The foundation said the western lowland gorilla, who developed an "Extraordinary mastery" of signing, died in her sleep aged 46. The Gorilla Foundation, a nonprofit aimed at protecting the species and their habitats, said in a statement that Koko will be remembered as "as the primary ambassador for her endangered species".
Koko was born on the Fourth of July, 1971, at the San Francisco Zoo; her full name, Hanabi-Ko, is Japanese for "fireworks child".
Born at the San Francisco Zoo, the almost 300-pound gentle ape began learning sign language as a baby in 1974 by Dr. Francine "Penny" Patterson as part of a Stanford University project.
Koko was believed to have had an IQ of between 75 and 95 and could sign more than 1,000 words.
Most gorillas like Koko live in the wild for 30 to 40 years; in the care of humans, they can live as long as into their 50s, according to the Smithsonian's National Zoo & Conservation Biology Institute.More news: Iran player fails on front-flip throw-in try
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The foundation said Koko opened the minds and hearts of millions as an icon for interspecies communication and empathy. "She was beloved and wil be deeply missed".
"It was awesome and unforgettable", said the actor, who killed himself in August 2014. She also guested on "Mister Rogers' Neighborhood", an encounter which was excerpted in the current documentary "Won't You Be My Neighbor?" She chose the gray-and-white kitten from a litter for her birthday in 1984, according to a 1985 Los Angeles Times article.
She quickly became famous after was taught sign language by Dr. Francine "Penny" Patterosn.
The Gorilla Foundation, which took care of Koko, also raised a number of other intelligent gorillas that were also able to use sign language.