|MadSci Network: Zoology|
Ducks, and other warm-blooded animals that remain active during the winter in cold climates, don't seem to mind having cold feet, noses, ears, etc. Apparently these areas of the body are not supplied with many of the nerve endings that respond to cold temperatures.
A more important question physiologically is: "How do these animals avoid losing precious body heat to the environment?" A duck's normal body temperature is in the 106 to 108 (degrees F) range, while the cold water it paddles around in is just above freezing (33 degrees F or so). If the feet remained at body temperature, they would constantly be "pulling" heat from the body core as they rapidly lost heat to the water.
The answer to this problem is something referred to as a countercurrent heat exchange mechanism. It is present in the extremities of warm- blooded animals that are active in the cold. Let's look at the duck's leg as a model of this process. Arteries carry warm blood to the foot from the body, while veins carry cooler blood from the foot to the body core. These two types of vessels run so close together in the leg that heat moves from the artery to the vein before it gets to the foot. (This kind of a system of adjacent veins and arteries is called a rete.)
Even many "cold-blooded" animals use something like this. Check out this web page for a good explanation, with diagrams and graphs, of how it works in the tuna:
Try the links in the MadSci Library for more information on Zoology.