Language in Apes:
How Much Do They Know and How Much Should We Teach Them

If you find yourself flummoxed by the terminology, you can look at my primate glossary.


Humans have often asserted a fundamental difference between themselves and other animals. One of these assertions which has had many proponents into the twentieth century is that humans differ from animals in their use of language. In the past thirty years this assertion has been the subject of much debate as scientists have researched language use by apes. (I use the term "ape" to refer to "great ape" in this essay, as many of my sources do. There have apparently been no language experiments with gibbons or siamangs.) Extraordinary claims have been made by some researchers about the linguistic capabilities of their subjects, mostly chimpanzees. These claims have been refuted and counter-refuted many times, and the literature on the subject is extensive. In this essay I will examine the question of how much, if at all, primates are able to communicate using language. I will then examine the ethical issues surrounding the teaching of language to apes.

What is language?

First, what is language, and how does it differ from other forms of communication? There does not exist a universally accepted definition of language, or criteria for its use; this is one of the reasons for the disagreement among scientists about whether apes can use language. Language consists of various aspects which people believe are more or less important, for example, grammar, symbol usage, the ability to represent real-world situations, and the ability to articulate something new (Wallman 1992: 6). Duane Rumbaugh describes language as "an infinitely open system of communication" (Rumbaugh 1977b: xx). Some people say that anything an ape can do is not language; of course, if these are the same people who say that language defines us as humans, and an ape can learn sign language, then they are saying that deaf people who use sign language are not human (Patterson & Linden 1981: 119-120). One famous view of language is Charles Hockett's seven key properties: duality, productivity, arbitrariness, interchangeability, specialization, displacement, and cultural transmission (Linden 1974: 137). I will return to these properties later in the essay.

Why teach language to apes?

Why should we try to teach language to apes? In their introduction to Language in Primates, the editors answer the question: "This project would shed light not only on the nature of language and cognitive and intellectual capacities, but also on such issues as the uniqueness of human language and thought" (de Luce & Wilder 1983: 1). Such projects also shed light on the development of language in early humans. A very different reason for teaching language to apes is that the research would discover better methods for training mentally retarded children "who for various reasons fail to develop expressive linguistic skills during their early years" (Rumbaugh, Warner & von Glasersfeld 1977: 90).

How apes communicate in the wild

In the wild, primates use a wide variety of methods of communication (Jolly 1985: 192-217). Many primates rely on olfactory communication, for example, scent marking or urine washing to mark their territory. Primates use tactile communication to develop or confirm relationships; mothers carry their young, adults may sit and/or sleep together, and adults of many species groom each other. Visual communication is important especially for higher primates, who look at what they are paying attention to, as we do. Some visual elements are facial expression, hair erection, general posture, and tail position. Primates use vocal communication, from soft grunts to whoop-gobbles, when they want to catch the attention of others. Often vocalizations signal emotional situations, like danger of an attack or the location of a large food source. The meaning of primate communication depends on the social and environmental context as well as the particular signals that are being used (Strum 1987: 263). Sue Savage-Rumbaugh notes that reports of the complexity and intentionality of chimpanzee communication in the wild have not yet had the recognition they deserve (Savage-Rumbaugh 1986: 400). The monkeys called vervets have the most sophisticated animal communication that we know of; the sounds they use are learned, not instictive (Diamond 1993:143). There are various studies of primates in the wild going on right now, but it is difficult to do an in-depth study of communication in apes.

