they do so because the moisture in the mud helps them to stay cooler.
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The pigs roll or toil in the mud during the hot summer because the mud is having a good amount of moisture. The pigs do that because when they roll or toil then the moisture can let them get cooler but when compared to that when the pigs do not roll or toil in the mud and stay normal then they will have to adapt the hot summer or else they will have to die. This things are even done by the stray dogs to get rid of the hot summer's heat. this moisture of the mud helps out the pigs to control the temperature of the body of the pig and the surrounding temperature.
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hope so my answer is useful and mark as best if possible plz plz
Hey are it is very hot in Summer pigs roll in the mud as there is lot of moistor in the soil which help them to regulate their body temperature 
nic ans