The western lowland gorilla appeared in many documentaries showing her mastery of 1,000 words of sign language. On hearing of her death, fans took to social media to describe the impact she had on humanity.
Followers of Koko, the gorilla who captured the hearts of millions by mastering sign language, mourned her passing on Thursday at the California-based Gorilla Foundation where she lived.
Born Hanabi-ko (Japanese for "Fireworks Child") at San Francisco Zoo in 1973, Koko became one of the most famous research subjects into how apes use language.
She reputedly learned more than 1,000 words in American Sign Language, and became the subject of many books and television shows.
Icon for interspecies understanding
"Koko touched the lives of millions as an ambassador for all gorillas and an icon for interspecies communication and empathy," the foundation said in a statement on its website.
Her impact is "profound" and "what she has taught us about the emotional capacity of gorillas and their cognitive abilities will continue to shape the world," the statement went on.
Within a few hours, news of Koko's death had been shared more than 25,000 times on the foundation's Facebook page, where hundreds of fans sent their condolences.
"So intelligent and sweet," wrote Kim Donohue, from Washington state. "She really meant a lot to me but I am grateful she passed easily in her sleep."
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Marion Burford, from Shropshire in the UK, wrote: "I watched several documentaries about her and thought what a wonderful example of love and compassion she was to the human race."
Another fan, Kimberley Morton, from the US state of Kentucky posted: "What a role she played in our humanity. She taught us that gorillas are gentle, compassionate and extremely intelligent."
A fan in Robin Williams
Among her many media appearances, perhaps most famous was her introduction to the late actor Robin Williams in 2001. The pair were filmed tickling each other, and in one shot, Koko playfully tried on Williams' glasses.
The star of Mrs. Doubtfire described their interaction as "mind-altering," while other actors including Oscar-winning Leonardo DiCaprio and Star Trek's William Shatner were similarly impressed upon meeting her.
Animal psychologist Penny Patterson began teaching sign language to Koko when the gorilla was just a year old. The project later moved to Stanford University.
Koko was among a handful of primates who could use sign language, along with Washoe, a female chimpanzee, and Chantek, a male orangutan. Her keepers said she understood some spoken English, too.
First online gorilla chat
In 1998, Koko took part in the first "interspecies" chat, relaying comments such as "I like drinks" via a human interpreter to tens of thousands of online participants.
In 2004, Koko used sign language to describe a pain in her mouth, and used a pain scale of 1 to 10 to show how badly it hurt, according to a story by the Associated Press.
Despite these efforts, some critics argued that Koko's signs were not spontaneous but prompted by Patterson, and questioned the extent to which apes use language in the same way as humans.
The foundation says it plans to publish a sign language mobile app featuring Koko for the benefit of gorillas and children.
mm/rc (AFP, AP, Reuters)