History of the apes & language question

The question of whether apes can use language has been asked for some time. In 1661 Samuel Pepys wrote in his diary about what he called a "baboone": "I do believe it already understands much english; and I am of the mind it might be taught to speak or make signs" (Wallman 1992: 11). In 1748 Julien Offray de la Mettrie published a document which speculated that apes could be taught to speak (Hewes 1977: 12). During the first half of the twentieth century, the first experimental forays into the area of ape language were, in fact, attempts to teach apes to speak. Robert Yerkes experimented with chimpanzees in the 1920's, and concluded that they could not learn speech. He made a suggestion that was not followed up on for forty years: perhaps apes could learn sign language (Rumbaugh 1977a: 76). Others continued to attempt to teach speech to apes, the most successful of whom were Keith and Cathy Hayes who taught chimpanzee Viki to speak four words (Gardner & Gardner 1989: 5). Experiments in which chimpanzees were raised as children were successful in other respects: the chimpanzees learned to understand a great deal of human speech, and they often communicated with their "family" to some extent by gestures.

Why apes cannot speak

There are several reasons cited why apes cannot speak. One is that apes are just not intelligent enough; this reason is losing popularity as ape language research continues to discover new facets of ape intelligence. Robert Yerkes believed that apes cannot speak because they lack "the tendency to reinstate auditory stimuli--in other words to imitate sounds" (Rumbaugh 1977a: 77). The reason that was commonly cited by scientists in the latter half of the twentieth century is that apes' vocal chords (or some other part of their anatomy) are not built for speaking (de Luce & Wilder 1983: 3). A relatively recent suggestion is that the vocal habits of apes prevent them from speaking. When apes use vocal communication, they are almost always very excited, perhaps "too excited to engage in casual conversation" (Gardner, Gardner & Drumm 1989: 29). Whatever the reason, once it was apparent that apes could not learn to speak, the apes' propensity for using gestures made sign language the next obvious choice.


Allen and Beatrice Gardner began teaching sign language to an infant chimpanzee named Washoe in 1966. The Gardners provided Washoe with a friendly environment that they thought would be more conducive to learning. The people who cared for and taught Washoe used sign language almost exclusively in her presence. Washoe learned signs by various methods, including imitation and instrumental conditioning. Washoe was able to transfer her signs spontaneously to a new member of a class of referents; for example, she used the word "more" in a wide variety of contexts (not just for more tickling, which was the first referent) (Gardner & Gardner 1979: 190). The Gardners noted that "Washoe has transferred the DOG sign to the sound of barking by an unseen dog" (191). They also reported that Washoe began to use combinations of signs spontaneously after learning only about eight or ten of them. The Gardners soon extended their experiments to several other chimpanzees: Moja, Pili, Tatu, and Dar. They needed to replicate their success with Washoe, and they did. All of these chimpanzees "signed to friends and to strangers. They signed to each other and to themselves, to dogs and to cats, toys, tools, even to trees" (Gardner & Gardner 1989: 24). Private signing by the chimpanzees has recently been studied systematically; the study confirmed that private signing is robust (Bodamer, Fouts, Fouts & Jensvold 1994). One of the most remarkable developments in this research occurred when Washoe adopted an infant named Loulis. For the next five years, no sign language was used by humans in Loulis' presence; however, Loulis still managed to learn over 50 signs from the other chimpanzees. Bob Ingersoll, who studied Washoe and Loulis during this time, believes that there wasn't much active teaching going on, but rather Loulis picked up the signs from the other apes' use of them. The learning of signs from other chimpanzees meets Hockett's criterium of cultural transmission. Because the chimpanzees continued to use sign language without any input from humans, the Gardners concluded that "once introduced, sign language is robust and self-supporting, unlike the systems that depend on special apparatuses such as the Rumbaugh keyboards or the Premack plastic tokens" (Gardner & Gardner 1989: 25).


