Koko, the famous sign-language-using gorilla, has become somber since news of his death came into The Gorilla Foundation in northern California, according to the foundation's website, koko.org.

At first, Dr. Penny Patterson said that Koko came with an "inquiring look" and then she explained to the gorilla that Williams had died.

As more calls came in, Koko, Patterson said, signed to her "CRY LIP" (lip is Koko's "word" for woman).

By the end of the day, Koko held her head bowed and her lip was quivering.

Koko and The Gorilla Foundation express our condolences for the loss of our special friend, Robin Williams — to... http://t.co/wx1kVlPrOc

In 2001, Williams met Koko after her lifelong gorilla companion died.  She had not smiled in over 6 months, until Williams came in.  Officials with the foundation said that not only did Koko cheer up, but she had transformed him from his usual manic self to a "mellow, sensitive, empathetic guy."