The French humorist and keen observer of human nature François Rabelais once famously declared that "laughter is the property of man." Yet serious doubts on this affirmation can arise on a visit to any city zoo, where we see a wide range of facial expressions and laughter-like sounds coming from the monkey cage.

But are our primate cousins really chortling? The question has nagged primatologists for decades. One expert says we should turn the question around: "When I hear people laughing, what I am basically hearing is the laughter of a primate," says Bernard Thierry of France's national center of research (CNRS), at the University of Strasbourg.

But while human laughter arises in many different situations, and has various meanings, what we generally regard as the equivalent in primates only occurs in very stereotypical play situations, such as chasing or tickling.

So what does apes' laughter tell us? Is it just an unsophisticated communication tool used to set the terms of engagement - which in this case is just play - or does it also have a more "conversational" function, to help the animals bond with each other?

In order to get a clearer picture, Marina Davila-Ross and her colleagues at Britain's University of Portsmouth studied the behavior of 59 chimpanzees living in four groups at a chimpanzee sanctuary, the Chimfunshi Wildlife Orphanage, in Zambia. The British researcher's team filmed and analyzed more than 500 scenes of play, carefully noting the spontaneous laughter triggered by a fellow chimp's laugh, which they then defined as "responsive laughter." The films showed that play sessions lasted significantly longer when one playmate joined in the laughter of another.

Davila-Ross says that the biggest surprise was to find that animals that belonged to the most recently formed groups often imitated those from the more established groups made up of chimpanzees that had known each other for a longer time. "This suggests responsive laughter might be playing a special role in reinforcing social bonds," she explained.

The researcher believes her findings have parallels with what is called "conversational laughter" in humans, because when chimpanzees echoed their fellow creature's spontaneous laughter, it was always shorter in length, just like with humans. In both human and primate, the laughter seems to be a way of underpinning the connection of the social interaction. "These sorts of responses may lead to important advantages in co-operation and social communication - qualities that help explain why laughter and smiles have become integral tools of emotional intelligence in humans," she argues.

This is not Marina Davila-Ross' first foray into experiments on primates' laugher – and the related families that seem to exhibit something similar. In 2009, she methodically tickled the babies of humans, chimpanzees, bonobos, orangutans and gorillas, and then analyzed their vocalizations. She found that the differences and similarities in the characteristics of each of their laughter – such as peak frequencies and patterns of inhalation and exhalation – matched with how closely related the species were.

Chimpanzees - our closest evolutionary relative – shared the most similarities with us. And two years before that, an orangutan's smile had caught her attention. She had been able to show that when one of them opened its mouth in one of their typical play expressions, its playmate would immediately respond in the same way.

Back in France, Bernard Thierry is not at all surprised by these latest observations from his colleague across the Channel. He is convinced that it is normal that an animal engaging in play activity with another would respond with similar vocalizations, or that these responses would not be symmetrical. Even if he doesn't agree with all of Davila-Ross' findings, he himself has focused for many years on the Macaque monkeys' expressions during play, which he believes are "the equivalent of the human smile."

Amongst the Macaque species he is studying, the ‘smile" where they bare their teeth can be used in several ways, either for a dominant monkey to introduce itself to another dominant, or as a sign to engage in peaceful activity. "In this sense, it is a meta-communication tool or cue, which gives information about the exchange," Thierry explains. "For example, it signifies that when they begin to exchange blows, it will be in a play mode and not an aggressive one."

All this begs the question about how deeply rooted laughter is in us. From the smiling Macaque monkey to the laughter of our own species, Homo sapiens, can we really conclude it is a continuum, with our chimpanzee cousins occupying some kind of intermediary position? Or have humans made an evolutionary leap into something completely new: where laughing as we do marks some key break with our primate past?

Defining what makes us singularly human still remains one of science's last frontiers. Maybe humor can help provide the answers? Because apart from some anecdotal evidence about performing chimpanzees (playing hide and seek), one thing is for sure: all monkeys, from the great apes to their smaller cousins, are completely impervious to the comedy of a situation. They don't do slapstick as we understand it.

Bernard Thierry explains: "From what we have observed, when animals stumble, they appear human and are consequently funny to us. But it means something altogether different to them." So when it comes to evolution and monkeying around, perhaps it is that little touch of cruelty that makes our laughter human.

Photo - tambako