1 - They have a lot of body fat (between their core and their exterior) which serves as an insulator (to their core) against cold.
2 - Each strand of hair is clear and hollow which allows sunlight to pass down through the core of the shaft, and...
3 - The skin is black, so when that sunlight passing down through the hair shafts reaches the skin, it is absorbed into and spreads throughout the skin's surface. This follows the same principle as black asphalt on a road or roof being warmed by the sun.
Another adaptation, a behavioral one, bears have adapted to keep warm is they "den" during the coldest part of the winter. This has a second benefit, too. It is when food is most scarce. Sleeping through it rather than exerting energy hunting and scavanging means they neither suffer through hunger pangs, nor the chill of coldest winter.
Denning (also known as "winter lethargy") is what many people inaccurately refer to as "hybernation," however, bears don't actually hybernate as their heart rate and metabolism doesn't drop low enough to be truly defined as hybernation. Basically, they are sleeping, but more deeply than typical sleep. A hybernating squirrel can actually be handled without being stirred out of their hybernation for brief periods, but If someone were to touch a bear while it was denning, it would immediately awaken and the results would be VERY unfavorable to the person who touched it! Also, in the case of polar bears, many don't den. The males almost never do. Typically only the females den, and some of them don't. Females who do usually have cubs or are going to have cubs while they're denning.
2 - Each strand of hair is clear and hollow which allows sunlight to pass down through the core of the shaft, and...
3 - The skin is black, so when that sunlight passing down through the hair shafts reaches the skin, it is absorbed into and spreads throughout the skin's surface. This follows the same principle as black asphalt on a road or roof being warmed by the sun.
Another adaptation, a behavioral one, bears have adapted to keep warm is they "den" during the coldest part of the winter. This has a second benefit, too. It is when food is most scarce. Sleeping through it rather than exerting energy hunting and scavanging means they neither suffer through hunger pangs, nor the chill of coldest winter.
Denning (also known as "winter lethargy") is what many people inaccurately refer to as "hybernation," however, bears don't actually hybernate as their heart rate and metabolism doesn't drop low enough to be truly defined as hybernation. Basically, they are sleeping, but more deeply than typical sleep. A hybernating squirrel can actually be handled without being stirred out of their hybernation for brief periods, but If someone were to touch a bear while it was denning, it would immediately awaken and the results would be VERY unfavorable to the person who touched it! Also, in the case of polar bears, many don't den. The males almost never do. Typically only the females den, and some of them don't. Females who do usually have cubs or are going to have cubs while they're denning.