Koko was born 46 years ago at the San Francisco Zoo. She was taught sign language by Dr. Francine Patterson starting in 1974, The Associated Press reported.
She had a vocabulary of about 1,000 words of American Sign Language, National Geographic reported.
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Koko's ability to communicate and the empathy she showed made the gorilla a national treasure. She was featured on National Geographic's cover twice, including one from 1978 that featured a photo that the animal took of herself in a mirror.
She had learned how to use a camera after watching a photographer and imitating him. At the end of the National Geographic photoshoot, she signed "love camera," according to National Geographic.
The second cover, from 1985, showed the large western lowland gorilla cradling a kitten. Koko had asked for a kitten for Christmas in 1984. Her caretakers gave her a stuffed animal, but that didn't work for the gorilla who told her human family that she was sad that she didn't get a real cat, The Los Angeles Times reported in 1985. So on her birthday, she was allowed to chose one and became famous being a gorilla who had a kitten as a pet. Researchers said that Koko would cradle and pet the small cat, while the feline would treat Koko like a human. Like any cat, she was independent, and would bite, or get loose when she had had enough love from her gorilla caretaker. When that happened, Koko would sign "obnoxious cat." But when the kitten, whom Koko named All Ball, wanted affection, Koko would sign,"soft, good cat." Koko and All Ball's family came to an end, though, when the cat wandered into a road and was hit by a car. Koko signed about death after the kitten was killed and cried, mourning her pet, the Times reported.
That wasn't the only time that Koko showed that she understood death and would mourn the passing of a friend, the "Today" show reported. When she was told of Robin Williams's death in 2014, she became sad, Patterson wrote on Koko.org. Williams and Koko became friends after they met in 2001.
The Gorilla Foundation said it will keep Koko's memory alive by continuing its conservation efforts in Africa, operating a great ape sanctuary on Maui and developing a sign language app that features Koko that will help gorillas and children learning to sign.
Koko died in her sleep at the age of 46, the AP reported.