The next step is to study how flexible sound-learning is in apes and whether they can adjust their sounds—pitch and intonation, for instance—depending on the context, Wich said.
"Those things are very important because they give us clues to understanding the evolution of human speech," he said.
(Related: "Animal 'Speech' Project Aims to Decode Critter Communication" [September 26, 2006].)
Although Bonnie appears to have learned to whistle just for the sake of whistling—not in direct imitation to communicate or get attention—she was willing to imitate zookeeper Stromberg's whistles for the study.
"Bonnie made a short whistle after a short whistle and a long whistle after a long whistle," Wich said.
W. Tecumseh Fitch of the University of St. Andrews in Scotland said the research shows that orangutans have better control over respiration than previously suspected.
Fitch, who specializes in the evolution of speech but was not involved with this study, said the research "provides further verification of an old idea: that apes have complex, voluntary control over the mouth, lips, and tongue, just like us.
"What is lacking is control over the larynx," the part of the throat containing the vocal cords.
Charles Snowdon of the University of Wisconsin noted that Bonnie's whistles are not as complex as the imitations of some birds and even dolphins.
"There has been lots of controversy over whether non-human primates can learn vocalizations or can modify vocalizations," said Snowdon, who studies acoustic communication and its development but was not involved in this study.
"Until now there has been little evidence of direct imitation of vocalizations by a primate," he said. "The really interesting question is why it is so difficult to find [more] good evidence of vocal imitation."
Lead author Wich noted that another captive orangutan that used to live with Bonnie but has since passed away apparently learned to whistle from Bonnie, according to zookeepers.
If orangutans can learn new sounds from each other, this could explain variations in the sounds made by different populations of orangutans in the wild.
Animal keeper Stromberg said it makes sense that this orangutan would have the skills and ability to pick up human sounds.
"Bonnie's very intuitive," Stromberg said. "She's very observant, and she always watches people and what they do."
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