“The world would be a nicer place if everyone had the ability to love as unconditionally as a dog”
- M.K. Clinton

A study published in Psychological Science conducted by Amalia P. M. Bastos, Patrick D. Neilands, Rebecca S. Hassall, Byung C. Lim, Alex H. Taylor found that jealousy is a commonly experienced behaviour between humans and dogs. 

Jealousy, a social behaviour linked to self-awareness, and theory of mind has been associated uniquely to humans for a long time. 

In this study, 18 dogs and their behaviours were assessed to understand whether jealousy is a behaviour exclusive to humans. They were presented with situations where they could observe social interactions between their owner and a fake dog, and then their owner and a fleece cylinder. 

Jealousy in dogs was perceived in three notable ways. It appeared when the dog’s owner interacted with a potential social rival. As well, it occurred as a result of the initial interaction actually happening rather than the simple presence of the relative parties. Finally, it emerged even when the dog witnessed an out-of-sight interaction between a social rival and its owner. 

The evidence of this study claims that jealousy is in fact a behaviour not exclusive to humans. Dogs can and do mentally represent jealousy-inducing social interactions.