50 Times Pets Hilariously Showed How Jealous They Are

Despite these pictures and other anecdotal evidence, scientists have long struggled to identify emotions in animals, especially because of the difficulty in separating out biases and anthropomorphisms.

That being said, research is starting to suggest that jealousy, at least, is a "primordial" emotion that both people and some animals (particularly dogs and primates) share.

Before we dive in deeper, it's important to understand that while the terms jealousy and envy are sometimes used interchangeably, psychologists see them as two very different emotions.

Envy is thought to be a two-entity emotion that occurs when we lack something — whether it's a specific personal attribute or an object — that someone else has. Jealousy, on the other hand, requires a social triangle and emerges when someone or something threatens a special relationship.

For someone to feel jealousy, they need to have the cognitive ability to recognize, at least on some level, the importance of a relationship and gauge potential threats to it — a tall order, no doubt.

In humans, research shows that infants as young as 6 months demonstrated it when their mothers interacted with another infant (which was, in reality, a realistic-looking doll). This suggests that jealousy can be an inborn (not learned) emotion that evolved to protect any type of social relationship from interlopers, and one that may exist in other social animals.

In 2014, researchers at the University of California, San Diego modified human infant experiments, adapting them to man's best friend.

They found that dogs acted far more jealous when their owners interacted with a fake dog (petting it and treating it as if it were real) than with jack-o'-lanterns or books. In fact, one-third of the dogs tried to get in between their owners and the fake dog, and one-fourth of them even snapped at the fake dog.

Scientists have also documented jealousy in coppery titi monkeys, a monogamous primate species.

In response to romantic rivals, male titi monkeys are known to become aggressive, place themselves between their mates and potential rivals, and occasionally physically restrain their mates to keep them from moving toward intruder males.