Cold, blustery weather makes me sick. Not like the sneezing and coughing kind of sick, mind you, but physically ill, as in, “Riding my bike in winter makes me want to throw up.” The problem lies in my extremities: my fingers and toes and ears get so cold, so fast, that they thoroughly blanch and lose feeling, and when eventually the blood gets to circulating back into them they throb painfully, almost nauseatingly. It might be Reynaud’s; it might simply be shoddy plumbing. No amount of layering works to mitigate the chill. My body seems not to mind that its phalanges and cartilaginous protrusions are at constant risk of frostbite in the winter, what with how quickly it draws from them the life-sustaining warmth of blood. And I don’t even live in a particularly cold climate! I wouldn’t last a week in wintry Alaska.
Clearly I am a wimp when it comes to winter weather. I try and limit my exposure to brisk, insulated jaunts between climate-controlled locales, such as from my house to the grocery store two miles distant, or, better yet, from my blanketed seat in the living room to the refrigerator not six feet away. I do go on walks to the nearby pond, however, and when I see those mallards and widgeons paddling around in Longfellow Creek, plunging their heads into its gelid murk, I can’t help but cringe. They appear to spend all their waking hours on the water—water I wouldn’t dare dip a toe into for longer than an instant. What’s keeping their feet from freezing off?
Birds, like all mammals and some fish, are homeotherms, meaning that they internally regulate their body temperature by burning calories to create and conserve heat. The challenge of maintaining a narrow, near-constant range of internal temperature during colder months is met by various means: some animals layer on heaps of energy-rich fat to metabolize in leaner times, others rely on thick, insulating coats or feathers; and some just eat continuously. (Humans, of course, avail themselves of each methodology.) Birds that spend their winters swimming, diving, and dabbling in cold water need to take particular care in keeping warm and dry, and thus many species sport a pillowy layer of down feathers overlaid by waterproof plumage. Their feet and beaks, bared to the elements, require warmth as surely as other parts of their bodies, but the route taken is a bit more circuitous (if you’ll pardon the anatomical pun).
In order to keep those exposed areas alive and warm, blood must flow freely to and from the extremities, nourishing tissues—and it must do so economically, with minimal loss of heat. Too much heat loss leads to hypothermia; too little blood flow invites frostbite and gangrene. The bird’s circulatory system achieves this balance by employing what’s called countercurrent, the side-by-side pairing of opposite flows—in this case, venous and arterial blood. Veins transport cool, oxygen-depleted blood back to the heart, while arteries furnish tissues with the warm, oxygenated stuff. In the beaks and feet of these cold-adapted birds, the veins and arteries are intertwined like a caduceus, so that arterial heat is transferred to venous chill and vice versa, ensuring a low, stable temperature in the extremities and minimizing the unavoidable loss of heat to the water. Of additional benefit is the fact that bird legs are mostly tendon, scales, and bone—the muscles are nestled close to the warm, feathered body and thus amply insulated. Researchers in North Carolina found that mallards, for all their incessant dabbling on slushy ponds in the dead of winter, surrender a paltry five percent of their total body heat through their orange, webby feet.
This physiological countercurrency was first discovered by Galen, the second-century Greek physician-surgeon-philosopher who called the convoluted networks of veins and arteries retia mirabilia, Latin for “miraculous nets”. It is a feat of natural engineering, remarkable in its efficiency, and since Galen’s time the countercurrent has been emulated and expanded upon by others in a number of decidedly unnatural processes, including petroleum refining, nuclear waste processing, and the extraction of gold from nickel-cyanide slurry. Biomimicry has perhaps not found its proudest expression in the appropriation of retia mirabilia.
Back at the pond, I feel a shiver coming on. My appendages are already tingling and I’ve been outside all of fifteen minutes—time to get moving. Miracle net notwithstanding, those ducks look pretty cold to me.