Researchers say their study indicated that jealousy in dogs appeared to be driven by a desire to protect the relationship with their owner.
The research team - Christine Harris, professor of psychology at the University of California-San Diego (UCSD), and Caroline Prouvost, a former honors student at UCSD - recently published their findings in the journal PLOS One.
According to the researchers, understanding the mechanisms underlying jealousy in humans is important. They point out that the emotion is often cited as the third leading cause of non-accidental homicide.
To date, there have been no studies looking at jealous behavior among dogs, note Harris and Prouvost.
With the belief that such studies may lead to a better understanding of jealousy among humans, the team conducted a series of tests on 36 dogs that were adapted from an experiment usually carried out with 6-month-old human babies.
Dogs demonstrated jealous behavior when owners' attention on stuffed dogs
In their own homes, the dogs' owners were required to ignore their pet and focus their attention on either a stuffed, animated dog that barked, whined and wagged its tail or a jack-o'-lantern pail. Owners were asked to treat each object as if it were a dog. They were required to pet them, for example, and talk to them like they would their real dog.
In another test, owners were asked to read a melodic pop-up book aloud, while again, ignoring their dogs.
All tests were videotaped, and independent raters were used to assess any aggressive, disruptive or attention-seeking behaviors the dogs demonstrated.
The researchers found that the majority of the dogs - 78% - pushed or touched the owner when their attention was focused on the stuffed dog, while only 42% demonstrated this behavior when their owner focused on the jack-o-lantern pail, and only 22% did this when owners read the pop-up book. In addition, 30% of the dogs attempted to push themselves between their owner and the stuffed dog, and 25% even snapped at the stuffed dog.
Based on the levels of aggression shown toward the stuffed dog, Harris and Prouvost believe the animals thought the fake dogs were real, noting that 86% of dogs even sniffed the fake dogs' rear ends either during or after the tests - a dog's way of investigating another.
"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," says Harris. "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."
Jealousy 'not specifically tied to sexual and romantic relationships'
Harris notes that most of the research on jealousy is among couples in relationships, but she says the emotion is also common among friends, siblings and even close work colleagues. In addition, she says early signs of jealousy are often seen among infants and young children.
For example, two sisters may compete for parental attention, indicating that we are hardwired to experience jealousy and that it can evolve with the desire to protect a social bond.
Harris says their findings support this theory:
"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. Our results challenge these ideas, showing that animals besides ourselves display strong distress whenever a rival usurps a loved one's affection."
In another study on dogs reported by Medical News Today earlier this year, researchers found that dog and human brains have "voice areas" in the same places, and both react in similar ways in response to emotional cues.