In the year after Project Washoe began, David Premack started an experiment with a different kind of language. The above-mentioned plastic tokens are those which Premack used to train a chimpanzee named Sarah. These tokens represented words, and varied in shape, size, texture, and colour. Sentences were formed by placing the tokens in a vertical line (an orientation which Sarah favoured). This language differs from sign language in that "the permanence of the sentence not only makes it possible to study language without a memory problem, but to study memory in the context of language by regulating the duration for which the sentence remains on the board" (D. Premack 1979: 233). Sarah was taught nouns, verbs, adjectives, pronouns, and quantifiers; she was also taught same-difference, negation, and compound sentences. The earliest words named "various interesting fruits, so that Sarah ... could both solve her problem and eat it" (A. Premack 1976: 79). Sarah exhibited displacement, the ability to think of something (in the following case, chocolate) when it is not immediately present. Presented with the sentence "Brown color of chocolate" without any chocolate present, and later presented with "Take brown," Sarah took a brown object (A. Premack 1976: 89). When a trainer put a question on Sarah's board and walked away, Sarah showed little interest in answering it--"in somewhat the way a conversation falters when one person ceases to pay attention to the other" (D. Premack 1971: 821). To show that Sarah was not merely responding to cues from her human trainers, she was adapted to a new trainer that did not know her language. When this trainer presented her with questions, she gave the correct answers less frequently than usual, but still well above chance. Ann Premack remarked that "it would be interesting to see how well a child at this language stage of about 150 words would do in a simple language test with a virtual stranger" (A. Premack 1976: 103). To test Sarah's view of words, Premack presented her with an apple and a set of features (for example, round vs. square and red vs. green). Then she was presented with her word for apple, and the same set of features. She choose the correct features for both the real apple and her word for apple, a light-blue plastic triangle (A. Premack 1976: 104). This demonstrates Hockett's property of arbitrariness; the symbol for apple is arbitrary (that is, there is no similarity between an apple and a light-blue plastic triangle).


The chimpanzee Lana learned to use another language system, an electronic keyboard. The Lana Project was headed by Duane Rumbaugh, who wanted to create a situation which would "allow for the systematic variations of training procedures that would differentially influence the course of acquiring and using linguistic skills" (Rumbaugh, Warner & von Glasersfeld 1977: 87). The language of lexigrams, each of which represented one word, was called Yerkish. When Lana pressed a key with a lexigram on it, that key would light up and the lexigram would appear on a projector. When keys were pressed accidentally, Lana used the PERIOD key (end of sentence) as an eraser so that she could restart the sentence; Lana did this on her own before it occurred to the researchers (Rumbaugh & Gill 1977: 167). Lana also started using NO as a protest (for example, when someone else was drinking a Coke and she did not have one) after having learned it as a negation ("it is not true that...") (Rumbaugh & Gill 1977: 169-170). Lana acquired "many linguistic-type skills for which she had received no specific training" which showed her ability to abstract and generalize (Rumbaugh & Gill 1977: 190). For example, she spontaneously used THIS to refer to things for which she had no name, and she invented names for things by combining lexigrams in new ways. Lana's trainers admit that

we cannot assume that Lana 'understands' the meaning of every word she uses as we do, but the consistent appropriateness of her novel sentence constructions ... support the conclusion that she has conceptual meanings for many of them and also for their relationships (Rumbaugh & Gill 1977: 192).


Herbert S. Terrace was skeptical of the reported success of the chimpanzees Washoe, Sarah, and Lana. He believed that there were simpler explanations for many of the reported interpretations of these apes' language use. Although Terrace admitted that the apes had achieved something significant, he compared their behaviour to that of pigeons who are taught to peck different colours in a certain order (Terrace 1979: 20). He also believed that the apes used signs only to receive rewards from their human trainers. When Terrace set up his own experiment with the chimpanzee Nim, "Nim's main reward for learning to sign was our approval and being able to sign about something that was important to him" (Terrace 1979: 145). Nim was raised like a human child and taught sign language in similar ways to those of Washoe. He was observed practicing his signs in the absence of their referents (Terrace 1979: 143). Nim often signed DIRTY (used when he had to go to the toilet) or SLEEP when he was bored and wanted a change. He used the signs BITE and ANGRY to express his feelings, and he tended not to attack if he perceived that his warning was heeded; this is an important substitution of an arbitrary word for a physical action, displaying Hockett's property of specialization (the speaker does not act out messages). Although Nim learned many words, Terrace concluded that Nim could not combine words to create new meanings on his own. He believed, from viewing videotapes, that the combinations of words that Nim used were prompted by prior utterances from his trainers. The other thing Terrace discovered from the videotapes was that Nim interrupted his trainers more than human children interrupt their parents. Terrace has a good point: if we are going to say that apes can create a sentence, we must eliminate the other possible explanations for the utterances (Terrace 1979: 219). Terrace is not as much of a skeptic as some others make him out to be, though; he believed that the conditions under which Project Nim were operated were not ideal, and future projects might have more success if they were able to instill a greater motivation to sign in their subjects (Terrace 1979: 223).

