Editor's Note: This story was published on Aug. 13, 2014. Koko died in her sleep at the foundation's preserve in California's Santa Cruz mountains on Tuesday, June 19, 2018.

Robin Williams brought joy to many — even a gorilla. The actor apparently formed such a special bond with one gorilla that she is said to be mourning the loss of her friend.

In 2001, Williams visited the Gorilla Foundation in Northern California, where he met Koko, a gorilla who is fluent in American sign language, TODAY reported. During his visit, the two shared laughs and tickled each other. “Robin’s ability to just ‘hang out’ with Koko, a gorilla, and in minutes became one of her closest friends, was extraordinary and unforgettable,” Koko’s mentor and surrogate mother, Penny Patterson said.

Monday, the primate overheard Dr. Patterson, talking on the phone about Williams’ death, and “became extremely sad” Patterson wrote on koko.org. In response to the actor’s passing, Koko was seen with her head bowed down, acting very somber. About half an hour later, Koko signed to Patterson, "cry lip," ("lip" is Koko's sign for woman).

The Gorilla Foundation

Williams raved about his transformative experience with Koko in a video for the foundation. “I recently had a mind altering experience communicating with a gorilla,” he said. “We shared something extraordinary: Laughter. It was awesome and unforgettable.”

The ape was so comfortable with Williams that she pulled his glasses off his face and tried them on. She also rifled through his wallet after picking his pocket.

Photos taken of the two friends show Koko smiling. According to koko.org, she had not smiled in months since her childhood gorilla companion, Michael, passed away. Before Williams left Koko, she kissed his hand and pulled him in for a big hug.