A small child next to us looked down at her snow-covered boots, then pointed to a duck that stood on the ice on the bank and asked her mother an extremely good question: “Why don’t his feet get cold?”…
It’s this: The bigger the temperature difference between two objects when they touch, the faster heat will flow from one to the other. Another way of putting that is to say that the more similar the temperatures of the two objects are, the more slowly heat will flow from one to the other. And that’s what really helps the ducks. As all that frantic paddling was going on, warm blood was flowing down the arteries of each duck’s legs. But those arteries were right next to the veins carrying blood back from the feet. The blood in the veins was cool. So the molecules in the warm blood jostled the blood vessel walls, which then jostled the cooler blood. The warm blood going to the feet got a bit cooler, and the blood going back into the body was warmed up a bit. Slightly farther down the duck’s leg, the arteries and the veins are both cooler overall, but the arteries are still warmer. So heat flows across from the arteries to the veins. All the way down the duck’s legs, heat that came from the duck’s body is being transferred to the blood that’s going back the other way, without going near the duck’s feet. But the blood itself goes all the way around. By the time the duck’s blood reaches its webbed feet, it’s pretty much the same temperature as the water. Because its feet aren’t much hotter than the water, they lose very little heat. And then as the blood travels back up toward the middle of the duck, it gets heated up by the blood coming down. This is called a countercurrent heat exchanger, and it’s a fantastically ingenious way of avoiding heat loss. If the duck can make sure that the heat doesn’t get to its feet, it has almost eliminated the possibility of losing energy that way.
So ducks can happily stand on the ice precisely because their feet are cold. And they don’t care.