Many biological anthropologists would argue that culture is an evolved human adaptation, said Knott.
An alternative explanation to culturally transmitted learning would be that each individual just figures the behavior out for itself, or that the behavior is simply an adaptation to the environment in which it lives.
"This, however, cannot explain the occurrence of 'arbitrary' signals [like kiss-squeaks]," Knott said. "Social learning must be involved to explain the transmission of such behaviors."
Some behaviors, like using a stick to dig out seeds, are so advantageous to an animal that they'd never give it up voluntarily, said van Schaik.
"On the other end of the scale there are signal variants, like the kiss-squeak on a leaf or a hand that are not that different in functionality, and can go in and out of popularity within a group," he said.
Having cultural behaviors requires strong mother-infant bonds and close interaction within a group. Orangutan offspring stay with their mothers until they're seven or eight years old, but orangutans are on the lower end of the sociability scale among great apes.
"Infants and young learn most cultural knowledge from their mothers, but if that was the only way of learning you'd have as much individual variability as you have matrilineal lines," said van Schaik. "You wouldn't see this checkerboard pattern across populations that we're seeing."
The researchers found that sites closest to one another showed more behavioral similarities than with more distant sites.
"Also, we found the biggest behavioral repertoires within sites that showed the most social contact, thus giving the animals the greatest opportunity to learn from one another," said van Schaik.
Orangutans once ranged throughout Southeast Asia, and may have numbered in the hundreds of thousands. Today they are found only in Borneo and Sumatra, and are listed as critically endangered by the World Conservation Union. Increasing population pressure100 years ago 10 million people lived in Indonesia; today that number is close to 220 millionillegal logging, slash-and-burn agricultural practices, and civil unrest all pose threats to the orangutan.
Conservationists estimate there may be as few as 15,000 orangutans left living in the wild.
"We're losing the race against timejust as we discover how to study the roots of human culture, we're losing the tools," said van Schaik.
"At new sites we find new things; there is enormous cultural variation between populations, and we're losing it. You cannot protect one population and discover the whole cultural phenomenon," he said. "And even if somehow you could restore the forest and the animals, just as with human cultures, once a culture is gone, it's gone."
Editor's note: For more than 30 years two organizationsthe National Geographic Society and the Leakey Foundationhave supported long-term orangutan studies that contributed data to this recent finding.
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