London, August 5 (ANI): A new study has found that Orangutans push leaves against their lips, making drawn-out "kiss squeaks", in order to sound more intimidating to predators.
According to a report in New Scientist, researchers recorded kiss squeaks between 2003 and 2005 near a research station in Indonesia's Central Kalimantan Province on the island of Borneo.
"When you're walking the forest and you meet an orangutan that not habituated to humans, they'll start giving kiss squeaks and breaking branches," said Madelein Hardus, a primatologist at the University of Utrecht in the Netherlands, who documented the practice among wild apes in Indonesian Borneo.
She contends that orangutans use leaves to make kiss squeaks to deceive predators, such as leopards, snakes and tigers, as to their actual size - a deeper call indicating a larger animal.
Orangutans also produce kiss squeaks with their lips alone or with their hands.
To determine if the leaves make a difference, Hardus's team recorded a total of 813 calls produced by nine apes, and then measured the pitch of the different kinds of kiss squeaks made by each animal.
Across all nine orangutans, the unaided kiss squeaks came out with the highest pitch, followed by calls produced when the apes put their hands over their mouths.
But, leaves lowered the high-pitched calls the most, Hardus's team found.
What's more, the orangutans that were unaccustomed to Hardus's team produced leaf calls at far higher rates than apes that were used to humans.
"It looks like orangutans try to deceive the predator when using the kiss squeaks on leaves, because orangutans only use it when they're highly distressed," she said.
That explanation implies that orangutans can guess at what others know and don't know - a cognitive ability known as theory of mind.
"An orangutan would have to understand how their calls are being perceived by other animals, a clear example of theory of mind," said Robert Shumaker, an orangutan expert at the Great Ape Trust of Iowa, in Des Moines. "If, in fact, this is what they're doing, deception is a perfectly plausible possibility," he added.
"It's a particularly interesting form of tool use, to me, because it gets away from a lot of the typical examples of foraging," said Shumaker. "It's really, really nice to see an example that has absolutely nothing to do with food," he added. (ANI)