An orangutan has surprised her keepers by learning to whistle.

Bonnie, is the first orangutan ever documented making the sound, according to the National Zoo in Washington DC.

The 32-year-old’s keeper, Erin Stromberg, said Bonnie's new-found skill has implications on ideas about the evolution of speech.

He said: “I think what makes it significant is that you can train apes to whistle, but no one trained her to do it. She decided to do it on her own,"

The zoo said that Bonnie taught herself by listening zoo keepers who whistle while they work. She is also thought to have taught another orangutan called Indah to whistle too.

Mr Stromberg helped researchers study her behaviour for a paper published in Primates by whistling basic patterns to see whether Bonnie could copy them.

They found that her behaviour disproves the argument that orangutans have no control over their vocalisations and their sounds are purely involuntary responses to stimuli such as predators.

Mr Stromberg said: “I think what makes it significant is that she decided to do it on her own. Something made her want to whistle, or at least try it out. And so to me, she was challenging herself to do something else."