"Eating like a pig" is usually meant as an insult.

But despite their dirty reputations, some pigs clean their food before eating it, a recent study shows. Researchers at University College London and Basel Zoo in Switzerland made their discovery after a zoo employee spotted several wild boar bringing dirty apple chunks to a stream and sloshing them around before eating them.

Follow-up experiments confirmed the pigs can tell when their food was dirty—and that they can delay gratification by washing their food, rather than chowing down immediately.

Pigs are intelligent animals known for their ability to adapt to new environments. So one might expect them to be capable of discriminating clean food from dirty food and delaying gratification in order to wash food items. Yet this is the first reported observation of pigs washing their food. The researchers recently reported their results in Animal Cognition.

The discovery could help change the way some people see pigs. Despite the animals’ reputation for greed and uncleanliness, they’re actually thoughtful and fastidious animals.

Cleaning Up

Pigs aren’t the only animals that dunk or manipulate their food in water before eating it. Raccoons, birds, and primates all do so. But not all these animals are truly washing their food.

Some bird species simply moisten their food to make swallowing easier. Raccoons look for food in water and often roll it around in their hands, but the behavior is a reflection of their constant need to use their hands to sense the world and look for food—not an urge to wash it.

To confirm that an animal truly aims to remove dirt and grit from food, researchers need to see that it can distinguish between clean and dirty food and that it will deliberately carry dirty food to a water source.

The discovery that some pigs wash their food came about by chance. Tanja Dietrich, the communications officer at Basel Zoo, noticed adult and juvenile wild boar—all newcomers to the zoo—picking up sandy apple halves in their mouths and carrying them to the edge of a creek that ran through their habitat. At the creek, the pigs placed the fruit pieces in the water and manipulated them with their snouts before eating.

Dietrich told Volker Sommer and Adriana Lowe, anthropologists at University College London, about the unusual behavior she’d seen, and the team set out to test whether the pigs were purposely washing their food.

The researchers provided the pigs with a pile of cut-up apples, some clean and some covered in sand. The pigs carried the sandy apples to the edge of the creek, set them down in shallow water, and pushed them around with their snouts for up to 30 seconds. When the apples were free of sand, the pigs ate them.

The pigs never took clean apples to the creek to wash them.

"The behavior with the apples does meet the criteria for true washing, most importantly distinguishing between clean and dirty foods," says Lowe. “It was a distinctive and interesting enough behavior to capture people’s attention.”

The pigs weren’t always such picky eaters. If they hadn’t been fed in the morning, they scarfed down a few dirty apple pieces without washing them, then took the rest to the creek. The pigs also devoured favorite foods such as maize cobs and sugar beets without washing them. (But they carried another delicacy—whole dead chicken—to the creek and washed it before eating it.)

Persnickety Pigs

Pigs are notorious for their love of the muck. So why might they be fussy eaters? The researchers speculate that they may dislike the taste or texture of the sand. The behavior may also protect their mouths, as prolonged exposure to abrasive grit could damage their teeth.

It's not clear whether the pigs came upon the idea to wash their food individually, through trial and error, or whether one pig started washing its food and others followed suit.

Whatever the reasons behind the behavior or how it emerged, the new findings show clearly that the pigs could both discriminate between clean and dirty foods and delay gratification long enough to carry food to a water source and wash it. Delaying gratification is a sophisticated skill, difficult even for highly intelligent animals like chimpanzees and people.

Erik Meijaard, an ecologist who chairs the International Union for Conservation of Nature’s Wild Pig Specialist Group, says the actions of these pigs sounds like it may be a "luxury behavior" observed in one group of captive boars. "They washed some apples, but not if they were very hungry, and neither did they wash more preferred foods," he notes.

Lowe believes the pigs only began washing their food after arriving at Basel Zoo. "The washing behavior wasn't observed at any of the pigs' previous zoos," she says, "although their keepers were not looking out for the behavior."

"Maybe this kind of research helps us appreciate what special animals pigs are, and thus benefits their protection in the wild," Meijaard says. “There are some 15 threatened pig species in the World, some nearly extinct in the wild, and they need all the attention they can get."