How animals keep warm and cool
Human beings belong to an animal group called the mammals. All the animals in this group can keep their body temperature steady just as we do. There is only one other group of animals which can do this. They are birds. Animals which can keep their body temperature steady are sometimes called warm-blooded animals.

None of the other animals on the planet can keep their body temperature steady. Their body temperature changes with their surroundings. They are sometimes called cold-blooded animals. It is not really accurate to use the word cold-blooded because during the day the temperature of cold-blooded animals can rise as high as the temperature of warm-blooded animals. The main difference is that when night comes and the air becomes cooler, the temperature of the warm-blooded animals stays steady but the temperature of the cold-blooded animal falls.

Imagine for a moment what it would be like if you were a cold-blooded animal. When you woke up your eyes would open slowly and you would look around. If you could see the Sun shining you would decide to move into the sunlight. When you tried to move your arms and legs you would find that they could only be moved very slowly. It would take you some time to crawl into the sunlight. When the Sun's rays struck your body, heat energy in them would pass through your body surface and warm it. The heat would pass into your blood by conduction and your heart would slowly pump it round the body. As you basked in the Sun, more and more heat would enter your body and it would start to warm up. After a while you would find that you could move your limbs quite quickly. You would also be able to think quite quickly and plan what you were going to do. Inside your stomach your last meal from yesterday would finish off being digested and the energy from the food would give you extra heat to get on with your life of finding food and avoiding animals that would eat you.

As the Sun sets and the air cools you would find yourself moving and thinking more slowly. The digestion of your food would also slow down. At this time you would find a safe place to hide until morning. You would try to make sure that it was near a sunny place so you could begin another day by basking in the sunshine.

Do warm-blooded animals all have the same body temperature?
No. There are many different temperatures. Our body temperature is 37°C. The temperature of a hedgehog is 35°C, the temperature of a dog is 38°C and the temperature of a sparrow is 40°C. Birds tend to have higher body temperatures than mammals.

Do birds sweat like us when they get too hot?
No. They do not have sweat glands. They open their beaks and let water evaporate from their tongues, mouths and lungs to cool them down.

How do mammals use their fur to keep warm?
They make it stand up a little. This traps a layer of air next to the skin which stops heat escaping by conduction.

How do mammals stay warm when their fur is wet?
Mammals have two sets of hairs. The inner hairs form a woolly coat which traps air. The outer hairs form a protective waterproof coating over the inner hairs. If the inner hairs were to get wet, they would stick together and the layer of air would collapse. Heat would then be lost quickly through the fur and the animal may die of hypothermia.

Whales do not have thick fur. How do they survive in the cold sea water?
Whales have a sparse covering of hair. It is not sufficient to trap a layer of air. The whale has an insulating layer below its skin. This is a very thick layer of fat. It is usually called blubber. Seals have a covering of fur but they also have a layer of blubber under their skins to keep them warm.

Why do some mammals have white fur?
Everything loses heat by radiation. Dark animals lose more heat by radiation than light-coloured animals. Mammals
with white fur lose the least heat of all by radiation. They are found in cold regions of the world. For example, the polar bear and the arctic fox are found in the region around the North Pole and the alpine hare lives high in the mountains.

Do any birds have white feathers?
Yes, they do. The snowy owl has white plumage. It lives in the arctic region. The ptarmigan, a bird rather like a pheasant, has white feathers in winter and lives on mountains.

Do birds' feathers trap air like the fur of mammals?
Yes, they do. In summer when the birds are hot they flatten their feathers against their bodies to reduce the layer of insulating air. In winter they raise or fluff out their feathers to make a thicker layer of air.

Do any warm-blooded animals ever have cold blood?
Yes. Animals such as the bat and the hedgehog hibernate in winter. They build up a layer of fat under their skin, which acts both as insulation and a food store. When they hibernate they fall asleep and their temperature falls to near freezing point. At this time their heart slows down and they breathe very slowly. They use the energy in their stored fat to keep them alive.

Can all cold-blooded animals survive at our body temperature?
No. They all have a range of temperatures in which they can survive. The frog, for example, can survive between 7°C and 30°C. If you hold a frog your hand will soon seem painfully hot so let it go as soon as you can. Lizards can have a temperature range from 18°C to 45°C. We can only survive temperatures of 43°C. Above that temperature, we die of heatstroke. The ice fish has a very small temperature range of -2°C to 5°C. It can survive in ice because its blood has a kind of anti-freeze in it.