Primates, in particular great apes, are candidates for being able to experience empathy and theory of mind. Great apes have complex social systems; young apes and their mothers have strong bonds of attachment and when a baby chimpanzee or gorilla dies, the mother will not uncommonly carry the body around for several days. Jane Goodall has described chimpanzees as exhibiting mournful behavior. Koko, a gorilla trained to use sign language, was reported to have expressed vocalizations indicating sadness after the death of her pet cat, All Ball.