Experiments by researchers revealed that dogs exhibited jealous behaviour such as snapping and pushing when their owner displayed affection for a stuffed dog that barked, whined and wagged its tail.
The University of California study used an adapted test for six-month-old babies to monitor the reaction of 36 dogs in their own homes when their owners ignored them in favour of the stuffed dog or a bucket with a Halloween design. In a third scenario, owners were asked to read aloud a pop-up book that played tunes.
The dogs' reactions were then filmed.
The study found the dogs were about twice as likely to push or touch their owner when he or she was petting and talking sweetly to the false dog as when the owner was displaying the same behaviour towards the bucket. Even fewer pushed or touched their owner when the book was being read aloud.
Just fewer than one-third of the dogs also tried to get between their owner and the stuffed animal and a quarter snapped at the "other dog", with only one doing so at the bucket and book.
Researchers psychology professor Christine Harris, from the University of California San Diego, and former honours student Caroline Provoust said the aggression displayed by the dogs suggested they believed the false dog was real.
They said the research supported the view that there may be a more basic form of jealousy which evolved as a protection from interlopers into social settings.