Orangutans have been observed using their hands as a soundbox to make their "kiss squeak" calls sound deeper – therefore themselves bigger – in order to ward off predators.
The behaviour was observed among orangutans in Sumatra and Borneo and appears to be a culturally learned behaviour – other orangutan populations have not been seen to use their hands in this way.
They place their hands in front of their mouths to make their warning calls sound deeper. This, theoretically, makes them appear larger than they are – and behaviour scientists believe it helps to warn off predators. The study, Acoustic Models Of Orangutan Hand-Assisted Alarm Calls, is published in the Journal of Experimental Biology.
Previously, scientists had observed this same group of orangutans putting leaves in front of their mouths when making the kiss squeaks – which experts at the time believed could also be a means of making their calls seem more intimidating.
However, the latest study looks specifically at the acoustic effects of using hands as a soundbox – a behaviour more likely linked with deepening the sound of their calls.
Lead author Bart de Boer told IBTimes UK: "It's extremely rare in that we haven't observed it anywhere else in other animals except humans of course. It's not all orangutan populations either. It's unique in the animal world if you like."
De Boer – who is an acoustics expert – said although he has not analysed the leaf sounds, he thinks this behaviour is probably more to do with display than actually deepening the sound of the kiss squeak.
He said: "Acoustically it's much more complicated to analyse the leaves because leaves are flexible. So I focused on the hand. I'm not entirely sure if the leaves make a big difference.
"[The earlier study] was published as using the leaves to modify the sound, but acoustically speaking I wonder if it's more of a visual display because I don't think using the leaves make that big of a difference."
The authors found that when using the hands as soundboxes, the acoustics are altered in such a way that makes them deeper. "The use of cylindrical wave propagation in animal calls appears to be extremely rare, but is an effective way to lengthen the acoustic system; it causes the number of resonances per kilohertz to increase," they wrote.
"This increase is associated with larger animals, and thus using the hand in kiss-squeak production may be effective in exaggerating the size of the producer."
In terms of what is happening acoustically, De Boer said waves propagate along the vocal tract to your mouth in a cylindrical tube. He said: "It's a linear motion of the wave that goes from A to B.
"If you have the hand in front, the waves move sideways – it's like ripples in a pond – it's like a circularly expanding wave very different from the motion in a tube. In between the orangutan's face and hand, the acoustic waves move like those ripples in a pond – mathematically it's almost the same."
It is not known where this behaviour originated as orangutans are not social in comparison with other ape species. "But then they're smart enough that they realised it has some effect and they keep doing it," De Boer said.
"In principal the next step would be to figure out how is this learned? Is it from parents? Or others doing it? The other thing is are predators are they actually more impressed with this bigger sounding kiss squeak than the one without the hand that sounds higher?"
He notes both of these aspects will be very difficult to study but added: "At least the acoustic analysis shows that's actually worth doing."