Gorillas grin 'to reassure friends'
Gorillas bare their teeth in a playful "grin" to reassure one another during play, scientists have discovered.
This "flash of teeth" seems to let their playmate know that they do not intend to harm them.
The researchers, from the University of Portsmouth, study the facial expressions of primates to uncover the evolutionary origins of human smiling and laughter.
Lead researcher Dr Bridget Waller explained that non-human primates have two expressions "that shed light on our smiling".
Their "playface", she explained, appears to be a foundation of human laughter.
Dr Waller told BBC Nature: "[During play, gorillas] open their mouths and cover their teeth as if to say, 'I could bite you but I'm not going to'."
Another expression the primates use, where they reveal both rows of "sparkly white teeth" is believed to show one of the origins of human smiling.Smiling signal
This is not a playful expression, Dr Waller said. "It's a greeting; a subordinate display."
The different contexts in which gorillas use these facial expressions reveals that smiling and laughing are probably rooted in very different "ancestral displays", as Dr Waller explained.
The gorillas would play for longer when they "grinned"
"People think we smile when we're happy, but that's not true," she told BBC Nature.
"You smile when its appropriate in a situation. You smile at someone in the corridor - you don't laugh at them."
Dr Waller and her colleagues wanted to find out more about the contexts in which these two expressions combined; when gorillas flashed their upper teeth as they played.
Watching the animals revealed that they would do this during particularly "rough" and intense play and they would play for longer when they bared their teeth.
"It's possibly because, when play gets rough, you need an extra signal to show each other that [you're] just playing," Dr Waller said.
The findings, she said, showed the foundation of people's social laughter; when humans laugh along in conversation to put one another at ease.
"I always think of facial signals as about reducing uncertainty," said Dr Waller. "We use [these] non-verbal signals all the time."
Prof Richard Byrne, an expert on primate communication and behaviour from the University of St Andrews said it was interesting to study the facial expressions of non-human primates because most of our our own expressions "seem to be 'primitive', in the main".
"Superficially [their] expressions may look a bit different because the ape or monkey faces are so different to our own," he said.
"But when examined properly, most human expressions have proved to be shared with quite distantly related primate species - and therefore must derive from an ancient shared ancestor."