Koko was born at the San Francisco Zoo and moved to the Santa Cruz Mountains eight years later, along with The Gorilla Foundation.
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Koko was featured in multiple documentaries and appeared on the cover of National Geographic twice. She touched the lives of millions as an ambassador for all gorillas.
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She was an icon for inter-species communication and empathy.
The Gorilla Foundation says Koko shared a special bond with Robin Williams. The two met for a video shoot in 2001.
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Koko's handlers said at that time, she hadn't smiled in six months since her lifelong partner passed away. Williams made her smile.
The two joked and wrestled, even got into a tickling fight.
The foundation said Koko expressed sadness after her handlers explained Williams had died in 2014.