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The Straight Dope



Cecil, how can dogs walk around in snow and subzero weather without getting frostbite on their feet?

--Lisa Burns, via the Internet

Consulting with the veterinary section of the Straight Dope Science Advisory Board, we found a diversity of opinion on this topic, which may be summarized as follows:

(1) Dogs do so get frostbite on their feet.

(2) They do not either. Once again, it seems, we've got our work cut out for us.

Heading up the do-not camp was Stuart Nelson Jr., head veterinarian for the famous Iditarod dogsled race currently under way in Alaska. This 1,100-mile event lasts two weeks and features several dozen dog teams and their mushers racing from Anchorage to Nome in some of the most grueling conditions imaginable. The temperature falls to 40 below (at which point, I might note, Fahrenheit and Celsius thermometers read the same). Dr. Nelson says he's seen lots of frostbitten canine parts, including the nipples, uvula, and prepuce--no wonder the Iditarod has been condemned by animal-rights groups.

But he's never seen any frostbitten feet. He thinks this may result from some peculiarity of the canine circulatory system. When people are exposed to extreme cold, vasoconstriction in the extremities reduces the flow of blood there, helping reduce heat loss and maintain the body's core temperature. Maybe this doesn't happen with dogs. Or maybe they just have more blood vessels down there. Whatever the case, it's not something peculiar to sled dogs. The doc says he's never seen frostbitten dog feet in private practice, either. He sees this as a survival mechanism: if a dog can't move, it dies.

Other vets doubt that dogs enjoy any special protection, though they do concede that frostbitten paws are rare. A 1975 veterinary account of the Iditarod, written before Dr. Nelson's time, mentions a few cases, but these occurred because booties intended to protect dogs with paw blisters were tied too tight. Researchers have induced frostbitten paws in the lab to learn more about human frostbite, suggesting there's no great difference between us and the pups circulationwise. One vet believes dogs are protected by the thick epithelial pads on the bottoms of their feet, their somewhat higher body temperature (100 to 102.5 degrees Fahrenheit), their thick fur (remember that the frostbitten areas noted by Nelson were hairless), and their practice of curling their paws next to their bodies when sleeping.

I doubt we need to look for an explanation any more exotic than that. Dogs don't wear clothes either, but they withstand conditions that would kill you or me.

For years I've heard that nickel-cadmium batteries which are not fully discharged will "develop a memory" upon recharging, and--after a time--if they are only partially used each time before recharging, eventually the full capacity of the battery will not be available. Is this true? --Hobberstad

It's a myth, bud, though one with a basis in fact. Many years ago in the space program it was discovered that a computer-monitored nicad battery repeatedly discharged to exactly 25 percent capacity did develop "memory"--eventually a quarter of its charge became permanently unavailable. But this kind of thing seldom if ever occurs in earthbound applications. More commonly one sees a condition that mimics true memory, called voltage depression. In some overcharged batteries the available voltage may drop partway through the discharge cycle, spoofing a low-battery monitor (on a laptop computer, say) into indicating that the battery is low. But there's still plenty of good juice left.

Fully discharging a single nicad cell (e.g., a flashlight battery) is usually harmless and may sometimes be desirable. But fully discharging a true nicad battery--that is, a series of cells wired together, which you typically find in your higher-voltage devices--is definitely a bad idea. One cell may become fully discharged, but the remaining cells continue to force current through it, causing permanent damage. So the best advice is, don't fully discharge your nicad batteries, and don't overcharge them either.

Art accompanying story in printed newspaper (not available in this archive): illustration by Slug Signorino.

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