Jealousy is such a powerful emotion that at least one study has characterized it as the third leading cause of non-accidental homicide in all cultures. Is it possible that this universal green-eyed monster evolved as a survival mechanism?
In a study published Wednesday in the journal Plos One, researchers at UC San Diego experimented with dogs to see whether they, like humans, were hard-wired for jealousy.
If so, the researchers suggested that human and canine jealousy might exist for similar "primordial" reasons.
Although many dog owners will attest to bouts of canine jealousy -- even evolutionary scientist Charles Darwin suspected that the creatures were capable of such emotion -- few have tried to prove it scientifically, according to psychology professor Christine Harris and researcher Caroline Prouvost, the study's authors.
In an experiment, the authors took 36 dogs of various breeds -- along with their owners -- and observed the dogs' behavior as their masters interacted with three non-living objects. One object was a children's book, which they read aloud; another object was a plastic Jack-o'-lantern pail; and the third was a mechanical stuffed dog that emitted a bark when the owner pressed a button.
The authors based their experiment on several studies that examined whether human infants are capable of jealousy. The studies, which concluded that infants were probably capable of jealousy, involved experiments in which their mothers showed attention to a life-like doll instead of their child, and other objects. The infants were reportedly more likely to respond with "negative" behavior if their mother diverted her attention to the doll.
In the dog experiment, authors instructed the dog owners to push the bark button on the stuffed dog's head, and then speak to it sweetly, while ignoring their own dog. After that, they showed attention to the pumpkin pail, and read the children's book, while also ignoring their dog.
Researchers said that the dogs were far more likely to act aggressively when their owners spoke to the stuffed dog than when they paid attention to the other objects. One-fourth of the dogs snapped at the stuffed dog, while only one dog snapped at the pail or the book. The dogs were also more likely to push or touch their owners as they interacted with the mechanical dog, and tried to get in between the owner and the stuffed dog more frequently than the other objects. Whining also occurred more frequently with the stuffed dog, authors wrote.
"The data present a strong case that domestic dogs have a form of jealousy," the authors wrote.
The researchers said that although the vast majority of studies that examine jealousy in humans focus on jealousy within romantic relationships, their findings suggest a deeper cause.
"One possibility is that jealousy evolved in species that have multiple dependent young that concurrently compete for parental resources such as food, attention, care, and affection," the authors wrote.
"It is easy to imagine the advantages that might be gained by a young animal that is not only alert to interactions between siblings and parents, but also motivated to interpose itself in such interactions."