(CNN) -- Sure, Fido is a brown Lab. But inside, he may also be a little green.
New research suggests that dogs can exhibit jealousy, a human emotion usually ascribed to squabbling siblings or the jilted third of a love triangle.
A study by scholars at the University of California, San Diego found that dogs showed jealous behaviors when their owners displayed affection toward an animatronic stuffed dog that barked, whined and wagged its tail. The dogs snapped at and pushed against the stuffed dog and tried to get between it and the human.
This may come as no surprise to any owner of multiple pooches who has seen them jostle for space on someone's lap. And it's not unusual for people to assign human feelings to their dogs, whose baleful eyes seem like deep pools of emotion when compared with those of, say, cats.
But animal-behavior experts say the study is a significant step forward in understanding our dogs' emotional lives.
"This is the first study I know of that directly asks this question: Do dogs get jealous?" said Marc Bekoff, author of "Why Dogs Hump and Bees Get Depressed: The Fascinating Science of Animal Intelligence, Emotions, Friendship, and Conservation."
The study by Christine R. Harris and Caroline Prouvost was published Wednesday in PLOS One, a peer-reviewed online scientific journal. For their research, the authors videotaped 36 dogs individually at their homes while their owners ignored them and interacted with a series of three objects: the fake dog, a children's book and a plastic jack-o'-lantern.
The canines included 14 small breeds such as pugs, dachshunds, corgis and terriers. Researchers chose small breeds so they could more easily control the dogs if they acted out violently.
The dogs acted jealous when their owners petted the stuffed dog and talked sweetly to it as if it was real, although they displayed less such behavior when the owner showered attention on the pumpkin or read aloud from the children's book, which had pop-up pages and played melodies.
In this way, the study suggests, the dogs' jealousy was triggered by social interaction and not merely by their owners' ignoring them for an inanimate object. Eighty-six percent of the dogs sniffed the butt of the toy dog during the experiment, so many of them may have seen it as real.
The findings mirror those of other studies that found human babies as young as 6 months displayed jealous behaviors when their mothers interacted with a realistic-looking doll. The infants did not act jealous, however, when their mothers attended to a nonsocial item such as a book.
"These results lend support to the hypothesis that jealousy has some 'primordial' form that exists in human infants and in at least one other social species besides humans," the study said.
Although most animals clearly demonstrate primal emotions such as anger or fear, studies have been less conclusive in determining whether dogs are capable of more complicated feelings such as guilt or shame, Bekoff said.
But research has shown that dogs do understand when they're being treated unfairly, he said.
"Dogs are really keen social observers," Bekoff said.
Animal behavioral expert Patricia McConnell, author of "For the Love of a Dog" and other books, said she was impressed with the new study's methodology.
But she's not surprised by its findings.
"I think we share a tremendous amount of emotional life ... with dogs," she said. "But I have never thought of jealousy as a particularly complex emotion (in animals). Is human jealousy exactly like dog jealousy? I'm sure it's not."
Does your dog ever act jealous? Share your experience in the comments section below.