In the video, starting at about the 20-second spot, the orangutan mother uses a hand to cover her mouth. This is what de Boers and his team noticed.
Analyzing the manipulated sounds further, the researchers determined that the orangutan was using her hand to lower the sound's pitch and make it deeper. Humans can do this too by speaking in a deeper than normal voice while cupping one or both hands over the mouth.
For a wild primate to accomplish this is no small feat.
Orangutans Use Charade-like Communication
"The problem is that the orangutan is not sitting still on a branch making its noises," de Boer reminded. ‘There are cicadas singing in the background, rustling leaves-all kinds of horrible stuff going on."
Even with all of these sounds and interruptions, the orangutan's manipulated calls surprisingly match those of much larger, formidable primates, the researchers discovered. The calls were also amplified, thanks to the hand manipulation.
Other studies on orangutans have found that some can whistle tunes, use their tongues to produce different clicking tones and produce sounds that resemble our consonants. So studying orangutans could help shed light on the emergence and evolution of human speech.
Photo: An orangutan manipulating its calls by cupping a hand over its mouth. Credit: Madeleine Hardus