Cooling down without sweat
Why Do Pigs Roll in Mud?
A pig cannot sweat much. Mud is how it gets rid of heat, and it does a better job than water.
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Step 1 of 5
Getting rid of heat usually means sweating.
Drying is the part that cools you, not the wetness Sweat sits on the skin and dries off, and drying takes heat with it. That is how you cool down on a hot day.
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Step 2 of 5
A pig has almost no sweat glands to do it with.
There are a few, and they are in the wrong places to matter. A pig that relied on sweating would overheat on a warm afternoon.
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Step 3 of 5
So it puts the water on from outside.
Rolling in a wallow coats the pig in wet mud. Now there is a layer to dry off, and drying it cools the pig underneath.
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Step 4 of 5
Mud beats plain water, and by a long way.
Water runs off and is gone in minutes. Mud clings and dries slowly, so the cooling keeps going for hours instead.
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Step 5 of 5
The dried crust does two more jobs.
It shades pale skin that burns easily, and when the pig scrapes it off the biting insects stuck in it come away too.
Pigs are careful animals, and given room they keep the wallow well away from where they sleep and eat.
The short version
Pigs roll in mud because they can hardly sweat, and a coat of mud dries slowly enough to carry their heat away for hours.
Try it yourself
Wet one hand with water and the other with thin mud, then wave both. The muddy one stays cool for much longer, because it takes much longer to dry.