The recent cold snap prompted many of us to don our winter garb: hats, gloves, and an extra layer of clothing or two. When the temperature drops, we rely on increasing the “R-value” of our clothing.
But what about the birds and mammals that are active throughout our winter season? Many of these species develop high R-value winter coats: extra long and dense hairs in the case of mammals, and denser, thicker feathers in the case of birds. In both cases, as with our long underwear and down coats, the goal is to trap warm air next to the body. And just as a winter camper will shake and fluff up his or her sleeping bag to increase its loft, birds will fluff up their feathers on particularly cold days to trap a little more warm air.
Many of our winter residents will also don a layer of fat, both for its insulation value and as a source of high quality energy to keep their internal furnaces burning efficiently. This is particularly important for our aquatic mammals—whales and seals—and diving seabirds, for whom a bulky outfit that trapped a lot of air would hinder their ability to swim and catch their fish prey.
There’s one common winter sight for which the above adaptations offer no explanation: a duck, gull or goose standing on ice. The feet of most of our winter birds have neither feathers nor insulating layers of fat. How do they avoid freezing, or excessive heat loss, while in contact with ice or near-freezing ground and water?
The answer lies in an ingenious design of the circulatory system. Warm, oxygenated, arterial blood leaving the feathered portion of the leg and heading toward the feet is shunted into a network of tiny blood vessels that engulf the much cooler veins exiting the feet and carrying carbon dioxide and other cellular waste products. Warmth from the arterial blood is conducted into the veins, resulting in a significant drop in the arterial blood as it continues further down the leg, and a significant rise in the venal blood as it heads up the leg and into the bird’s warm core.
This system is called a countercurrent heat exchange. Since the arterial blood entering the feet is at or very near the temperature of the ice and icy water, very little heat is lost to the environment. This concept has been copied in the design of energy efficient heating and venting systems for buildings: warm exhaust from furnaces, dryers and stoves is directed into an outgoing duct that comes in contact with the incoming duct containing cold, fresh, outside air. The contact point is designed to maximize heat exchange between the two so that the incoming cold air is warmed by the exhaust that it is replacing.
This is just one example of the many ways we can mimic the natural world, design with nature, conserve important resources and minimize our impact on the environment.