Like humans, orangutans have behavioral 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 6 or 7, 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 behavior 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.