Even when they are very young, orangutans may start to form ideas about their world - specifically, how and when to use certain tools. That's the conclusion of a new study, which indicates that ape cultural traditions may not be that different from our own.
Like humans, orangutans have behavioural traditions that vary by region.
Orangutans in one area use tools, for example, whereas others don't.
Take the island of Sumatra, in western Indonesia. By the age of six or seven, orangutans from swampy regions west of Sumatra's Alas River use sticks to probe logs for honey.
Yet researchers have never observed this "honey-dipping" among orangutans in coastal areas east of the water.
How do such differences arise? Many experts say that social learning is key - that the apes figure out how to honey-dip by watching others.
But even the most careful field researcher can have difficulty proving this, says Yale University anthropologist David Watts.
Wild apes are always responding to their environment, he says, and it may be influencing their behaviour far more than social learning.
An unfortunate series of events has finally allowed scientists to test social learning's importance.
Deforestation has caused a large number of orangutan orphans, many of whom come from both sides of the Alas River, to wind up at the Batu Mbelin shelter in northern Sumatra.
At first they're quarantined, and then they move to large social groups.
Psychologist Thibaud Gruber of the University of Zurich's Anthropological Institute & Museum in Switzerland, and his colleagues, began studying Batu Mbelin's quarantined apes because political unrest made it unwise for the researchers to work in the field.
The team gave the orangutans two stick-based challenges: raking food into their cage and dipping for honey.
Apes from both sides of the river picked up the raking behaviour relatively quickly.
This suggests that all of the animals could understand sticks as tools, Gruber says.
But while nine of 13 west-side apes "knew" to honey-dip, only two of 10 east-side apes did, Gruber's team reports this month in Current Biology.
What's more, the savvy west-side apes were just four years old on average - too young to have begun honey-dipping when they were in the wild.
Gruber says this indicates that specific ways of using tools come from observing others.
The young orangutans who "knew" to honey-dip likely formed the idea of honey dipping in their heads before they were physically able to do it, Gruber says.
And when it came to applying this idea years later, they had little trouble.
Gruber calls such mental representations of stick use "cultural ideas".
If they really exist, he says, then behaviour differences among apes are closer to human cultural differences, which also often stem from ideas.
Primatologist Dorothy Fragaszy of the University of Georgia in Athens says orangutans might have ideas of the sort that Gruber describes, but that's not the most plausible explanation.
Fragaszy cautions that Gruber cites only one study that discusses orangutans developing stick skills in the wild - and she says that work isn't conclusive about when orangutans begin experimenting.
"I would say [the orphans] were somewhere along the normal process of learning about [tool use], which involves watching and trying," when they left the wild.
"They had enough practice," she adds, "that they [could] do it later, in this simpler situation."
For Watts, it's the site that Gruber chose that stands out.
He credits Gruber with making a "good logical case" for social learning by running experiments "partway between" lab work and fieldwork.
He and Fragaszy both say that Gruber's study spotlights a valuable, if regrettable, new type of place to conduct research.
Gruber fears it may one day be the only place to do such research in northern Sumatra. Fires are burning in the forests where the tool-using apes live, he says.
"The loss of their habitat," he adds, "probably also means loss of their culture."
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