How long could you stand on Antarctic ice before your bare feet froze solid? A minute, maybe two? If you're an emperor penguin, you can do it for two months, and in wind chills as low as -75ยบ F. Those naked bird feet may look positively frigid, but their special circulation acts as a kind of antifreeze to keep them just warm enough that they don't freeze. Penguins' legs and feet have evolved to lose as little heat as possible. They hold onto heat by restricting blood flow in really cold weather, keeping foot temperature just above freezing. Their legs work like a heat exchange system — blood vessels to and from the feet are very narrow and woven closely together, cooling the blood from the body on the way to the feet and heating the blood as it returns to the body. Feet get cool blood so there's less heat to lose, while the body stays toasty. Humans have the ability to restrict blood flow to their extremities in cold weather too, although not as dramatically as penguins. Your hands get whiter during freezing weather because there's less blood in them because it has been redirected to the core of your body to make sure your vital organs stay warm.
