating  mechanisms, no increased metabolism to keep them  

warm. Instead, they have an adaptation found  nowhere else in the animal kingdom - blubber.

Blubber is a specialized layer  of fat that lives under the skin,  

and is anatomically and biochemically  adapted to be an efficient thermal insulator.

Like other adipose tissue, blubber is composed of  numerous fat cells called adipocytes, which are  

filled with lipids - aka fat. Blubber can be up  to 93% lipid with very little water content, and  

because lipids have a low thermal conductivity,  it does not transfer heat as well as other tissues  

like muscle or skin. This makes it an excellent  insulator. Blubber covers the entire body of  

animals such as seals, whales, and walruses—except  for their fins, flippers, and flukes.  

Animals with the thickest blubber, like right  whales, can have blubber as much as 50cm thick.

Other animals, like polar bears,  

have up to 11cm of fat surrounding them  to help keep them warm in the water.  

But when on land, a different adaptation exists  to keep them warm - their heavy fur coat.

Close to their body is fluffy white fur, which  traps dry, warm air next to the skin. And all  

around that is their outer coat, which is made up  of hollow, transparent hair called guard hairs.  

And, because of these hairs, polar bears are  almost completely invisible in infrared vision.

Infrared radiation is a part of the  electromagnetic spectrum that we can't see  

but can feel as heat. Warm bodies emit this  radiation, which is what infrared cameras  

can pick up on. Polar bear guard hairs absorb  this radiation more effectively than almost  

any animal, hanging on to the warmth rather  than letting it be lost to the cold environment.  

And because nearly all of the infrared radiation  is absorbed, it won't show up on infrared cameras.  

This makes aerial surveys and population counts  challenging for researchers - their white color  

camouflages them, and only their nose, eyes,  and breath appear in the infrared spectrum.

A polar bear's specialized fur is a very  effective way to trap heat against the body,  

but some animals aren't lucky enough to  have such an effective coat - most notably,  

us. This is why a more hidden adaptation  courses through the body of many endotherms .

The body's surface is the main  site for heat exchange with  

the environment - especially  in those of us with no fur.

As warm blood flows to the outside  of our body, toward the skin,  

heat radiates away and is lost. And so, when  the temperature outside plummets, the flow of  

blood to the skin has to be controlled to keep  the body's core temperature at a safe level.  

And while hidden beneath the surface, you have  probably felt this effect. Once the body senses  

cold, it constricts the thin web of capillaries  in your extremities, starting with your fingers  

and toes, and then moving farther up your arms  and legs. This shrinking of the blood vessels  

is called vasoconstriction, and its goal is  to keep warm blood around the vital organs,  

keeping them safe, even if it means  risking frostbite in the extremities.  

Many endotherms have this vasoconstriction ability  for when the weather gets cold. But to keep their  

extremities from being damaged, some animals have  countercurrent heat exchange systems that allow  

heat to be transferred from blood vessels  containing warmer blood to those containing cooler  

blood. Animals like wolves use this extensively  in their legs and feet to keep them from freezing.

Blood leaves the wolve's core at a warm body  temperature and travels towards the feet,  

while the blood returning from the feet  has been cooled down from the environment.  

As the cold blood runs up the leg from  the foot and passes by the warm arteries,  

it picks up most of the heat from the  arteries from conduction. And conversely,  

the blood flowing down from the body is  cooled. This means that less heat will be lost  

from the feet due to the now reduced temperature  difference between the blood and the surroundings,  

and that the blood moving back into the body's  core has been warmed, helping maintain the body's  

core heat. Humans can do this to some extent,  but not as well as many of the arctic endotherms.

Instead some people - particularly  people whose ancestors lived in arctic  

climates - take the vasoconstriction  of the blood vessels one step further.

After some time in the cold, certain people's  constricted capillaries will suddenly  

dilate - sending a rush of warm blood into the now  freezing extremities, briefly warming them, before  

constricting again. This prevents the extremities  from being severely damaged by frostbite,  

while still ensuring that the vital organs stay  warm. The cycle of constriction and dilation is  

called the Hunter's response. People who live in  cold environments, or people whose ancestors did,  

have this automatic response. Inuit  hunters, for example, can raise the  

temperature of the skin in their hands from  almost freezing to 10C in a few minutes.

Cold tolerant animals, like those found  in the polar extremes of the earth,  

are at constant war with freezing temperatures.  Behavioral strategies, physiological processes,  

and anatomical features all work together  to keep the cold out of their bodies.

But what if avoiding freezing wasn't the only  option? What instead, you just let the ice take  

you over? For us, and most animals, this would  mean certain death. But for some strange animals,  

giving into the cold entirely is not only  possible, but advantageous. Some animals even  

take it so far as to freeze completely solid.  Their heart stops, their brain activity ceases,  

and yet - come spring, they completely recover  from what should have been certain death. In the  

next video, we'll explore how scientists have  realized that preventing freeze is not the only  

way to survive the cold, and see how we humans  are trying to harness these powers for ourselves.

Behind every piece of research, every scientific  story - there is a human one. Much of the ecology  

and physiology explored in this video was only  made possible by the people in the field, in these  

icy landscapes taking measurements or observing  animal behavior. These human stories fascinate  

me as much as the science ones, and this is why we  decided to start a podcast to explore just this.  

Modulus - hosted by me, and Brian from Real  Engineering, is a podcast about the people  

behind the scientific stories we tell you here  on YouTube. We will talk to the scientists who  

are on the cutting edge of research, and the  people who are affected by the topics we discuss.  

From people who have spent months as  saturation divers at the bottom of the ocean,  

to the people pioneering liquid battery  technology, to the people affected by the decision  

to introduce genetically modified mosquitoes  in Florida, this podcast will show the real  

life people behind these topics, and the real life  impact these scientific stories have on the world.

The first episode of Modulus launched today  on Nebula, the streaming platform made by me  

and several other educational YouTube content  creators. It's the place to watch and listen  

to our videos and podcasts ad free, along with  original content that is not available anywhere  

else like the Real Engineerings' the Logistics  of D-day, or Tom Scott's gameshow Money.  

We can take more risks on Nebula, where we  don't have to worry about the YouTube algorithm.  

There is so much original content there,  with more being added all the time.

And to make it even better, Nebula  has partnered with CuriosityStream,  

the streaming platform with thousands of high  budget, high quality documentaries. There are  

loads of documentaries about ecology and animal  behavior, like this one called “Polar Bears.” It's  

a beautiful overview of polar bear biology, with  stunning cinematography of the Arctic landscapes.

If you've hesitated before to get CuriosityStream  and never quite pulled the trigger- now is  

definitely the time to do it. For a limited  time, CuriosityStream is offering 41% off  

their annual plans, making a yearly subscription  just 11.79. That's less than a dollar per month!

So by signing up at  curiositystream.com/realscience,  

you will get a subscription to CuriostyStream  and a subscription to Nebula, for just  

$11.79 for the entire year. Signing up is  also the best way to support this channel,  

and all of your favorite  educational content creators.

Thanks for watching, and if you would  like to see more from me the links to  

my instagram, twitter, and patreon are below.