Miles, H. Lyn White. The cognitive foundations for reference in a signing orangutan. in Parker, Sue Taylor (Ed); Gibson, Kathleen Rita (Ed). (1990). "Language" and intelligence in monkeys and apes: Comparative developmental perspectives, (pp. 511-539). New York, NY, US: Cambridge University Press
columbia.edu
Ann James Premack and David Premack (1972) "Teaching Language to an Ape"Scientific American. Vol. 227 (4), pp. 92-99.
Savage-Rumbaugh, E. S. et al (1985) "Language learning in two species of apes"Neuroscience & Biobehavioral Reviews. Vol. 9 (4), pp. 653-665; doi:10.1016/0149-7634(85)90012-0