Between verbal jokes, slapstick comedy, and funny animal pictures, there are numerous reasons we laugh. But are humans the only species that can find humor in life?

The tricky part is that philosophers and psychologists have long struggled to come up with an exact definition of what constitutes humor in the first place.

They've presented numerous theories over the years, one of the most popular being the "incongruity theory" of humor. At its basic level, this theory states that humor arises when there's an inconsistency between what one expects to happen and what actually happens — and this includes comedic tools like puns, irony, and twists of fate.

According to this definition, it sounds like the vast majority of animals probably don't have a sense of humor, as they lack the cognitive mechanisms and networks that would allow them to identify such inconsistencies.

One known exception is Koko, the famous western lowland gorilla who understands more than 1,000 American Sign Language signs and 2,000 spoken English words.

This clever primate is known not only to use language to humorous effect by playing with different meanings of the same word, but also to understand slapstick comedy — she's reportedly signed the word "chase" after tying her trainer's shoelaces together and made laughing noises at her trainer's clumsiness.

But there are various issues with the incongruity theory of humor and related theories, including the fact that they fail to take into account that jokes with predictable punch lines can also be funny and that certain incongruous things aren't funny.

Furthermore, they also seem to evade the primary reasons why we laugh: being tickled by a friend or family member.

Recently, a few psychologists came up with a new theory. They propose that humor arises from the so-called benign violation or "something that threatens a person’s well-being, identity or normative belief structure but that simultaneously seems okay," they wrote in a paper about the theory.

Benign violation can explain why a number of things make us laugh, including being tickled: that is, tickling benignly invades someone's physical space.

You can't tickle yourself because that doesn't constitute a violation and complete strangers can't tickle you to the point of laughter because you won't perceive it as a benign act.

Under this theory, various animals do, in fact, have "humor," if only they can be tickled.

Research shows us that our primate relatives — chimpanzees, bonobos, gorillas, and orangutans — all produce laughter-like sounds when tickled (as well as when they're wrestling and play-chasing).

This also suggests that humor and our ability to laugh likely came from humans' and great apes' last common ancestor.

Additionally, another study discovered that chimpanzees can make silent "laugh faces" just like humans.

Dogs are also known to have a kind of panting laugh and play face that's suggestive of humor.

But as far as these pictures go, we're probably the only creatures that can appreciate them. Which makes them even more special.