Rolling around in the mud helps to keep pigs from overheating when it gets too hot outside. Some pig farmers now use water sprinklers instead of mud to keep pigs cool in warm months. Mud, however, tends to keep pigs cooler longer because the water in mud evaporates more slowly than pure water. Just because pigs like to roll in mud, don't assume that they're dirty animals. On the contrary, pigs tend to be very clean animals.

Source: Wonderopolis