Published on Wednesday in Scientific Reports, a Nature publication, researchers from Britain, Germany and the United States provided evidence that orangutans are able to learn sounds from humans, then reproduce them at will. It had yet to be conclusively demonstrated that a great ape could control the pitch and volume of its vocalizations — until Wednesday.

Though it’s understood generally that apes can communicate, some believe spoken language is a uniquely Homo sapiens trait, said co-author Shumaker. Adriano Lameira of the University of Durham in Britain led the study, with independent researcher Madeleine Hardus, Alexander Mielke of Germany’s Max Planck Institute for Evolutionary Anthropology and Serge Wich of Liverpool John Moores University.

“There are people, even academics, who will assert that great apes have little to no control over their vocalizations,” Shumaker said. These people believe that a chimp, gorilla, bonobo or orangutan can only produce sounds in response to stimuli, similar to how we might yelp if a mischief-maker unexpectedly pokes us with a needle or how we weep at a sad event. Those reactions are not controllable.

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Rocky, who lives in the Indianapolis Zoo, has indicated that great apes could go beyond uncontrollable reactions. Just as humans can chose when and how to use their vocal cords, Rocky was able to choose when he made his vocalizations. Shumaker noted in the video that Rocky could imitate his trainer’s pitch and volume and reply immediately to her in an almost conversational banter.

“It’s abundantly clear that, for orangutans, it is possible to very carefully and directly control those vocalizations in terms of volume and pitch,” Shumaker said.

Shumaker noted that this shouldn’t be equated with the words and communication of humans. Rocky’s grunts have no deeper meaning — other than that, Shumaker believes, Rocky originally learned these sounds to get human attention.

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Myriad studies dating from the late 20th century indicate that apes do understand language, including users of sign language, such as the chimpanzee, Washoe, the orangutan, Chantek, and the gorilla, Michael. The chimpanzee, Sarah, used plastic symbols to communicate concepts.

The learning aspect of Rocky’s grunts are another crucial component. It’s something he likely learned from humans, rather than an organic sound that all orangutans make. Researchers determined this by comparing his sounds with what they call “the largest database ever assembled of orangutan calls,” which comprises more than 12,000 hours of observing 120 orangutans both wild and captive. Rocky’s sounds were confirmed to have an entirely different frequency range.

“We’ve demonstrated that apes are able to learn a new vocalization that is unknown in their species,” Shumaker said. “We don’t know how he learned it, but he either innovated it on his own or was able to reproduce it, then learn it. There’s no doubt now that apes are able to acquire or learn new vocalization.”

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Shumaker attributes this interesting finding to how he and his co-authors, who conducted a similar study with two female orangutans to show they can create whistles by choice, work with the apes. “Because we think of them as partners, I think they are much more likely to engage with us and open these capabilities and show them off,” he said. “We are in no [way] creating abilities, we are simply creating an environment in which they are comfortable revealing those abilities to us.”