Life

"Man is distinguished from all other creatures by his faculty of laughter," wrote the English essayist Joseph Addison in 1712. Modern science has a different take - up to a point.

We're pretty sure that no other animal laughs quite like we do. That's down to our unique status as an ape that has learned to stand on its own two feet. "Bipedalism was the breakthrough," says Robert Provine, the doyen of laughter research. Four-legged mammals must synchronise their breath with their stride. By taking pressure off the thorax, bipedalism gave us the breath control needed for speaking and the ability to chop up our exhalations, giving the characteristic ha-ha-ha sound of human laughter.

If laughter really is just a social lubricant (see "What are you laughing at?"), you might expect our equally social great-ape cousins to do something similar. "Laughter is literally the sound of rough-and-tumble play," says Provine - and great apes at play do indeed produce something akin to a laugh. But their playful pants are not as musical as ours and instead of being made up of extended exhalations, they are produced by breathing in and out. As a result, ape laughter doesn't sound much like our own. When Provine played a recording of chimp laughter to his students, most of them thought it was a dog panting, a few had it down as noisy sex and some even heard sawing or sanding.

This is probably about as close as we are going to get to human laughter. Last year Marina Davila-Ross from the University of Portsmouth, UK, and colleagues tickled three babies and 21 orang-utans, gorillas, chimps and bonobos, measured various acoustical features of the sounds they produced and used these to create a family tree of laughter. It was remarkably similar to the evolutionary tree (Current Biology, vol 19, p 1106). "The strongest acoustical differences were between humans and great apes," says Davila-Ross. But the laughs of the African great apes - the chimps and gorillas that are our closest genetic cousins - were acoustically more similar to ours than the squeaks produced by orang-utans.

So where should we draw the line? Should we define a laugh simply as any vocalisation made during play? Opinion is divided.

One researcher who advocates a liberal definition is Jaak Panksepp of Washington State University in Pullman. By making recordings of rats using bat detectors, he discovered that they produce characteristic ultrasonic chirps at a frequency of 50 kilohertz when tickled. Not only does he consider this to be laughter, he argues that studies on rats could help us understand the neurobiology of human laughter (Behavioural Brain Research, vol 182, p 231).

Read more: The secrets of laughter

Kate Douglas is a feature editor for New Scientist.

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