Other Ape Language Experiments: Sherman & Austin, Chantek, Kanzi, Koko

Many other ape language experiments were done other than the four described above; I will briefly list several of these. Sherman and Austin were two chimpanzees who were able to communicate specific information to each other through the use of symbols, information that they could not communicate without the symbols (Savage-Rumbaugh, Rumbaugh & Boysen: 1978). Chantek, an orangutan, learned about 150 different signs, and used them spontaneously and without undue repetition. Chantek internalized a minimal value system, using signs for GOOD and BAD in appropriate contexts (Miles 1993: 47, 52). A bonobo named Kanzi learned at a faster rate than the chimpanzees; he learned his first words by merely watching lessons directed at his mother. Sue Savage-Rumbaugh describes Kanzi as "the ape at the brink of the human mind" in her book of that title. Savage-Rumbaugh asserts that Kanzi uses sentences; that is, he follows structured rules in his multi-word utterances (showing the property of duality). He even makes up his own rules, such as first using a lexigram to specify an action and then using a gesture to specify an agent (Savage-Rumbaugh & Lewin: 1994: 161). Francine Patterson has been raising Koko, a gorilla, since 1972 and teaching her sign language. Koko has learned a greater vocabulary than Nim, uses more words per utterance on average, and "a great deal of creativity, spontaneity, and structure characterize her utterances" (Patterson & Linden 1981:116). Koko also rhymes and jokes; on one occasion she used a metaphor of an elephant to refer to herself when she pretended a long tube was her "trunk" (Patterson & Linden 1981: 143). These characteristics of Koko's utterances show the property of productivity, in which a speaker says something never heard or said before and is understood by the audience. (It is interesting how one automatically uses the language of speech when describing any form of language use, including those which are not spoken. Many of my sources also exhibit this trait.) The Nova program Can Chimps Talk? did a good job of exploring the various experiments and issues involved in ape language.

Dichotomy in the Scientific Community

Scientists seem to be divided into two camps on the subject of ape language research. In the one camp are the researchers who treat their apes more like children; these people tend to focus on the accomplishments of their subjects, and the similarities between ape and human language. In the other camp are the researchers who treat their apes more like experimental subjects; these people tend to focus on the failures of their subjects, and the differences between ape and human language. Both groups have problems with the research methods of the other. Francine Patterson, Koko's trainer, believes that "one cannot really understand the mental workings of other animals or bring them to the limits of their abilities unless one first has true rapport with them" (Patterson & Linden 1981: 211). Herbert Terrace and others believe that the accomplishments of apes who are taught language are less than what is reported because the apes' utterances are cued by humans. Sue Savage-Rumbaugh characterizes this difference of opinion as sometimes generating "more heat than light"; she believes that people accepted the results of the early ape language experiments too readily, and rejected them too readily after Project Nim (Savage-Rumbaugh 1986: 398, 10). George Johnson summarizes the views of both sides in his article Chimp Talk Debate: Is It Really Language?.

