What evidence suggests that chimpanzees understand some of the symbols they use?

Evidence suggesting that chimpanzees understand the symbols they use—rather than merely performing rote, rewarded behaviors—includes several key observations from language research:

• Abstract Representation: The chimpanzee Sarah demonstrated that symbols could function as abstractions. When asked "What colour of apple?" she correctly used a token for "red" even when the apple was represented by a different, arbitrary symbol (a blue triangle), indicating the token stood for the object's property rather than just the physical item.1
• Categorization: Chimpanzees have shown the ability to sort symbols into distinct conceptual categories, such as "foods" and "tools," even when the actual objects were absent.1
• Spontaneous Understanding: The pygmy chimpanzee Kanzi demonstrated the ability to understand spoken English words under rigorous testing conditions that eliminated gestural cues from trainers, and he acquired these skills without the use of food rewards.1
• Syntactic Awareness: While debated, some interactions suggest an understanding of word order. For example, when a trainer reversed the order of signs (e.g., "Lucy tickle Roger" versus "Roger tickle Lucy"), the chimpanzee Lucy eventually adapted to the change, suggesting she recognized that the sequence of symbols altered the meaning.1