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.

The scientists had male titi monkeys watch their mates interact with stranger males for 30 minutes as well as watch stranger females interact with stranger males for the same amount of time.

When observing their mates, the monkeys experienced increased levels of the hormones testosterone (associated with mate-related aggression and competition) and cortisol (an indication of social stress).

Brain scans also revealed the primates had heightened activity in an area associated with social exclusion in humans (the cingulate cortex) and another area associated with aggressive behavior (the lateral septum).

Complementing these pictures, a large percentage of pet owners report consistent signs of jealousy in other domestic pets, including horses, birds, and cats.

For more "evidence", check out Bored Panda's earlier piece 50 Shameless Pets That Stole Their Owners’ Partners And Didn’t Even Feel Sorry About It.