Koko (born July 4, 1971) is a female gorilla who, according to
Francine "Penny" Patterson, is able to understand more than 1,000
signs based on American Sign Language and understand approximately
2,000 words of spoken English.
The bonobo (a type of ape), Kanzi, who learned to speak using a
keyboard with lexigrams, picked up some sign language from watching
videos of Koko; Kanzi's researcher, Sue Savage-Rumbaugh, did not
realize he could sign until Kanzi began signing to anthropologist
Dawn Prince-Hughes, who had previously worked closely with
gorillas.
Washoe (c. September 1965 Ellensburg - October 30, 2007) was a
female chimpanzee who was the first non-human to learn to
communicate using American Sign Language, as part of a research
experiment on animal language acquisition.
A few other gorillas and chimpanzees have supposedly learned
sign language.
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Of course, they can have no fingers, but dolphins and some
whales have been taught to respond to sign language as well.