Koko, the beloved gorilla who was able to communicate in more than 1,000 signs, has died at 46 in California's Santa Cruz mountains.

The Gorilla Foundation said the 46-year-old western lowland gorilla died in her sleep at the foundation's preserve on Tuesday.

Koko was born at the San Francisco Zoo, and Dr Francine Patterson began teaching the gorilla sign language that became part of a Stanford University project in 1974.

According to Dr Patterson, Koko was able to understand more than 1,000 signs.

The foundation said Koko's capacity for language and empathy opened the minds and hearts of millions.

"Koko touched the lives of millions as an ambassador for all gorillas and an icon for interspecies communication," the Gorilla Foundation said in a statement.

"She was beloved and will be deeply missed."

Koko appeared in many documentaries and twice in National Geographic.

The magazine's 1978 cover featured a photo that Koko had taken of herself in a mirror.

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For her 44th birthday, the gorilla chose a grey kitten and a black-striped kitten — Ms Grey and Ms Black — to join her family, signing the words "cat" and "baby".

The foundation said it would honour Koko's legacy with a sign language application featuring Koko for the benefit of gorillas and children, as well as other projects.

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ABC/AP