Hair Length and Temperature Tolerance

Robert Jay Russell, Ph.D.

Copyright; all rights reserved

Jay Russell and Josie
Jay Russell, Ph.D. with his Coton de Tulear
Anatolian Shepherd
LGD - An Anatolian Shepherd
If your Livestock Guardian Dog (LGD) is dark-coated, then it will absorb heat from solar radiation quickly; if your LGD is light-coated, it will absorb heat from solar radiation slowly. Eventually, all things will attain an equillibrium, but it is the speed at which that equillibrium state occurs that usually chills us mammals to the bone or fries us like bacon. Color aside, your LGD's hair length and coat condition are crucial factors determining its ability to tolerate the daytime heat load. It is actually all biophysics (mostly Newton's Law of Cooling) and it is not difficult to get a grasp of the concepts involved (trust me on this).

Ta is the Temperature at the tip of the hair.
Ts is the Temperature at the surface of the skin.
L is the distance between the tip of the hair and the surface of the skin.


               (Ta minus Ts)   C =  (F) x  -------------                     L
C is "conductance," the speed at which the Temperature at the tip of the hair (Ta) will be equal to the Temperature at the skin's surface (Ts).

F is a number, determined through experiment, which estimates the ability of a hair to conduct heat. Dry oiless hair (like that of a Coton de Tulear) has a very small number, while silky, oily hair has a very big number. The bigger the number "F", then the faster the dog will gain or lose heat at the surface of the skin. I'll ignore it here. NB: to my knowledge, this number has never been determined for any dog.

Okay, now a few facts. Most mammals attempt to keep their skin temperature at about 85 degrees F (29 deg C). The sun can heat the tip of a hair to more than 150 degrees F (66 deg C). Plug in those temps to the formula and you get (Ta - Ts) is (150 - 85) which is "65," a big number. Now, divide that number by the length of the hair (for metric-philes, just use cms).

If the hair is six inches long, then the the number "C" shrinks to 10.8. The dog's skin will stay cool longer. If the dog has been clipped to 1-inch, then conductance, C, will remain at 65 and the dog's skin will heat quickly. And, if you surgically clip Fido to say 1/8th of an inch, then you'd divide 65 by 0.125 and get a conductance value, C, of 520. The dog's skin temperature would equal the hair tip's temperature in a twinkling of an eye (i.e., divide 520 by 65 and you see that heat will transfer eight times faster than it would if you had left the hair at 1-inch long).

Now just play with numbers. Want to see what would happen if the dog's usual 6-inch coat was heavily matted? Just decrease "L." What happens if it is nighttime and the ambient temperature drops to 40 degrees? Just substitute "40" for "Ta" ("C" will be negative number, indicating that your pooch is loosing skin heat towards the tip of its hair and the environment beyond). If a dog exercises, then its Ts will increase as its body attempts to dump the heat load to the oustside. A dog will pant when its Ts is high and the Ta is high and/or when its Ts is high and "F" is low.

If a dog is really suffering, soaking it with water quickly makes "F" a big number (water is much more conductive than cotton or air or normal hair, for example) and simultaneously flattens the coat making "L" a small number. A matted coat impedes the dogs own ability to vary "L" (which it normally does by erecting or flattening its topcoat hairs using the follicle's tiny "erector" muscles) and mats also alter the "F" value, probably making "F" a smaller number. Matted dogs cannot dump heat from their skin surface to the outside air, a potentially dangerous situation.

Pretty nifty, no? With Newton's formula, you can get a glimpse of what your dog might be feeling (no small accomplishment for us thin-haired, sweaty-skinned primates who are obsessed with adulterating hair, changing its reflectivity, varying its length, and otherwise messing with Mom Nature).

BTW: bird fanciers can play with this formula and gain insight into why a bird fluffs up its feathers or flattens them. Bird feathers are known to have a really small value for "F" showing that feathers (modified reptilain scales) are waaaaay better than typical mammalian hair at slowing down thermal conduction. That's why you might have a down jacket rather than one stuffed with Anatolian clippings. There's no substitute for a big "L" and a little "F" when it comes to outerwear. Oh yes, one last point. For those of you with a hairless breed, the formula doesn't work because there is no "L" to consider. A Xolo or Chinese Crested is at the whim of the sun and the air. A naked LGD would be a pretty unhappy dude.


Above, I noted that long hair usually protects a mammal from heat gain through solar radiation. And indeed, most LGDs, created to stand outside and protect hoofed stock from predators, are either white, fawn, or light-brown colored, hence solar energy absorbtion isn't high on their list of concerns. But I just received an excellent series of email questions and comments from a person whose black Standard Poodle is estactic when clipped short in the summertime. Why? I thought I'd share some of my response to her email with you....

