Do Kerries Overheat?

By Craig Bohren, bohren@ems.psu.edu
Copyrighted Craig Bohren, 2002

There are some deep-seated misconceptions about the temperature of black dogs.

What is called "steam" rising from a dog is what I call a mixing cloud (which you can read about in my book Clouds in a Glass of Beer). It's the same kind of cloud you see from your cup of coffee, from hot soup, from the exhaust of an automobile, from airplane contrails). You can't see steam. The presence of this cloud does not indicate that anything harmful is happening to the dog. Putting water on a dog can only lower its temperature, but we have to be careful to specify what temperature we are talking about. The temperature of a dog's fur is largely irrelevant. Dogs (and humans) suffer from hyperthermia because their skin temperature, and, more important, deep core temperature rises.

I have done the following experiment with dogs. I have measured the temperature of the fur on their backs while standing in direct sunlight. Then I have put water on their backs and measured the temperature. It drops markedly, especially under conditions of low humidity. But, alas, this is the surface temperature of the fur, and if the fur on a dog's back is fairly thick, the reduced surface temperature doesn't have much effect on the dog's skin and core temperatures. You can understand this by doing a simple experiment. Take a thick piece of styrofoam, put your hand on one side, and on the other side pour ice water. Your hand won't feel any different. It is insulated from the cold surface.

I have done the experiment with dogs dousing them with HOT water. Same result. This is because the cooling mechanism is evaporation. The initial temperature of the water is largely irrelevant.

There are two reasons for putting water on a dog's belly instead of its back:
(1) the fur on bellies is usually much thinner than on backs (less insulation);
(2) the blood vessels on bellies are closer to the surface (as evidenced by the pink bellies on lots of dogs).

Thus it is more effective to put water on the bellies of dogs than on their backs, but this does not mean that putting water on their backs is inimical in any way. We have to distinguish between what is effective and what is not rather than what is harmful and what is not.

Evaporative cooling is very effective provided that the humidity is sufficiently low. But when the humidity is not low, and you really have to cool a dog, the only way to do this is by direct immersion in a large body of cool water.

Now to the issue of black dogs. I have to restrain myself when I read the messages about the poor black dogs in the summer sun. Maybe they are at a disadvantage, but I doubt it. Here is what happens (or perhaps I should say
can happen). The SURFACE FUR of a black dog in the sun undoubtedly gets hotter than that of a white dog in the same environment. And by heaven you can FEEL this with your hand, which conveys the mistaken notion that the
dog is hotter (by which is meant that its core temperature rises). But again, surface temperature is largely irrelevant.

Many years ago ornithologists puzzled over a disturbing fact: in desert environments, most birds are black. Why hasn't evolution had the good sense to make them white, because as everyone knows, black animals are hotter in
sunlight than white animals? So some experiments were done in which skin temperatures under white plumage and black plumage were measured. And guess what? The temperatures under the black plumage were lower. What happens is that the incident solar radiation is absorbed in the outer layers of the black plumage, and these layers are insulated from the underlying skin. The white plumage does indeed reflect a fair amount of solar radiation, but it also transmits such radiation to the underlying skin.

I have done simple experiments to demonstrate this. In fact, as I recall I posted a message about this many years ago to this list. I placed towels in direct sunshine. Thermometers were placed under each towel. On top of one towel I placed a thin layer of black cloth. The surface temperature of this black cloth was indeed higher than that of the white towel. But the temperature underneath the towel with the black cloth was lower by about 0.5 C.

I could go on and on, but I'll try to restrain myself and end with one more puzzler. You might wonder why it is that dogs with thick coats can survive at all in summer heat. If we were to don thick fur coats in summer and march around in the noonday sun we'd probably keel over.

The reason, I believe, why dogs can wear fur coats in summer and we can't lies in the differences in our evaporative cooling mechanisms. We cool evaporatively from more or less our entire skin surface. If we put on a fur coat, we impede evaporative cooling from most of our body. A dog, however, cools evaporatively mostly from its nasal passages (not, as is commonly believed, from its slobbering tongue). As a consequence, the dog's fur on its body cannot impede that which does not happen (much) anyway.

Editor's note: Not withstanding the above scientific explanation, it is a good idea to keep your animals and yourself well protected from excessive sunshine.

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