How did researchers teach chimpanzees such as Washoe and Sarah to use signs or symbols?

Researchers have employed various methods to teach chimpanzees nonvocal languages, moving away from failed attempts to teach them human speech due to the structural limitations of the ape's vocal tract.1

• Washoe: Taught by Beatrice and Allan Gardner, Washoe was instructed in the sign language used by the deaf. This method was chosen because the gestures were better suited to the chimpanzee's anatomy than vocalization. Washoe learned over 150 signs, which she used to identify objects, make requests, and express verbs.21
• Sarah: Taught by Premack, Sarah was trained to use plastic tokens that functioned as words. These tokens represented nouns, verbs, adjectives, and prepositions. Sarah demonstrated an ability to associate these symbols with their referents, such as using a token to identify the color of an apple even when the object itself was not present.1

These studies highlight a fundamental tension in animal learning research: the choice between informal, social interaction—which risks anthropomorphic interpretation—and rigorous, reward-based experimental control, which may fail to capture the true nature of communication.1