Los Angeles: Scientists have found that gorillas can learn new vocal and breathing-related behaviours, a finding that may change the perception that humans are the only primates with the capacity for speech.
Koko the gorilla - which has spent more than 40 years living immersed with humans at The Gorilla Foundation - is best known for a lifelong study to teach her a silent form of communication, American Sign Language.
But some of the simple sounds she has learnt may change the perception that humans are the only primates with the capacity for speech.
In 2010, Marcus Perlman started research work at The Gorilla Foundation.
"I went there with the idea of studying Koko's gestures, but as I got into watching videos of her, I saw her performing all these amazing vocal behaviours," said Perlman, now a postdoctoral researcher at University of Wisconsin-Madison.
Scientists believed that the calls apes make pop out almost reflexively in response to their environment - the appearance of a dangerous snake, for example.
The particular vocal repertoire of each ape species was thought to be fixed. They did not really have the ability to learn new vocal and breathing-related behaviours.
These limits fit a theory on the evolution of language, that the human ability to speak is entirely unique among the nonhuman primate species still around today.
"This idea says there's nothing that apes can do that is remotely similar to speech," Perlman said.
"And, therefore, speech essentially evolved - completely new - along the human line since our last common ancestor with chimpanzees," said Perlman.
However, Perlman and collaborator Nathaniel Clark of the University of California, sifted 71 hours of videos of Koko and found examples of Koko performing nine different, voluntary behaviours that required control over vocalisation and breathing.
Among other things, Perlman and Clark watched Koko blow into her hand when she wanted a treat, blow her nose into a tissue, play wind instruments, huff moisture onto a pair of glasses before wiping them with a cloth and mimic phone conversations by chattering wordlessly into a telephone cradled between her ear and the crook of an elbow.
Koko can also cough on command - not particularly groundbreaking human behaviour, but impressive for a gorilla because it requires her to close off her larynx.
"The motivation for the behaviours varies," Perlman said.
"She often looks like she plays her wind instruments for her own amusement, but she tends to do the cough at the request of Penny and Ron," he said.
These behaviours are all learned, and the result of living with humans since Koko was just six months old.
This suggests that some of the evolutionary groundwork for the human ability to speak was in place at least by the time of our last common ancestor with gorillas, estimated to be around 10 million years ago, researchers said.
The study was published in the journal Animal Cognition.