Viki was the subject of one of the first experiments in ape language. Viki was raised by Keith and Catherine Hayes in the same manner as a humaninfant, to see if she could learn human words. She was given speech therapy, which involved the Hayeses (her adoptees) manipulating her lower jaw. Eventually, she was able to voice four words:
This extremely limited success was at first interpreted as evidence that apes were not capable of using human language. However, further experiments in which chimpanzees were instructed in the use of American sign language indicated that Viki's achievements had been significantly hampered by physiological limitations—chimpanzees are not able to produce the sounds that make up human speech.
Catherine Hayes (1951), The Ape in Our House. New York: Harper.
K.J. Hayes and C Hayes, C (1952). "Imitation in a home-raised chimpanzee". Journal of Comparative and Physiological Psychology, 45, 450-459.
K.J. Hayes and C.H. Nissen. (1971). "Higher mental functions of a home-raised chimpanzee". In Schrier, A.M. and Stollnitz, F. (eds). Behaviour of Non-human Primates, 4,50-115. New York, Academic Press.