Ask Sam: If My Dog Doesn't Sweat, Why Does The Good Boy Sit In Front Of The Fan?

Sep 21, 2018

This guy looks like he wishes he could sweat.

Every other Friday on Morning Edition NHPR’s Sam Evans-Brown tracks down answers to questions about the environment and outdoors for our listeners in a segment we call “Ask Sam.”

Finn from the White Mountains asks: “My question is about dogs. My understanding is that dogs do not sweat, they just pant to cool down. So my question is whether being in front of a fan would actually cool them off?”

Okay, you might think this question is about dogs, but really it’s about thermodynamics. (Which I also, coincidentally, love thinking about.)

So for starters, if you had a room that was completely sealed so no heat could get in or out, and ran a fan in that room, it would actually just get hotter and hotter in the room. Moving around a bunch of air that is the same temperature doesn’t make it cooler, but running an electric motor, will create waste heat. 

So then, why do fans cool you off?

  1. If it’s cooler in one place you can put the fan in a spot that will push the cooler air toward you. (For our dog case let’s assume that this is just a fan on the floor somewhere)
  2. As the listener points out, because it evaporates your sweat, and evaporation has the powerful cooling effect. (Dogs do actually sweat, but only a tiny bit on their paws as a way to add traction, not in order to cool off, so this won’t help.)
  3. And for the third factor, let’s turn to Yana Kamberov who studies the genetic evolution of human sweatiness at the University of Pennsylvania

“All creatures are able to dump body heat through their skin,” she says, “And so sticking your dog in front of a fan certainly would help the dog cool off, because — what it’s going to do is it’s going to increase airflow over all of those surfaces.”

Heat moves in three ways: Radiation (think the heat from the sun reaching the earth); conduction (think of the handle of your frying pan getting hot); and convection (think about the warm air that flows out into a room from a hot stove). This is convection, and it doesn’t require sweat to work. Indeed, we use fans blowing across heat sinks on computer chips to cool them off after all of those computing cycles, and servers definitely don’t sweat.

But the fan wouldn’t just be moving cooler air past your dog, it would also be making your dogs’ panting more effective. Panting works, in part, because of evaporation. That’s why dogs dangle their tongue out into the air — it’s a big, floppy, slimy heat sink that is evaporating saliva, and all of that open-mouth panting is more powerful when you have air blowing over it.

In fact, even though the dog’s mouth is much smaller than the rest of its body, this might actually be the main way that a fan cools your dog down, and that’s because evaporation is just so dang powerful.

Our superpower is WHAT?

A calorie is the amount energy required to make one gram of water become one degree warmer. So one calorie heats up a gram of water one degree. But if you want to make one gram of water change from liquid to gas it takes somewhere in the neighborhood of 580-600 calories. So evaporating a gram of sweat removes 580 times as much heat energy from your skin as cooling that same gram of sweat by one degree.

So sweating is really a super-power, and in fact it’s OUR superpower. There are other mammals that sweat, but none of them sweat like we do. We are the sweatiest... the best sweaters.

“This is one of the most defining traits of our species. The reason we look so different from a chimp, when you look at our skin, is essentially because we sweat,” says Yana, “There’s debate as to why we evolved this particular mechanism, why is it that humans have this heavy reliance on sweating? In humans, sweat gland density is roughly ten times that of chimpanzee.”

There are basically two explanations — and this is from a rather famous Harvard researcher Daniel Lieberman who was actually Yana’s advisor. The first is that we evolved in Africa, where it’s very hot, and all the other animals would just chill out and hide in the shade in the heat of the day. Our ability to run around out in the heat let us go out in the sun and get food while all the predators were like ‘“Nah, too hot.”

Theory number two is that the ability to really efficiently cool ourselves off, let us evolve to be the ultimate cursorial hunter. Which is to say, we aren’t super fast, but we could just run and run and run until our quarry — which has to rely on panting to cool down — overheated and had to stop.

So, Finn! Yes, the fan would cool your dog off, but it would cool you off even more, and that makes us special.

That’s Sam Evans-Brown, host of NHPR’s Outside/In which you can subscribe to where-ever you get your podcasts,  if you’d like to submit a question you can record it as a voice memo on your smartphone and send it to oustidein@nhpr.org, OR call the hotline, 1-844-GO-OTTER.

**Correction: A previous version of the audio for this story suggested that humans are the only mammal that sweat. This is not correct, as horses are definitely very sweaty mammals. This suggestion was removed.**

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