Scientists were surprised to learn that Bonnie the orangutan, shown here, can whistle. Turns out she's not the only one, it seems.

When the mood strikes her, Bonnie whistles. She's not very good at it — she utters only single notes and can't carry a tune. But don't judge her too harshly; as an orangutan, she's the first nonhuman primate ever documented to whistle, or to spontaneously mimic the sound of another species. Now thirty years old, Bonnie lives at the Smithsonian's National Zoological Park in Washington, D.C. In the 1980s, she probably heard a happy caretaker whistling, and she soon made whistles of her own, seemingly just for the fun of it. Recently, a team of primatologists, led by Serge A. Wich of the Great Ape Trust of Iowa in Des Moines, took a closer look at Bonnie's abilities.

By comparing recordings, they confirmed that the sounds she makes are nothing like normal orangutan sounds or vocalizations, and that her whistling tends to be imitative. For example, she usually replicates the duration and number of whistles (one or two) that caretakers produce in front of her. Other orangutans and chimpanzees known to produce unusual sounds have typically received extensive training — yet Bonnie isn't alone in her spontaneous whistling. Another National Zoo orangutan named Indah also took up the habit, but died before she was recorded. And Wich says that since publishing, he's heard from workers at other zoos with whistling orangutans in their care.

The research was detailed in the journal Primates.