Why are some pigs more optimistic than others?

Scientists studied whether different pigs had sunnier or cloudier outlooks by testing how differences in housing and mood impacted porcine decisionmaking. 

Melanie Stetson Freeman/Staff/File
Free range pigs hang out on a farm with their shelters visible in the distance in the Scottish Border region in Scotland.

A new study by British scientists finds that humans aren’t the only animals who can be optimists or pessimists – pigs are, too.

That similarity affects porcine decisionmaking, the authors wrote in a study released this week in the journal Biology Today. Whether a pig can find the silver lining in its day can determine their reaction to stimuli, according to researchers. [Editor's note: This paragraph has been updated for clarity.]

"This finding demonstrates that humans are not unique in combining longer-term personality biases with shorter-term mood biases in judging stimuli," the authors wrote.

While researchers have done many studies on animal mood and decisionmaking, most studies have ignored the role of personality in that decisionmaking in animals. In this study, scientists hypothesized that personality and mood could play a role in a pig's cognitive bias, or patterns of thinking that deviate from normal "good judgment."

In order to examine the relationship between mood, personality, and decisionmaking, researchers studied 36 pigs and tested their response to bowls of food. Study authors chose pigs because of their high intelligence and keen sense of smell.

Researchers at the University of Lincoln in England initially set out two bowls for their test subjects: one containing sugar-covered chocolates, and the second containing bitter coffee beans. Like most humans, pigs consider chocolate delicious, and were likely to associate positive feelings with that tasty bowl.

On the other hand, researchers anticipated that the pigs would associate negative outcomes with the less desirable coffee beans, placed on the other side of the room from the preferred chocolate.

Pigs were labeled optimists if they then looked into a third empty bowl, which was placed between the two other bowls in the room, and pessimists if they stuck to the two full bowls, despite the (empty) potential of a treat. Scientists also labeled pigs either reactive or proactive – flexible and passive, or active and less flexible.

In humans, proactive personalities have been linked to extraversion, while reactive personalities have been linked to neuroticism.

The final component of the tests was mood. Scientists know that, in humans, decisions can be affected by moods. In this study, researchers sought to alter the pigs' moods by housing them differently prior to each test.

Some of the pigs were given the animal equivalent of an economy hotel room – comfortable but not lavish accommodations – while others checked in to a luxury suite, with extra comfy straw on the floor and more space to move around.

For proactive, optimist pigs, their housing didn’t make a difference in which bowls of food they investigated – they invariably checked out the empty bowl, optimistically searching for that one extra treat, regardless of the quality of their beauty sleep.

Among the reactive pigs, however, housing made a real difference. The reactive pigs that had a chance to enjoy the luxury straw bedding in the deluxe accommodations were more likely to go investigate the empty bowl, while the pessimistic pigs who had experienced merely adequate housing were less likely to do so.

Cognitive bias has long been tricky to test in animals, with different experiments yielding conflicting results. Researchers say, however, that this study could help detangle some of the confusion.

"Because proactive pigs behaved differently to reactive pigs, these findings could explain some of the inconsistent results between animal cognitive bias tests," researchers wrote in the study. "Accounting for personality differences between individuals may reduce some of this otherwise unexplained variation, making cognitive bias test outcomes more reliable and robust."

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