She communicates in sign language, using a vocabulary of over 1000 words. She also understands spoken English, and often carries on ‘bilingual’ conversations, responding in sign to questions asked in English. She is learning the letters of the alphabet, and can read some printed words, including her own name. She has achieved scores between 85 and 95 on the Stanford-Binet Intelligence Test.

She demonstrates a clear self-awareness by engaging in self-directed behaviours in front of a mirror, such as making faces or examining her teeth, and by her appropriate use of self-descriptive language. She lies to avoid the consequences of her own misbehaviour, and anticipates others’ responses to her actions She engages in imaginary play, both alone and with others. She has produced paintings and drawings which are representational. She remembers and can talk about past events in her life. She understands and has used appropriately time-related words like ‘before’, ‘after’, ‘later’, and ‘yesterday’.
She laughs at her own jokes and those of others. She cries when hurt or left alone, screams when frightened or angered. She talks about her feelings, using words like ‘happy’, ‘sad’, ‘afraid’, ‘enjoy’, ‘eager’, ‘frustrate’, ‘made’ and, quite frequently, ‘love’. She grieves for those she has lost—a favourite cat who has died, a friend who has gone away. She can talk about what happens when one dies, but she becomes fidgety and uncomfortable when asked to discuss her own death or the death of her companions. She displays a wonderful gentleness with kittens and other small animals. She has even expresses empathy for others seen only in pictures.

Many people react with scepticism to such descriptions of a non human animal, but the abilities of the gorilla Koko described here are broadly similar to those reported quite independently by observers of other great apes, including chimpanzees and orang-utans.

Peter Singer, Rethinking Life and Death: The Collapse of Our Traditional Ethics, New York, 1994, pp. 181-182