How does animal fur keep warmth in?

1. It is thirty degrees below zero, yet this arctic fox is warm inside, about as warm as you are. It has no heater and no house. What keeps its heat in?
2. Heat always moves from warm things to cold things. A fox in a thin summer coat would lose its heat to the icy air very fast, and it would have to eat far more to stay warm.
3. Zoom in on the winter coat. It has two layers. Long guard hairs form the outside. Under them is the underfur: short, fine, crinkly hairs packed very close together.
4. The secret is the air. The crinkly hairs hold thousands of tiny pockets of air and stop it from moving. Still air is very bad at carrying heat, so the warmth stays near the skin.
5. When it gets colder, tiny muscles pull the hairs upright. The coat gets thicker and holds more air. Then the fox curls into a ball and wraps its bushy tail over its nose.
6. Wind and water are the enemies of fur. Wind blows the warm air out, and water takes its place. So the long, oily guard hairs act like a raincoat and a windbreaker for the underfur.

This is a tiny piece of the fox's coat, seen from the side. Tap each part.

• Guard hairs. Guard hairs are the long, stiff hairs you see on the outside. They block wind, shed rain and snow, and protect the soft fur underneath.
• Underfur. The underfur is made of short, thin, crinkly hairs growing very close together. Its job is to trap air.
• Trapped air. Between the hairs are countless little pockets of air. Air that cannot move is a very poor carrier of heat, so it works like a blanket.
• Warm skin. Warm blood flows through the skin. This is where the body's heat arrives, and where the fur has to hold it back.
• Hair muscle. A tiny muscle is attached to each hair. When the animal is cold, the muscles pull the hairs upright, and the coat gets thicker.
• Oil gland. Next to each hair is a small gland that makes oil. The oil coats the hair, so water runs off instead of soaking in.
• Fat layer. Under the skin is a layer of fat. It is a second blanket and also a store of food for hungry times.

Switch between the thin summer coat and the thick winter coat. Then add wind or soak the fur, and watch how much heat escapes.

Experiment: The fox is wearing its thick winter coat. What happens to its heat loss if the fur gets soaked?

1. Choose the thick winter coat and look at the heat arrows.
2. Switch on "Soak the fur" and look again.
3. Dry it, then try the wind slider.

What we found: In a thick, dry coat the fox loses very little heat. When the fur is soaked, water takes the place of the trapped air, and heat pours out almost as fast as with a thin coat. Wind does something similar by blowing the warm air away. It is the still, dry air that keeps the fox warm.

Turn on JavaScript to try it yourself.

Choose the fox's coat, then send it through a cold calm night, an icy wind and wet sleet. It must stay warm in all three.

Mammals make their own body heat by burning food, and they keep their bodies at about the same warm temperature all the time. In cold weather, that heat keeps leaking away into the cold air. The faster it leaks, the more food the animal has to find.

Fur slows the leak. It does not make any heat itself. Fur works because it traps air. The fine, crinkly hairs of the underfur hold thousands of tiny pockets of air and keep the air from moving. Still air is very bad at carrying heat, so the warmth stays close to the skin. A thicker coat holds a thicker layer of air.

Animals have clever ways to help their fur. They grow a thicker coat for winter and shed it in spring. They fluff up their hair to trap more air. They curl up so that less of the body faces the cold. Long, oily guard hairs on the outside keep out the wind and the wet.

That last job matters a lot. If fur gets soaked, water pushes out the air, and water carries heat away much faster than air does. A wet animal can get cold very quickly. That is why you see animals shake themselves dry.

Key words

Insulation

Anything that slows down heat moving from a warm place to a cold place.

Underfur

The layer of short, fine, dense hairs next to the skin that traps air.

Guard hairs

The long, stiff outer hairs that keep wind and water away from the underfur.

Conduction

Heat passing through something that is not moving, like warmth through a spoon.

Convection

Heat being carried along by moving air or water.

Molt

To shed an old coat and grow a new one, as many animals do in spring and autumn.

Check your thinking

1. What is it in fur that does most of the work of keeping heat in?

   Show the answer

   The still air trapped between the hairs. Fur makes no heat. It holds air still, and still air lets heat pass only slowly.

2. Why does an animal fluff up its fur when it is cold?

   Show the answer

   A thicker coat traps a thicker layer of air. Standing hairs make the coat deeper, so more still air sits between the skin and the cold.

3. Why does wet fur keep an animal much less warm than dry fur?

   Show the answer

   Water pushes out the air, and water carries heat away much faster. The warmth of fur comes from trapped air. Soaked fur holds water instead, and heat passes through water easily.

4. What are the long guard hairs for?

   Show the answer

   Keeping wind and water away from the soft underfur. Guard hairs are a raincoat and a windbreaker. They keep the air in the underfur still and dry.

5. In the model, the fox loses about 16 watts in 48 mm of dry fur. Roughly what does it lose in 24 mm?

   Show the answer

   Nearly twice as much. The coat's resistance is nearly proportional to its thickness, so half the thickness lets almost twice the heat through.