Aug. 5, 2009 -- Wild orangutans have invented a sound modification tool that makes a "kiss squeak" noise and fools listeners into thinking the individual is larger than he or she actually is, according to a new study.
The study, published in the latest Proceedings of the Royal Society B, presents the first evidence concerning how and why non-human primates alter their own calls using tools and even their own hands.
In this case, the researchers describe the kiss squeak call as a sharp intake of air through pursed lips. Hardly a love call, the noise is produced by orangutans when predators startle them, or they are otherwise disturbed.
"As far as I and my colleagues know, no other primate emits kiss squeak sounds," lead author Madeline Hardus told Discovery News. "Male great apes can use their hands during the production of calls, such as buttress drumming in chimpanzees, chest beating in gorillas and snag crashing in orangutans, but in these examples, calls are not modified; they're added with other acoustic elements."
"Only kiss squeaks are known to be modified," added Hardus, a researcher in the Behavioral Biology Group at the University of Utrecht.
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For the study, Hardus and her colleagues recorded kiss squeaks made by wild orangutans at Central Kalimantan in Borneo, Indonesia. The orangutans will make the sound themselves, make it against their hands or produce it against leaves stripped from twigs. The latter method constitutes the sound modification tool.
Often orangutans will hold the leaves in one hand while making the noise, freeing the other hand to shake and throw twig "missiles," rocks and more.
Detailed analysis of the digitized recordings found that the hand and leaf-modified kiss squeaks lowered the maximum frequency of the calls, with the leaf tool version producing the lowest sounds. Since larger animals tend to emit such low noises, the scientists think the orangutans are trying to make themselves sound as big and imposing as possible for approaching predators that may only hear, but not see, the tree-dwelling primates.
The kiss squeak sound likely then "deters (others) by indicating that the presence of the predator has been noticed, probably leading the predator to leave the hunt faster and give up its hiding position."
The researchers found that using the leaf tool didn't increase the volume of the sound or change it in respects other than reducing its frequency.
An individual through "trial and error" probably first invented this whoopee cushion-like orangutan tool, Hardus believes. The innovation then "spread locally and became cultural."
Rob Shumaker, a scientist at the Great Ape Trust of Iowa, told Discovery News, "I firmly agree with the tool use portion of the study."
"As for deception," he added, "I think that Hardus et al. provide a very careful and logically constructed argument that propel the discussion of 'theory of mind' forward most appropriately."
"Theory of mind" asks whether or not an individual recognizes and can keep track of information among individuals in their group, and whether or not they can attribute psychological states, such as fear or aggression, to their peers.
"As threats to orangutan survival continue in Sumatra and Borneo -- the only two islands the Asian ape still inhabits -- and as their numbers continue to decline, orangutans continue to surprise scientists," Hardus said.
She added, "After studying them for years, and despite language being considered the hallmark of mankind, this study clearly indicates that ape communication abilities have been traditionally undervalued."
Both she and Shumaker hope conservation efforts will, as Hardus concluded, "lead to the survival of this emblematic species," permitting experts to study and better understand orangutans, paving the way to additional discoveries that can "also open doors to understanding the evolution of our own species."
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Jennifer Viegas' Blog: Born Animal
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