Is Fido jealous? Dogs fall prey to green-eyed monster, say scientists.

Is your dog acting especially pushy? It could be that your furry companion is feeling a bit green with envy.

According to a study published this week in the journal PLOS ONE, dogs exhibited jealous behavior when their owner played with another "dog."

Researchers from the University of California San Diego asked volunteer dog owners to interact with three items – a stuffed dog that barked and whined and wagged, a plastic jack-o'-lantern bucket, and a musical pop-up children's book – while their pet watched. The researchers videotaped the (real) dog's reaction to each scenario.

The volunteers were instructed to treat the stuffed dog and the jack-o'-lantern pail as though they were pets. They pretended to read the book to an invisible child.

When the owners read the book or talked to the plastic pail, the dogs acted relatively indifferent. But when their people gave affection to the "other dog," the real pets had to make their presence known.

Most of the dogs pushed, nudged, or otherwise touched their owner or the rival dog as if to say, "Hey, this is my person!" Some vocalized their displeasure by whining or barking. Some physically inserted themselves between their owner and the stuffed dog, in an effort to prevent a relationship between the two.

Dogs touched their owners twice as often when they were ignored for the "other dog" as with the pail. Almost a third of the dogs tried to physically separate their people from the rival.

"Our study suggests not only that dogs do engage in what appear to be jealous behaviors but also that they were seeking to break up the connection between the owner and a seeming rival," said study author Christine Harris, a psychology professor at UCSD in a news release. "We can't really speak to the dogs' subjective experiences, of course, but it looks as though they were motivated to protect an important social relationship."

Did the dogs really believe that the stuffed animal was alive? Nobody really knows, but the researchers noticed that 86 percent of the dogs sniffed the stuffed dog's backside, just as they would when meeting a real dog.

The dog – a subspecies of wolf – is the first animal known to have been domesticated by humans, firmly establishing themselves as man's best friend by at least 15,000 years ago. Although this bond has been mutually beneficial, scientists are conflicted over which animal initiated that relationship.

Some say humans adopted tiny wolf pups and dedicated time to train them, while others think friendly wolves entered human encampments themselves looking to feed on scraps.

Any dog owner would attest to having an emotional bond with his or her furry friend, but scientists have questioned the level to which dogs feel emotions. Some researchers have suggested dogs’ emotional development is limited to the level of a two-and-a-half-year-old human. This would mean dogs feel a range of emotions including excitement, fear, anger, distress, contentment, shyness, affection, and love.

And jealousy? The UCSD study was modeled after one used to measure jealousy in human babies. In test scenarios, mothers ignored their child in favor of a doll or a book. The results suggested that jealousy leads to increased attention and interest in the mother’s interaction, so Harris and her colleagues investigated those same characteristics in dogs.

"Many people have assumed that jealousy is a social construction of human beings – or that it's an emotion specifically tied to sexual and romantic relationships," Harris said. "Our results challenge these ideas, showing that animals besides ourselves display strong distress whenever a rival usurps a loved one's affection."