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  • The last glacial period on earth  began to fade around 11,000 years  

  • 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 areproblem 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 reactionsand endotherms can use this to their advantage.

  • When it gets particularly cold outsideendotherms 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 dayshivering. 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 muscleThis 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 heatburning 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 fatwhich 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 conductivityit 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 walrusesexcept  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, eyesand 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 extentbut 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 temperaturesBehavioral 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 solidTheir 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 therewith 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 calledPolar 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.

The last glacial period on earth  began to fade around 11,000 years  

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隐形北极熊和其他北极适应(Invisible Polar Bears and Other Arctic Adaptations)

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    joey joey 發佈於 2021 年 06 月 09 日
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