Eli5: Why do small animals like squirrels not lose appendages to frostbite when temperatures reach below freezing, and they walk around on ice and snow all day?
In: Biology
Essentially, it’s because they have fur that we do not. In the case of a fox, they have a thick layer of fur that grows in to trap their body heat. Other animals such as whales or walruses, have a thick layer of fat to keep them insulated.
To add, frostbite does not directly affect the skin, it affects the internal body temperature. Frostbite is caused when a layer of the body reaches a temperature low enough to freeze a deeper layer of skin. Because humans do not have a thick fur, our skin is exposed much more to the cold and is more likely to freeze. However by insulating the body, fur can keep the body heat trapped, preventing freezing.
I think most people’s answers here are wrong. The main reason four-legged creatures can walk without shoes in freezing temperatures, in general, is that they have different vasculature in their feet than we do. Their feet get more blood flow to stay warm, and their fur keeps their blood warm enough to keep their feet warm. They also have thicker skin on their feet which protects the deeper “alive” layers of skin from frostbite. Someone else would have to tell me the answer to this, but I would bet that people that don’t wear shoes have at least somewhat better resistance to frostbite than those who don’t due to skin thickness and the increased vasculature in their feet from using them more. I know there are some people of Scandinavian descent that have a gene variant that allows them more resistance to coldness in their hands due to increased vasculature, which causes your hands to cripple when exposed for too long in you and me. It’s a survival adaptation.
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Bird legs work like the kidney’s Loop of **Henle**
Basically, warm blood going down into the foot is right next to the blood going up. The body stays warm but the foot is nearly frozen. The heat goes from one side to the other. Near the top of the loop, warm blood from the body loses heat to colder blood coming back from the foot. Here are a couple of pictures.
https://en.m.wikipedia.org/wiki/Countercurrent_exchange
https://images.app.goo.gl/mKbMbikocBAaXoJ88 (Kidney: loop of Henle)
Edited: thanks to folks below who corrected my spelling and apologies to Mr. Friedrich Gustav Jakob Henle It’s HENLE not HENLEY.
Studied vertebrate physiology for a year in college, the basics boiled down to this. Small furred animals have enough insulation with their fur, fat built up ready to hibernate as well as just store energy and protect them in the winter time, and better circulation due to their small size. A squirrels heart beats 300 times per minute in the winter, that’s roughly triple a normal persons. Rodents such as squirrels, rabbits, raccoons, etc. eat extra food and build up fat during the winter, whether you can visually tell or not is irrelevant. You try to tell people’s body shapes from how and what they were outside right now and you’d probably be wrong.
TLDR: Faster heart beats for smaller animals mean more heat is pushed around the body keeping them warm. They also have fur that thickens in the winter usually and eat extra food to have some fat that helps insulate.
Evolution.
We are adapted to living in the hot grasslands of Africa. That’s why we suck at living in the cold. If you grabbed another animal from that same area and made it wander around Alaska in December it would soon die.
First, animals in cold areas have fur adaptations, both long guard hairs and small ‘fluffy’ hairs You can find these between the toes of may animals that have developed their winter coats.
Second – animals have different levels of bloodflow in their furthest extremities. Birds have very little bloodflow through their feet, meaning when exposed to cold they aren’t as impacted as other animals (although they will still stand on one foot holding the other close to their bodies to conserve heat…and cold weather birds have feathers on their feet for warmth). Just like low bloodflow, some animals address it with high levels of bloodflow.
Third, there are other adaptions like ‘pads’ on paws. Think about a dog’s foot. Those pads are composed of keratin (the same substance that makes up hair ,fingernails, and horns) collagen, and a special fat. Think about the thickest most calloused hands you’ve ever seen. Do you think that they could a hot plate a second longer than normal before the heat soaked through? The pads on a dog’s feet are much thicker than that, plus with fat and collegen mixed in. This works good for cold surfaces, hot surfaces, rough surfaces, etc.
Fourth, when it gets REALLY cold, animals seek shelter, frequently curling up in a ball to protect those extremities.
Countercurrent heat exchange **does not** prevent frostbite, if anything it reduces peripheral limb temperature as well as promote vasoconstriction. It does however preserve core body heat, reducing the risk of hypothermia.
PS it’s the same stuff that keeps yer balls cool.
[https://en.wikipedia.org/wiki/Countercurrent_exchange#Countercurrent_exchange_of_heat_in_organisms](https://en.wikipedia.org/wiki/Countercurrent_exchange#Countercurrent_exchange_of_heat_in_organisms)
Good explanations here as to why most animals don’t lose their feet to frostbite but I should point out one thing that anyone living in a rural area will know:
They can and they do.
Doesn’t happen as often as it could for reasons others have stated here but animals absolutely can suffer miserably in cold conditions.
Because of the metabolic ability, like the bird’s bare feet on ice, to thermoregulate their blood flows (think different ratio than us of volume/size of the arteries carrying blood into the legs) and heat exchange vascular system (think counter current heat exchange system) to achieve homeostasis (the condition of optimal functioning of the body). In short, this heat exchange system allows for the tissues in the feet to receive just enough heat to prevent cell death, and can greatly reduce heat loss, up to 90% depending of the species. It really is a marvel of adaptation.
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