On Tuesday while we were still in Crestline, despite the frigid weather, intemittent rain, snow and wind, Jerry and I ventured from the house for a post office run and a quick jaunt around Lake Gregory. The wind was howling, but it wasn’t raining, so while Jerry sat in the car and read the newspaper, I took my camera and traipsed a short distance along the shore line of our beautiful Alpine lake. Birds and ducks came flying and swimming toward me as soon as I stepped from the car, but when they saw I had no food offering, they settled back onto the water and into the routine way of birds on a winter lake.
How can those birds swim in this icy lake and appear so comfortable? I wondered, as I often have before, and I thought of down comforters, duck down, and other insulation the birds possess, but still…it was awfully cold. And what about their feet? Skinny feet with little sign of fat insulation. Gusts of wind blew so hard that once I nearly lost my footing, and after a bit, I walked back to the warm car, reached over and said to Jerry, “Want to feel some cold hands?” and before he could protect himself I pressed my icy hands on his warm neck. The icy hands were the result of my wanting to be ready to snap a promising picture to such degree that I had walked about without gloves.
But the birds of Lake Gregory were quite content to paddle around in that bitterly cold water. They appeared to be perfectly comfortable. How? Why? I continued to wonder, so I did a little research and want to share the information with you. Much of this I found on a site named Quarks, Quirks and Quips.
The secret for ducks (warmth in cold water) 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.
Caputa M, Folkow L, Blix AS. (1998) Rapid brain cooling in diving ducks. Am J Physiol.275(2 Pt 2):R363-71.
de Leeuw JJ, Butler PJ, Woakes AJ, Zegwaard F. (1998) Body cooling and its energetic implications for feeding and diving of tufted ducks. Physiol Zool. 71(6):720-30.
Koeslag JH. (1995) Countercurrent mechanisms in physiology. Continuing Medical Education 13: 307-315.
Reite OB, Millard RW, Johansen K. (1977) Effects of low tissue temperature on peripheral vascular control mechanisms. Acta Physiol Scand.;101(2):247-53.
Schmidt-Nielsen K. (1981) Countercurrent systems in animals. Scientific American 118-128.