We geeks are happy paddlers this time of year, having LI Sound all to ourselves. Sort of. In fact, we have lots of company (mostly Canadians, sans drysuits). What’s up with that? we’ve always wondered. Why don’t the feet of waterfowl freeze? For the answer, we asked a naturalist. Here’s what we learned:
It’s all about heat exchange, and the smaller the temperature difference between two objects, the more slowly heat will be exchanged. Ducks, as well as many other birds, have a counter-current heat exchange system between the arteries and veins in their legs. Warm arterial blood flowing to the feet passes close to cold venous blood returning from the feet. The arterial blood warms up the venous blood, dropping in temperature as it does so. This means that the blood that flows through the feet is relatively cool. This keeps the feet supplied with just enough blood to provide tissues with food and oxygen, and just warm enough to avoid frostbite. But by limiting the temperature difference between the feet and the ice, heat loss is greatly reduced.
Cool, huh? Literally! More interesting stuff:
Bird’s legs and feet are relatively free of soft tissue and even the muscles that operate the foot are mostly located higher up in the leg and connected to the bones of the feet with long tendons. Because there isn’t much soft tissue in the lower legs and feet, there is less need for warm blood.
By the way, birds aren’t the only animals to have evolved this counter-current blood flow trick. It can also be found in the flippers of whales and sea turtles, some reptiles, and some have even suggested that the proximity of the veins and arteries that supply human arms is a simple counter-current heat exchanger.