This dichotomy in the scientific community still exists today. It is exemplified by Joel Wallman's recent book, Aping Language, and the responses to it. In his book Wallman states that he does "not believe that any of the ape-language projects succeeded in instilling even a degenerate version of a human language in an ape" (Wallman 1992: 109). Whether the point of the ape language research is specifically to teach apes a human language is questionable. In her review of Aping Language, Patricia Greenfield argues that Wallman exaggerated differences between ape and human language, used human language as a standard to measure ape language by, ignored published results that did not agree with his thesis, and used unscientific evidence to discuss accomplishments by apes (Greenfield 1994: 940-942). In another review, Justin Leiber concludes that "Wallman has written a clear, helpful, even definitive, book" (Leiber 1995: 374). Ironically, it seems that scientists involved in the study of ape language need to communicate better among themselves.

Ethical implications of ape language research

In addition to the disagreements about what apes have learned, there are disagreements about the ethical implications of ape language research. Although not a primatologist, author Douglas Adams has some perceptive comments about "this business of trying to teach apes language" (Adams & Carwardine 1993: 23). While sitting four feet away from a wild silverback mountain gorilla, he asks

Why [try to teach apes language]? There are many members of our own species who live in and with the forest and know it and understand it. We don't listen to them. What is there to suggest we would listen to anything an ape could tell us? Or that he would be able to tell us of his life in a language that hasn't been born of that life?... Maybe it is not that they have yet to gain a language, it is that we have lost one (Adams & Carwardine 1993: 23).

Although it might not seem like it to their trainers (who are biased towards any semblance of "humanity" in apes), apes who are domesticated may be less intelligent than their wild counterparts. De Luce and Wilder point out that "while most investigators have assumed that the primate language experiments develop and enhance the intelligence and linguistic abilities of the apes, just the opposite may be the case" (de Luce & Wilder 18). One problem is that we have no way of communicating with apes unless we teach them some language that humans also know; the only other possibility is them teaching us their code. Roy Harris speculates that "human infants would fare no better at the language game if subjected to comparably bizarre experiments (involving removal from natural habitat, control by members of another species, ...)" (Harris 1984: 204). Perhaps further study of apes in the wild (while there are still some left) would be more profitable.

Are you responsible for what you tame?

Shirley Strum asks the question "are you responsible for what you tame?" and answers in the affirmative (Strum 1987: 199). Apes are cute and cuddly when they are small, but they get rather large and strong as they mature. One problem with apes raised by humans is that it is very difficult to teach them how to survive in the wild. I wonder what their trainers will do when Kanzi and Koko themselves start demanding more rights--we certainly cannot let a bonobo or a gorilla walk around town on its own in today's society.

The best interests of the apes

There is an interesting paradox in ape language research: the more successful the findings are, the better the case for not doing the research at all. If language use gives beings rights and freedoms, we should ask the apes' permission before experimenting on them--of course, it is difficult to see a situation in which an ape would understand the experiment before it was undertaken. Some scientists may claim that it is in the apes' best interests to teach them language, but Thomas Simon points out that we do not even know what is in the best interests of humans when it comes to language (Simon 1983: 106). Simon believes that the use of computer models, human subjects, and field studies are more appropriate than ape language research (Simon 1983: 108).

Basic rights for all great apes

If apes can use language, in some sense, then what is the significance of this? The collaborators of The Great Ape Project declare that apes should be included in a community of equals with humans: each member of this community should have the right to life, liberty, and freedom from torture (Cavalieri, Singer et al. 1993). This declaration may seem radical to many (and not radical enough for a few), but the trend has been for people to agree with more and more rights for animals. The contributors to The Great Ape Project argue from various viewpoints that apes and humans should be classified together at some level. James Rachels argues that Darwinism implies that we should treat the other great apes (we are also great apes) as equals (Rachels 1993). Christoph Anstotz compares the linguistic competence of apes favourably with that of profoundly mentally disabled humans (Anstotz 1993). Ingmar Persson argues that the same basis on which we justify equality among humans can be applied to other species (Persson 1993). In their epilogue, Cavalieri and Singer compare our treatment of nonhuman animals to that of slaves in former times (Cavalieri & Singer 1993). Even disregarding the apes who use language, "the behavior of wild chimpanzees is not so different from that of non-technological groups of humans" (Fouts & Fouts 1993: 39). What all of these people are saying is that there should not be an arbitrary line drawn between humans and other great apes.


