The reason why dogs pant so much is because they do not have sweat glands in the skin of their bodies except on their feet. This, incidentally, is the same as for domestic cats. Only domestic cats cool themselves by licking their fur so that their saliva evaporates and cools them.
Dogs also rely on the principles of evaporation to cool themselves but they use their tongue to carry it out. As soon as a dog overheats they open their mouths and their large tongue drops out and they start panting. They keep their tongue moist with saliva to maximise the cooling effects of evaporation. They may also drink water at this time to once again moisten the tongue to improve the evaporation.
During the long evolution of the dog, nature decided that keeping warm was more important than keeping cool which is why they developed thick coats. When this happened it precluded the effectiveness of sweat glands in their skin. The skin is covered by thick fur and therefore any sweat produced by glands in their skin could not evaporate and therefore there’d be no cooling effect. Sweat glands disappeared over time.
This only left the soles of their feet where glands could produce sweat, and the evaporation effect cool them. The trouble is that it’s a very small part of their anatomy and the glands are in the wrong place. Once again, you see the same thing in domestic cats. When domestic cats go to a veterinarian and become hot and aggitated you will see sweaty paw prints on the vet’s consultation table. I’m sure the same thing happens with dogs.
This evolution of a lack of sweat glands across the dog’s body is so firmly fixed that even hairless dogs have not re-evolved sweat glands on their body where they would be useful. It could have happened over the tens of thousands of years of dog domestication but it didn’t.
It is said that the panting ability of dogs has saved them many times from dying from heatstroke. Three major domesticated animals in our lives; the dog, the cat and the horse have different methods of keeping cool. As mentioned the dog pants highly effectively, the domestic cat licks their coat to deposit saliva and the horse sweats copiously as we so often see at the end of a horse race.
Because of selective breeding, today, there are many dogs with thin single coats which would allow sweating from sweat glands in the skin to be effective but they have not re-evolved.
Why does evaporation cause cooling
As mentioned above the only reason why dog panting cools the dog is because of the physical properties of cooling through evaporation. It’s quite a difficult concept to understand. As I understand it, it works like this.
In the dog’s saliva, or in the water on their tongue, there are particles which are hotter than the other particles. These hotter particles have more kinetic energy than the others. They break free of the liquid mass and become a gas. When they do this they carry heat with them which effectively cools down the water or the saliva on the dog’s tongue. This makes the tongue cooler and that area of the dog’s anatomy is sufficiently large to cool the dog down as a whole.