The famed gorilla Koko, known for her mastery of sign language, has died in California, a research centre announced on Thursday.
Koko, 46, was born Hanabi-ko - Japanese for "Fireworks Child" - on July 4, 1971, at the San Francisco Zoo. She was a western lowland gorilla.
Koko became the subject of books, television shows and press reports after learning more than 1,000 words in American Sign Language from animal psychologist Penny Patterson.
She passed away on Wednesday morning in her sleep, The Gorilla Foundation announced.
"Koko touched the lives of millions as an ambassador for all gorillas and an icon for interspecies communication and empathy. She was beloved and will be deeply missed," the foundation said.
Patterson started to teach Koko sign language a year after she was born, and with collaborator Ronald Cohn moved her to Stanford in 1974, going on to establish The Gorilla Foundation.
Koko appeared in numerous documentaries and twice appeared on the cover of National Geographic. The first cover, in 1978, was a photograph that Koko had taken of herself in a mirror.
In 1985, the magazine wrote about Koko and her kitten, called All Ball.
Their relationship became the subject of a book, "Koko's Kitten," taught in schools.
"Her impact has been profound and what she has taught us about the emotional capacity of gorillas and their cognitive abilities will continue to shape the world," The Gorilla Foundation said Thursday.
In 2005, two women sacked from jobs caring for Koko sued the foundation for allegedly ordering them to bare their breasts to the gorilla in a bonding effort.
The foundation strenuously denied the allegations and an out-of-court settlement was reached.