What do the apes say? Eugene Linden reported that Viki and Washoe thought of themselves as human; when categorizing humans and animals, they placed themselves with humans and other chimpanzees with animals (Linden 1974: 50). The problem of interpreting apes' signs will always be that we really do not know what they are thinking; of course, we can say the same for humans. The philosopher and mathematician Descartes believed that language separated humans who have souls from animals who do not (de Luce & Wilder 1983: 13). If it is language that makes us human, then surely at some level apes are human too. Many ape language researchers hope, as do I, that this research will lead to a better understanding of the relationships among all animals, including humans, and that we will work harder at giving the other animals their space on this earth.

For further information, here are a few web sites related to this topic.

References Cited

Adams, D. and Carwardine, M. "Meeting a Gorilla," in The Great Ape Project: Equality Beyond Humanity. Ed. P. Cavalieri and P. Singer, pp. 19-23. New York: St. Martin's Press, 1993.

Anstotz, C. "Profoundly Intellectually Disabled Humans and the Great Apes: A Comparison," in The Great Ape Project: Equality Beyond Humanity. Ed. P. Cavalieri and P. Singer, pp. 158-172. New York: St. Martin's Press, 1993.

Bodamer, M.D., Fouts, D.H, Fouts, R.S. and Jensvold, M.L.A. "Functional Analysis of Chimpanzee (Pan troglodytes) Private Signing." Human Evolution, 9:4, 1994. pp. 281-296.

Cavalieri, P., Singer, P. et al. "A Declaration on Great Apes," in The Great Ape Project: Equality Beyond Humanity. Ed. P. Cavalieri and P. Singer, pp. 4-7. New York: St. Martin's Press, 1993.

Cavalieri, P. and Singer, P. "The Great Ape Project--and Beyond," in The Great Ape Project: Equality Beyond Humanity. Ed. P. Cavalieri and P. Singer, pp. 304-312. New York: St. Martin's Press, 1993.

de Luce, J. and Wilder, H.T. "Introduction," in Language in Primates: Perspectives and Implications. Ed. J. de Luce and H.T. Wilder, pp. 1-17. New York: Springer-Verlag, 1983.

Diamond, J. The Third Chimpanzee: The Evolution and Future of the Human Animal. New York: HarperCollins Publishers, 1993.

Fouts, R.S. and Fouts, D.H. "Chimpanzees' Use of Sign Language," in The Great Ape Project: Equality Beyond Humanity. Ed. P. Cavalieri and P. Singer, pp. 28-41. New York: St. Martin's Press, 1993.

Gardner, R.A. and Gardner, B.T. "Teaching Sign Language to a Chimpanzee," in Language Intervention from Ape to Child. Ed. R.L. Schiefelbusch and J.H. Hollis, pp. 171 - 195. Baltimore: University Park Press, 1979.

Gardner, R.A. and Gardner, B.T. "A Cross-Fostering Laboratory," in Teaching Sign Language to Chimpanzees. Ed. R.A. Gardner, B.T. Gardner and T.E. Van Cantfort, pp. 1-28. Albany: State University of New York Press, 1989.

Gardner, R.A., Gardner, B.T. and Drumm, P. "Voiced and Signed Responses of Cross-Fostered Chimpanzees," in Teaching Sign Language to Chimpanzees. Ed. R.A. Gardner, B.T. Gardner and T.E. Van Cantfort, pp. 29-54. Albany: State University of New York Press, 1989.

Greenfield, P. "Book Review: 'Aping Language'." International Journal of Primatology, 15: 6, 1994. pp. 939-943.