Black is the PERFECT solar radiation energy absorber, hence that fact overrides any idea that simple insulative changes in hair length or conductance will protect against too much CONDUCTIVE heat gain. In nature, black mammals tend to be either nocturnal ("solar avoiders") or they are creatures of the deep forest (who are protected from the sun anyway). Rain forests, for example, are very, very dark even on the brightest of sunny days (they have the awesome feel of a moldering gothic catherdral). To give you some idea how dark: even on the brightest summer day, a typical rain forest will be so dark that ISO/ASA 400 (rather high speed) film is barely adequate to capture an image without a tripod.

A black dog (or a big, dark-coated mammal on a summer day) benefits from short hair or even no hair at all. That's why Bison and Musk Oxen (amongst others) shed. The reason: these creatures attempt to cool through CONVECTIVE heat loss. Air, passing over the surface of the skin, carries away the heat at the surface of the skin PROVIDED that the air passing-by is cooler than the skin surface (otherwise, the skin surface would gain heat from the hot air passing over it). Sunburn is reduced in black-skinned creatures (melanin is protective to some extent against ultra violet radiation -- but the NEW global conditions of our diminishing ozone layer has made this relationship -- UV and melanocytes -- a deadly one). Most mammals roll in dirt or mud for additional protection against sunburn.

Some mammals (like us) have yet another convective cooling trick. We sweat. An extra amount of energy is carried away from the surface of the skin as liquid evaporates (changes state from a liquid to a gas -- I think about 25 per cent more, but my physics is rusty. Too much sweating I guess ). There is some draw backs: if the skin surface is too wet, the surface water may act as a magnifyer (water bends light, changing the skin's "refractive index"). Magnifying the sun's rays is bad news. And in areas where the humidity is high (the air is saturated or nearly so with water vapor), evaporative cooling loses its efficiency dramatically.

Surface water cannot change state to water vapor is the air above it is already full of molecules. So, mammals who live in Houston must resort to air conditioners (chilling dehumidifyers) in office buildings, period! Alternatively, move to Los Angeles where there's less water vapor in the air (albeit other things like stray bullets seem to fill the space between air molecules there.. :-).

By the way: sweating is best done when you cannot see streams of water on the skin's surface. If the whole evaporative engine is working precisely, a "damp" skin will have water molecules changing state so quickly that the offensive streams of sweat will not be noticeable. We humans, especially human males, sweat like stuck pigs in our youth, but loose that ability to thermoregulate well when we age. Elderly people and dogs have a lot of trouble with both heat and cold, so take extra care of your aging canines (or people) in hot weather.

Thermoregulation aside, some dogs -- even those with long, white, dry hair that slows heat gain -- like to be shaved down because: (1) hair is heavy (shaving produces a feeling of freedom probably much like taking off a tight wool jacket and running around with just a t-shirt on); (2) hair, either matted or clinging to adjacent hairs quite naturally, will tug on the skin's surface. That constant tugging as the flank stretches and contracts during motion can really annoy some dogs. And dogs, like people, will often do something (like really enjoy short hair even in summer) that is not necessarily in their best interest. Natural selection does not necessarily shape tastes appropriately (consider that many primates will consume dangerously toxic foods or drugs until they croak. The term "forbidden fruit" has a genesis quite far removed from Genesis).

One of our little Cotons is Black & White. She is of two minds in the bright summer sun. Her long, white, dry hair protects her from heat gain while her prominent patches of long black hair fries her. She succumbs to panting (a form of evaporative core body cooling) far sooner than her light-coated daughter.

Incidently, you may see some black birds (like redwings, starlings, or crackles) sitting in the sun, exposing their rumps (a gland there needs sunlight) and panting like mad all the while. They are technically "gullar fluttering" -- the equivelent of a mammal's pant -- in an attempt to evaporate water on the tongue and cool their skyrocketing core temperature. Some birds actually act drunk after long exposure to the sun (their wee-tiny brains are truly addled). We had a Coton who so liked to sun on hot days (Cotons were originally from a desert area) that she'd practically fry her brain. We'd have to retrieve her from the back lawn out of concern. Even long-haired bright white dogs who know nothing about "Baywatch suntan envy" can overdo solar radiation exposure when the sun angle decreases and solar radiation (called "insolation") increases precipitously (summer).

A final point: dog breeds and mammal species who normally dwell in far northern or far southern climes have had little need to adjust to overly warm summers or to sun angles that reach the steep angles we see in more equatorial regions. Take special care of these creatures; it is we humans who have transplanted them outside their element. So here's a few conclusions:

Jay Russell, Ph.D.

This article originally appeared on the mailing list LGD-L in May, 1997 and is copyright by the author.