They're known as our best friends - so do dogs get jealous when we pat another pooch?
Given surveys suggest most owners think this is the case, Kiwi researchers set out to test the claim by looking at how a group of dogs reacted when an interloper seemed to steal their human's attention.
In humans, jealousy is closely linked with self-awareness, which is one reason animal-cognition researchers are so interested in studying this and other secondary emotions in animals.
The researchers' new study, published in the journal Psychological Science, showed that dogs did indeed show jealous behaviour - even when they had to imagine that their owner was interacting with a potential rival.
Fascinatingly, the findings may also shed light on jealousy and attachment in humans.
The study showed how dogs exhibited three human signatures of jealous behaviour. This behaviour emerged only when their owner interacted with a perceived social rival and not an inanimate object.
"Research has supported what many dog owners firmly believe - dogs exhibit jealous behaviour when their human companion interacts with a potential rival," said study lead author Amalia Bastos, of the University of Auckland.
"Dogs appear to be one of the few species that might display jealous behaviours toward members of another species in a way that is similar to how human children experience jealousy when their mothers show affection to another child."
To test the potential for dogs to display jealous behaviour, the researchers presented 18 dogs with situations where they could imagine a social interaction between their human companions and a realistic fake dog or a fleece-covered cylinder.
The fake dog would serve as a potential rival for attention while the cylinder would serve as a control.
Dogs observed the interloper positioned next to their owner - and then a barrier was moved into place obscuring the interloper.
Dogs could only observe the humans' upper body.
Despite this, dogs pulled harder on a lead to reach their owner when they owner appeared to stroke the interloper behind the barrier, compared to situations where dogs could see the fake dog and the owner was stroking the fleece cylinder.
"These results support claims that dogs display jealous behaviour," Bastos said.
"They also provide the first evidence that dogs can mentally represent jealousy-inducing social interactions."
She noted that previous studies confounded jealous behaviour with play, interest, or aggression, because they never tested the dogs' reactions to the owner and the social rival being present in the same room but not interacting.
"There is still plenty of work to do to establish the extent of the similarities between the minds of humans and other animals, especially in terms of understanding the nature of nonhuman animals' emotional experiences," Bastos said.
"It is too early to say whether dogs experience jealousy as we do, but it is now clear that they react to jealousy-inducing situations, even if these occur out-of-sight."