While New York is currently having amazingly warm weather, I still remember last winter in Central Park. Then I watched the ducks landing on the pond and skidding across the icy surface. Looking rather peeved, they eventually found a liquid patch of water.
But think of their feet. They are not thick enough to have an insulating layer of fat, nor are they covered in feathers. Thus blood must flow close to the skin, cooling rapidly in the freezing water, or on the ice Why don’t they get frostbite like humans do?
Frostbite is caused by severely reduced blood flow to the extremities in cold weather. Over extended periods, tissues in the fingers and toes do not get warmth or nutrients from the blood and die, causing gangrene and other nasty problems.
The secret for ducks is in the blood flow system. 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.
In this way, the blood in the foot of a duck remains very cool at all times, yet warm enough to keep the tissue healthy. By maintaining blood flow, nutrients required by the foot tissue are also provided. That being said, ducks can still get cold if they stay in the water too long.
It turns out that birds are not the only creatures to use countercurrent to survive in the cold. Marine mammals such as whales, seals and dolphins have arteries surrounded by a web of veins. This makes heat transfer between arterial and venous blood even more efficient, protecting flippers which do not have a juicy layer of blubber to insulate them. People, too, have a rudimentary system for countercurrent. Deep in the arms and legs, arteries and veins run together. When cold, only these protected arteries and veins are used. This restricts blood to extremities and causes – yes, frostbite. However it protects our core body temperature so that we survive (minus a few appendages). The reason our system is less developed is that we just don’t need the system that often – we are more used to trying to dissipate excess heat (by sweating or running blood close to the skin).
Back to ducks. Living in a winter climate is very costly, with an enormous amount of energy needed to reheat ducks after a cold swim or an icy meal. However ducks have adapted to gain advantages from the chill.
Cooling may allow ducks to dive deeper and swim further. By cooling the brain, less oxygen is required and thus a duck can stay underwater longer. In one study, ducks diving in 10 degree centigrade water could stay under 14% longer than those diving in 35 degree water.
Though looking at how irritated they appear when their pond freezes, I personally think ducks prefer summer.
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