No matter how many laughing cat memes threaten to swamp the Internet, no matter how many grinning dog videos have you LOL or ROFLing, scientists have yet to prove that cats or dogs can really laugh.

Now, chimpanzees and rats laugh. That's been scientifically proven — or as close to scientifically proven as scientists get.

But cats and dogs? Or, say, laughing hyenas or laughing gulls? Do any animals (save for you, me and those happy-go-lucky rats and chimps) bust-a-gut laughing?

Better yet: Do animals have a sense of humor at all?

Studying animal laughter

So far, the answers are murky. In the early 2000s, animal behaviorist Patricia Simonet discovered what was pegged as "dog laughter," a "breathy pronounced forced exhalation" that dogs used to initiate play and that, in one study, was shown to calm other dogs.

Was that really laughter? Or just heavy panting?

As far as cats, it's easy to say that a purring cat is happy and content, but it's a big leap to describe that purr as "cat laughter." In fact, it's been shown that cats purr for lots of decidedly un-funny reasons.

"Although it is tempting to state that cats purr because they are happy," Leslie A. Lyons, now a professor in the College of Veterinary Medicine at the University of Missouri, told Scientific American in 2006, "it is more plausible that cat purring is a means of communication and a potential source of self-healing."

So dogs and cats may do something that, perhaps, could be construed as laughter. But taking that seemingly simple leap is tricky. Any attempt to ascribe a human trait to something that isn't human — it's called anthropomorphizing — is inherently risky.

Because animals, let’s not forget, are…different.

A smiling dog with its tongue sticking outDogs perform a 'breathy pronounced forced exhalation' that could be a form of laughter, but it could also just be heavy panting. (Photo: Allen Skyy/flickr)

Finding the funny bone

Over the past 10 or 15 years, studies with rats and chimpanzees have convinced many experts that some animals — rats and chimps, mainly — can, indeed, break out a good guffaw once in a while.

A 2000 study concluded that rats, when tickled, emit a high-pitched “chirp” and will follow, even chase, the pleasure-inducing hand that tickles. In 2009, in a paper entitled "Reconstructing the Evolution of Laughter in Great Apes and Humans," researchers revealed that young primates like orangutans and chimpanzees, when tickled, let out "tickle-induced vocalizations."

In other words, both rats and chimps laugh.

Just last month, in another study, scientists concluded that chimpanzees use the same type of welcoming "laugh face" when they're not being tickled as when they are, suggesting those faces "may present the apes with the opportunity to communicate with their social partners in more explicit and versatile ways." Just like humans do, the study says.

Researchers took it another step: "We predict, based on the current findings, that the ability of humans to flexibly combine facial expressions with vocalizations evolved directly from such capability of ancestral apes."

It's easy, some say, eliciting a reaction that we can call a laugh out of animals just by tickling or rough-housing. But, remember, that kind of play — and that kind of laughter — is common in young humans, too, even infants, suggesting a deep-seated bond between humans and other animals.

"[N]eural circuits for laughter exist in very ancient regions of the brain, and ancestral forms of play and laughter existed in other animals eons before we humans came along with our 'ha-ha-has' and verbal repartee," Jaak Panksepp, a neuroscientist from Washington State and author of the landmark 2000 study, told in 2005.

The more difficult question is whether animals — even those happy-go-lucky chimps and rats — are advanced enough to actually have a "sense" of humor. Whether they can laugh at something that doesn't include physical stimuli. That's been harder to determine.

Still, the simple idea that animals can laugh should bring a smile to any grump’s face.

"The power of recognizing that another species has a mirthful response or is clearly enjoying something...we see ourselves in that," biologist Jonathan Balcombe told the Huffington Post. "We can see that that experiencing something akin to what we have."

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