The idiom “sweat like a pig” has been around for a long time. But, as it turns out, pigs don’t sweat. In fact, very few animals do sweat like people do. Other animals that sweat include, not surprisingly, primates, but also horses and hippos. Instead, animals have other ways to release heat, including panting. Some species of storks and vultures have more interesting way to cool down – they poop. On their own legs. Elephants and rabbits use their large ears to regulate heat. Elephants flap. In rabbits’ ears, the blood vessels dilate, which dissipates heat.
But back to the pigs…they cool down by rolling in the mud. This mimics sweating. As the mud dries and the water evaporates, it draws heat away the body. Wallowing in mud is preferred to a dip in water because the water in the mud evaporates slowly which allows pigs to stay cooler longer. Not only that, being covered in mud helps protect pigs’ skin from the harsh rays of the sun. It also smothers parasites on the skin.
So the idiom, “sweat like a pig,” has nothing to do with pigs at all. Instead it is a reference to pig iron. During the smelting process hot iron was shaped into molds, which to some looked like a sow and piglets. This thus the term “pig iron.” As for the idiom, when the iron cools, it reaches its dew point and condensation forms on the surface of the iron – it sweats. When the iron sweats, that means it’s cool enough to handle. So to cool down like a real pig, you’d have to take a wallow in the mud.