The cold morning mist rises off the winter pond, as quacking ducks paddle playfully around the chilly water. On the bank nearby a mallard watches contentedly, its bare feet planted firmly on a thick layer of snow. If ducks are warm-blooded like we are, why don’t they freeze to death?
Their feathers provide excellent insulation for the rest of their bodies. But even if we bundled up in the warmest down coats in the world, we wouldn’t last more than a few minutes in bare legs and feet.
The answer revolves around heat loss. In humans, our blood is heated in the core organs and muscles, and then the heart pipes it straight down our legs to our feet, where a good deal of that heat passes through our skin into the snow. Cold blood then returns by another pipe up to the core, where it is reheated. But ducks and other winter birds take advantage of physics designed to minimize heat loss.
Chemical engineers have long understood that you can transfer heat between two pipes that are touching each other. Experiments show that if fluids in two different pipes move side-by-side in the same direction, about one-half of the heat is gradually exchanged between the two pipes. However, if the two fluids move in opposite directions past each other, nearly one hundred percent of the heat is transferred from the warmer pipe to the other.
That’s what happens in ducks, before the blood ever reaches their feet. As the pipe for warm blood (the artery) leaves the heart and runs down toward the feet, it passes next to the pipe for cold blood (vein) coming back from the feet. These arteries don’t just touch each other. They are closely woven together by a web of tiny branching blood vessels that brings the warm and cool blood into close contact.
As a result, the blood transfers most of its heat before it ever reaches the feet. When the blood does reach them, it is often just above freezing temperature. Conversely, the returning blood is re-warmed before it even reaches the duck’s heart.
Virtually no heat is lost. In fact, the duck loses twenty times more heat through its feathers than through its feet!
Scientists call this engineering principle by the technical term counter-current exchange. But the Creator not only knew all about it from the beginning, He also put it to use in His creatures—long before modern human engineers discovered the principle and applied it to industry.
Perhaps you’re still wondering why duck feet don’t get frostbite from the cold. Unlike our legs, the legs on ducks are mostly hard parts, which don’t get hurt when frozen. Most of their muscles and soft parts are safely stored farther up the body.
Next time you see a mallard splashing contentedly in freezing water, stop and remember the wisdom of its Creator, and tell the person next to you about how wonderful He is!