Polar bears are found in very cold parts of the world where temperatures can drop as low as -20° Fahrenheit (-29° Celsius). Without proper protection, this weather can be deadly, so polar bears stay warm by utilizing their thick fur and fat, or blubber. Polar bears have evolved along with other Arctic animals to take advantage of minimal warmth, and can sometimes actually become too warm because their bodies are so efficient at trapping heat.
A polar bear's fur is the first step in keeping warm. Polar bears actually have two types of fur: long oily guard hairs and short insulating hairs. Polar bears stay warm by combining the properties of these two hairs. The guard hairs are actually hollow, and look like very small tubes of glass. The hollow guard hairs trap warmth and bring it close to the skin while also providing an oily outer layer that prevents the polar bear from getting wet. The insulating hairs trap heat close to the skin, much like insulating underwear on humans.
Blubber also helps polar bears stay warm, by providing an insulating fat pad on certain parts of the body. While polar bears do not use blubber as efficiently as some Arctic animals, it certainly adds a layer of protection and can also be used for energy when food sources are minimal. Blubber is distributed unevenly across the bear's body, and ranges from 2 to 4 inches (5 to 10 cm) in thickness. Blubber also combines with the air trapping guard hairs to keep polar bears afloat in the water.
Scientists have also seen polar bears stay warm by using the snow. Snow crystals trap small amounts of air, and polar bears will burrow in the snow to create an insulating blanket which, while cold, is still warmer than the outside world. This is sometimes combined with a biological process called winter sleep, which slows down metabolic processes. Winter sleep is not quite hibernation, as the bears can be woken, but it does create lower energy demands on the body that will keep the bears warmer.
Using this variety of physical and physiological adaptations, polar bears stay warm during even the coldest winters. In the summer, polar bears shed part of their winter coat and blubber, but they may overheat in extremely warm weather. Polar bears stay cool in the heat by spreading out to increase their surface area and going for dips in the Arctic waters, which are still chilly, even in the summer.