Harris, R. "Comment: 'Language in apes' by H.S. Terrace," in The Meaning of Primate Signals. Ed. R. Harre and V. Reynolds, pp. 204-205. Cambridge: Cambridge University Press, 1984.

Hewes, G.W. "Language Origin Theories," in Language Learning by a Chimpanzee: The Lana Project. Ed. D.M. Rumbaugh, pp. 3-53. New York: Academic Press, 1977.

Jolly, A. The Evolution of Primate Behavior. 2nd ed. NewYork: Macmillan, 1985.

Leiber, J. "Apes, Signs, and Syntax." American Anthropologist, 97:2, 1995. p. 374.

Linden, E. Apes, Men, and Language. NewYork: Saturday Review Press, 1974.

Miles, H.L.W. "Language and the Orang-utan: The Old 'Person' of the Forest," in The Great Ape Project: Equality Beyond Humanity. Ed. P. Cavalieri and P. Singer, pp. 42-57. New York: St. Martin's Press, 1993.

Patterson, F., and Linden, E. The Education of Koko. New York: Holt, Rinehart and Winston, 1981.

Persson, I. "A Basis for (Interspecies) Equality," in The Great Ape Project: Equality Beyond Humanity. Ed. P. Cavalieri and P. Singer, pp. 183-193. New York: St. Martin's Press, 1993.

Premack, A.J. Why Chimps Can Read. New York: Harper & Row, 1976.

Premack, D. "Language in Chimpanzee?" Science, 172, 1971. pp. 808-822.

Premack, D. "A Functional Analysis of Language," in Language Intervention from Ape to Child. Ed. R.L. Schiefelbusch and J.H. Hollis, pp. 229-259. Baltimore: University Park Press, 1979.

Rachels, J. "Why Darwinians Should Support Equal Treatment for Other Great Apes," in The Great Ape Project: Equality Beyond Humanity. Ed. P. Cavalieri and P. Singer, pp. 19-23. New York: St. Martin's Press, 1993.

Rumbaugh, D.M. "The Emergence and State of Ape Language Research," in Progress in Ape Research. Ed. G.H. Bourne, pp. 75-83. New York: Academic Press, 1977a.

Rumbaugh, D.M. "Preface," in Language Learning by a Chimpanzee: The Lana Project. Ed. D.M. Rumbaugh, pp. xix-xxii. New York: Academic Press, 1977b.

Rumbaugh, D.M., and Gill, T.V. "Lana's Acquisition of Language Skills," in Language Learning by a Chimpanzee: The Lana Project. Ed. D.M. Rumbaugh, pp. 165-192. New York: Academic Press, 1977.

Rumbaugh, D.M., Warner, H. and von Glasersfeld, E. "The LANA Project: Origins and Tactics," in Language Learning by a Chimpanzee: The Lana Project. Ed. D.M. Rumbaugh, pp. 87-90. New York: Academic Press, 1977.

Savage-Rumbaugh, E.S. Ape Language: From Conditioned Response to Symbol. New York: Columbia University Press, 1986.

Savage-Rumbaugh, E.S., Rumbaugh, D.M., and Boysen, S. "Symbolic Communication Between Two Chimpanzees (Pan troglodytes)." Science, 201, 1978. pp. 641-644.

Savage-Rumbaugh, S. and Lewin, R. Kanzi: The Ape at the Brink of the Human Mind. New York: John Wiley & Sons, 1994.

Simon, T.W. "Limits of Primate Talk," in Language in Primates: Perspectives and Implications. Ed. J. de Luce and H.T. Wilder, pp. 97-111. New York: Springer-Verlag, 1983.

Strum. S.C. Almost Human: A Journey into the World of Baboons. New York: W.W. Norton, 1987.

Terrace, H.S. Nim. New York: Alfred A. Knopf, 1979.

Wallman, J. Aping Language. Cambridge: Cambridge University Press, 1992.

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