Orangutans Kiss Leaves to Sound Bigger and Badder?

Matt Kaplan
for National Geographic News
August 4, 2009

Like Bruce Wayne switching to his Batman voice, orangutans may be going deep to deter predators, and some are even using tools to sound more intimidating, a new study says.

Wild orangutans often make drawn-out "kiss squeaks" when predators approach, apparently to let potential attackers know they've been spotted.

(Related: "Whistling Orangutan May Hint at Language Evolution."

Sometimes the apes use just their lips; sometimes they kiss their fingers. But in some communities, orangutans—male and female, young and old—go so far as to push leaves against their lips as they kiss-squeak.

Why the multiple methods?

To find out, researchers recorded kiss squeaks between 2003 and 2005 near a research station in Indonesia's Central Kalimantan Province on the island of Borneo. The team noted whether the sounds had been made with hands, leaves, or lips alone.

"Classic" kiss squeaks—lips only—were fairly high pitched, registering around 3,500 hertz. When hands were used, the frequency dropped to an average of 1,800 hertz. Leaves further deepened the sound, to an average of around 900 hertz.

Deeper, Bigger, Badder

In most animals vocal pitch and body size are tightly connected: Larger vocal organs create larger, deeper sound waves.

So perhaps, the new study says, the orangutans are trying to fool predators into thinking the apes are bigger than they actually are.

"This effect [probably] further discourages the predator from the hunt," said study co-author Madeleine Hardus, a behavioral biologist with the University of Utrecht in the Netherlands.

Though the team didn't gauge predator reactions to the various kiss squeaks, she thinks the popularity of the leaf method hints at its effectiveness.

But Hardus doesn't expect leaf kissing to sweep Borneo.

"Most of the areas where it is currently used are isolated, and we do not expect it to spread to other populations," she said.

"Although, where it currently exists, almost all of the orangutans use it."

Findings to be published tomorrow in the journal Proceedings of the Royal Society B.




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