During heavy snowstorms geese and ducks accumulate snow on their back feathers, deer may become buried with snow, and birds stand with their feet directly on the ice. Grouse and quail will often fly directly into and become buried inside snowbanks in the evening to emerge in the morning bright-eyed and chipper. Even more dramatic is that swimming birds are quite comfortable with their bellies, legs, and feet in direct contact with water at temperatures that rapidly draw heat out of unprotected humans and cause them to become hypothermic.
These activities are made possible through anatomical and metabolic changes. Body temperatures are at least 70 degrees warmer than ice and snow. Yet, the snow on a bird or deer's back and the ice under a bird's foot do not melt appreciably. These animals have several mechanisms that reduce radiant (from between feathers and fur) and conductive (through direct contact with feet) heat losses to the environment.
Feathers, Fur, and Radiation
As winter approaches birds grow new downy feathers that fill the space below their contour feathers while mammals grow short, frizzy underhairs and produce long, coarse, air-filled guard hairs that lay above them. These changes in feathers and fur trap air between the skin and the outer environment preventing radiant heat loss.
Most animals tend to rest facing into the prevailing winds because their feathers and hairs point toward the animal's rear. By facing the wind, air currents press the upper feathers or hairs together forming a relatively smooth, unruffled surface that reduces heat transfer from the body to the air. The underlayer of downy feathers or frizzy underhairs further reduces air flow across the skin to decrease heat radiation.
The combination of outer feathers or guard hairs and inner down or frizzy hairs reduces heat radiation to a trickle across the greater part of their bodies and homeotherms (animals that produce their own heat) are able to maintain high body temperatures while resting – even in extremely cold weather, if they can remain dry and unruffled. If the hairs or feathers become saturated with water or erratic winds keep their surface agitated, their insulative properties are dramatically reduced and these animals must often resort to extreme measures to stay alive under such adverse conditions. Smaller animals exhibit behavioral modifications (such as demonstrated by ruffed grouse above and chickadees) that help them survive the rigors of winter nights. Birds and some mammals (mice, bats, and opossums) enter a torpid state overnight wherein their body temperatures drop several degrees below their active temperature. Lesser nighthawks (Chordeiles acutipennis) in the southwestern deserts will even enter a temporary state of hibernation if temperatures fall below freezing. This torpor allows them to conserve energy through the long winter nights.
Feet and Convection
Standing on ice in bare feet sounds like an exquisite form of torture. But birds do it all the time and seem to think little of it. While most birds and mammals cope with bare feet and legs, some birds (such as ptarmigans and grouse) and mammals (lynx, and their prey the snowshoe hare) grow feathers or hair between their toes in winter that both insulate the toes and serve as snowshoes to support the animals on powdery snow.
All cells require a constant supply of blood. Placing our feet in ice water rapidly chills the blood. The blood then returns to the heart, warming up somewhat on the way, but the cooler blood can chill a stressed animal causing hypothermia.
The arteries and veins in the legs of birds and mammals move toward each other during the winter, establishing a countercurrent flow of blood that transfers most of the heat in the arterial blood moving away from the body over to the venous blood moving toward the body. By the time the blood reaches the feet, its temperature is almost the same as the surface the animal is standing on. Thus, although the temperature in the feet may hover between 10 and 40 degrees F, the temperature at the top of the veins in the legs will stay between 95 and 100 degrees F. Most importantly, the core temperature of the animal's body is only negligibly affected by the extremely cold temperature of the feet.
Because blood has a high concentration of dissolved minerals, proteins, and other molecules, the blood does not freeze until it reaches a temperature of about 5 degrees F. Except for the heat, the blood in the extremities has all the nutrients, dissolved gases, hormones, and essential molecules that the blood in the interior has. Thus the feet of the animals can reach a temperature of about 10 degrees F with little change in function. To prevent freezing, the cells in the feet produce antifreeze molecules much as do overwintering insects or produce isoenzymes that work in colder weather.
In addition to acclimatizing isoenzymes, producing antifreeze, and modifying their behaviors, animals have developed physiological and mechanical adaptations that help them survive the rigors of winter. Each of these adaptations provides only a small advantage for survival, and each is produced at a cost to the animal. If the balance of the costs and the advantages is greater than the costs, the animal survives. When the costs exceed the advantages, the animal perishes.
Albert Burchsted - Ph.D. in animal behavior, field biologist, and photographer. Al leads nature study walks and is an environmental consult in SE ...
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