Porcine skin carries sweat glands, but they are largely apocrine and contribute little to evaporative cooling. Combined with a thick subcutaneous fat layer and sparse hair, this leaves pigs poorly equipped to shed heat, and they are vulnerable to heat stress at temperatures other livestock tolerate easily. Panting provides some relief but is limited.
Wallowing solves the problem externally. A coating of mud evaporates from the skin surface over a longer period than water because the solids retard evaporation and the layer holds more liquid. Studies of pig wallowing behaviour find that mud-covered animals maintain lower body temperature for longer than those wetted with water alone. The coating also blocks ultraviolet radiation, to which pale pigs are susceptible, and deters biting insects.
The behaviour is often read as evidence of dirtiness. It is the opposite of indiscriminate: given a choice, pigs keep their dunging area separate from their lying area, and wallow deliberately in a designated spot. The association between pigs and filth reflects confinement conditions rather than the animal's own preferences.