By Victoria Gill
Science and nature reporter, BBC News
It is a true picture of contentment, and now a scientist is suggesting that a pig's love of mud is more than just a way to keep cool.
A researcher in the Netherlands has looked at wallowing behaviour in pigs' wild relatives to find out more about what motivates the animals to luxuriate in sludge.
His conclusions suggest that wallowing is vital for the animals' well-being.
The study is published in the journal Applied Animal Behaviour Science.
It is already well accepted that pigs use wallows to keep cool. The animals do not have normal sweat glands, so they are unable, otherwise, to regulate their body temperature.
The scientist who carried out the study, Marc Bracke from Wageningen University and Research Centre, trawled the scientific literature for evidence of what motivates other animals to carry out similar behaviours.
He examined closely related "wallowers", including hippos, which spend their time in water to keep cool.
Dr Bracke also looked at other hoofed animals, such as deer. Although these animals do not wallow, they roll on the ground in order to "scent mark", which has an important role in attracting a mate.
That analysis has led Dr Bracke to propose that mud wallowing, like rolling, could play a role in reproduction in pigs.
But more fundamentally, Dr Bracke suggests the behaviour could have evolved in pigs' most ancient relatives.
Pigs are related to water-loving hippos
"We all evolved from fish, so it could be that this motivation to be in water could be something that was preserved in animals that are able to do so."
For many animals, this would be too dangerous, because watering holes are ideal places for predators to ambush their prey.
"But pigs, like many carnivores, are relatively large animals with enlarged canine teeth, so they would be better able to fend off an attack."
So rather than pigs needing to cool down in mud because they do not have [functional] sweat glands, Dr Bracke thinks that they "did not evolve functional sweat glands like other ungulates because they liked wallowing so much".
"Pigs are genetically related to particularly water-loving animals such as hippos and whales," Dr Bracke said.
He explained: "It seems to me that this preference to be in shallow water could have been a turning point in the evolution of whales from land-dwelling mammals."
He concludes that the desire to wallow is probably hardwired and rewarding in itself.
"If so, wallowing could be an important element of a good life in pigs," said Dr Bracke.