We all know swimming in freezing ponds during winter isn't the brightest idea. Hypothermia can set in in seconds for us humans, and we go to great lengths to avoid contact with freezing water as a matter of survival. Usually, only those crazy enough to do polar bear plunges would take a dip in a pond in the middle of winter for fun. But while it's miserable for humans, ducks don't seem to be bothered at all by the chilly water. How is it they can hang out in an ice-cold pond and their thin, bare feet don't sustain irrepairable damage from exposure to the cold?
The trick, it turns out, is all in how they circulate blood through their feet. Ducks' feet aren't equipped with insulating layers of fat or feathers, so they have to be able to minimize how much heat they lose through their feet through blood circulation.
Quarks, Quirks and Quips explains it succinctly: "To maintain healthy tissue, and prevent frostbite, you need to provide nutrients to the tissue and keep it warm enough so that it doesn’t freeze. In ducks (and other cold-weather birds), this is done by a physiological set up called “countercurrent”. Think of venous blood, cold from exposure to the air, flowing back into the body from the feet. Too much cold blood will bring the core body temperature down, leading to hypothermia. Then think of warm, arterial blood rushing from the heart. In animals adapted to the cold, the veins and arteries run very close together. As cold blood runs up the leg from the foot and passes by the artery, it picks up most of the heat from the artery. Thus, by the time arterial blood reaches the foot, it is very cool, so does not lose too much heat in transfer with cold water. Blood flow is carefully regulated to maintain the delicate balance of providing blood but maintaining core body temperature."
Through this clever heat exchange system higher up in the leg, there is never a reduced blood flow to the feet and thus not much of a risk of frostbite. In fact, the system is so effective, researchers have found that mallards in freezing temperatures loose only about 5 percent of their body heat through their feet, according to Ask A Naturalist -- which also points out that the system works just as effectively for keeping a duck cool when it is in water warmer than its body temperature.