Orangutans cup their mouths to alter their voices
Orangutans use their hands to alter their voices and make themselves sound bigger, say scientists.
The animals cup their mouths when they produce kiss squeaks - alarm calls that often signify a predator is nearby.
Researchers have now studied the acoustics of these "hand-modified kiss squeaks" and shown that the animals sound bigger and "more impressive" when they use their hands in the call.
The findings are published in the Journal of Experimental Biology.
If the orangutans do this deliberately - knowing it exaggerates their size - the researchers think it may be a glimpse of an early precursor to language; an animal intentionally changing a sound it makes and, in turn, changing its meaning.
This same team of scientists - from the Netherlands, Belgium and the UK - first noticed that the apes changed their kiss squeaks with their hands back in 2009.
But that change they spotted was very subtly different, so the team decided to analyse it in much more depth, to understand exactly what effect it had on the orangutan's voice.
Lead researcher Bart de Boer from the University of Brussels, an expert in bioacoustics, worked out that the animals' hands were creating what he called a "a cylindrical extension of the lips".
"This has the same effect as lengthening the sound box of a musical instrument," he explained, "so you get resonance of the lower pitches".
In short, this makes a kiss squeak sound like it is coming from the puckered lips of a bigger orangutan.
"So they aren't doing this for nothing," said Dr de Boer, "It makes them sound more impressive."
Dr de Boer's main scientific interest is the evolution of speech. By applying detailed acoustic examination of the sounds these great apes produce, he says that researchers are gradually piecing together the puzzle of how language developed.
But, he said, "there is still a lot to be found out".
"It's unlikely that they're [making themselves sound bigger by accident], but it's still possible," Dr de Boer cautioned.
Dr Wendy Erb, an anthropologist from Rutgers University in New Jersey pointed out that non-human apes' gestural signals are thought to be more likely precursors to language because they are "more flexible and intentional" than vocal sounds.
And the use of gestures in apes is well known. Recently, a team from the University of St Andrews published a study that attempted to translate the many gestures wild chimpanzees use.
But, as Dr Erb pointed out, the orangutans' use of their hands to change a sound they make is an even more intriguing clue in this ongoing search for the earliest signs of language.
"Orangutans are thought to be unique in their use of a gesture that actually modifies another acoustic signal," she told BBC News.
Finding out whether the animals are deliberately "lying about their size", perhaps to scare away predators, when they cup their mouths will require many more hours of watching, recording and studying the wild animals.
But Dr de Boer is excited by what he says is a small, but important piece of that puzzle.
"The beginning is there - the evolution of language is not such an unsolvable mystery," he told BBC News.
"And these apes can clearly do more than we thought."