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Dogs are capable of feeling a basic form of jealousy, according to a study published in the PLOS ONE scientific journal.
The research, said to be the first experiment on canine jealousy, could redefine the view that the complex emotion of envy is a human construct, said Christine Harris, University of California, San Diego psychologist and an author of the study.
The owners of 36 small dogs were asked to do three things in the test - shower affection on a plush animatronic dog, shower affection on a plastic jack-o-lantern pail and read a children's book aloud - while ignoring their pet.
Researchers then watched how the dogs reacted.
Roughly 80 percent of the dogs pushed or touched their owner when they were coddling the toy, almost twice as often as when the owner played with the pail and about four times as often as when the owner was reading.
A quarter of the dogs even snapped at the toy, which barked, whined and wagged its tail, while the owner was playing with it. Only one dog snapped at the pail and the book.
"We can't really speak to the dog's subjective experiences, of course, but it looks as though they were motivated to protect an important social relationship," Harris said in a statement accompanying the study.
The research, based on a similar study to gauge jealousy in infants, suggests dogs and possibly other animals exhibit a primordial form of the emotion, the study said.
Researchers said jealousy may have evolved as a way for paired animals to protect their sexual relationships or for baby animals to compete for food and affection from their parents.
They said it also may have developed in dogs during their long domestication by humans.
"Humans, after all, have been rich resource providers over our coevolution," they wrote in the study.
Understanding jealousy is an important scientific task, they wrote, noting that jealousy is often considered a cause of homicides across cultures.