I think the most likely answer to this question is that chimps do not ask questions. Sagan is an expert on many matters, but he seems to rely too much on the testimony of the trainers in this case. People who train apes in sign language and other forms of communication tend to ignore all the nonsensical information they get from the apes and focus on the times when they get the information they are looking for. For instance, in this video, the ape Koko is asked when she does in her free time. The first answer she gives is blanket belly, which are two words that don't make sense together and don't answer the question. When Koko begins to sign about her baby (Koko never conceived and clearly wanted a baby), her trainer picks up on the sensical answer and rolls with it, despite the fact that this statement also does not answer her question. When Penny, the trainer, talks about Koko's level of communication, she brushes over all the nonsense strings of words that Koko makes and focuses on the times when she communicates an understandable idea.

Steven Pinker's book The Language Instinct covers the topic of ape language really well. He quotes a native American Sign Language speaker who was tasked with recording Washoe's use of signs "Every time the chimp made a sign, we were supposed to write it down in the log. . . . They were always complaining because my log didn't show enough signs. All the hearing people turned in logs with long lists of signs. They always saw more signs than I did . . . . I watched really carefully. The chimp's hands were moving constantly. Maybe I missed something, but I don't think so. I just wasn't seeing any signs. The hearing people were logging every movement the chimp made as a sign. Every time the chimp put his finger in his mouth, they'd say "Oh, he's making the sign for drink," and they'd give him some milk. . . . When the chimp scratched itself, they'd record it as the sign for scratch. . . . When [the chimps] want something, they reach. Sometimes [the trainers would] say, "Oh, amazing, look at that, it's exactly like the ASL sign for give!" It wasn't." Researchers are generous in the leeway they give when interpreting the signs of apes, and they define gestures that the apes naturally make (chimps gesture quite a bit in the wild) as signs.

The most important problem with chimp communication is that they do not use syntax in a meaningful way. Their trainers have to interpret the signs they use and impose syntax upon it for a human audience. I don't know about the syntax of questions in sign language, so I do not know what to look for in what little footage is available for chimps like Washoe, but seeing as how most claims about the length of statements demonstrated by chimps seems to top out at 3-4 symbols, it seems unlikely that they are producing any statement long enough to be a definitive question anyway. There is a world of difference between the statements made by chimps (read Herbert Terrace's 1979 paper, "Can an ape create a sentence?" for lots of examples of the syntax-less statements made by apes) and the sentences generated by children, which follow a sophisticated set of rules for different types of sentences. Chimps place the subject of the sentence indiscriminately before or after the verb, and the symbols that follow or precede the subject and verb also seem to obey no coherent rule. Once human children begin to talk in statements long enough to be considered sentences, they quickly be...

I think the most likely answer to this question is that chimps do not ask questions. Sagan is an expert on many matters, but he seems to rely too much on the testimony of the trainers in this case. People who train apes in sign language and other forms of communication tend to ignore all the nonsensical information they get from the apes and focus on the times when they get the information they are looking for. For instance, in this video, the ape Koko is asked when she does in her free time. The first answer she gives is blanket belly, which are two words that don't make sense together and don't answer the question. When Koko begins to sign about her baby (Koko never conceived and clearly wanted a baby), her trainer picks up on the sensical answer and rolls with it, despite the fact that this statement also does not answer her question. When Penny, the trainer, talks about Koko's level of communication, she brushes over all the nonsense strings of words that Koko makes and focuses on the times when she communicates an understandable idea.

Steven Pinker's book The Language Instinct covers the topic of ape language really well. He quotes a native American Sign Language speaker who was tasked with recording Washoe's use of signs "Every time the chimp made a sign, we were supposed to write it down in the log. . . . They were always complaining because my log didn't show enough signs. All the hearing people turned in logs with long lists of signs. They always saw more signs than I did . . . . I watched really carefully. The chimp's hands were moving constantly. Maybe I missed something, but I don't think so. I just wasn't seeing any signs. The hearing people were logging every movement the chimp made as a sign. Every time the chimp put his finger in his mouth, they'd say "Oh, he's making the sign for drink," and they'd give him some milk. . . . When the chimp scratched itself, they'd record it as the sign for scratch. . . . When [the chimps] want something, they reach. Sometimes [the trainers would] say, "Oh, amazing, look at that, it's exactly like the ASL sign for give!" It wasn't." Researchers are generous in the leeway they give when interpreting the signs of apes, and they define gestures that the apes naturally make (chimps gesture quite a bit in the wild) as signs.

The most important problem with chimp communication is that they do not use syntax in a meaningful way. Their trainers have to interpret the signs they use and impose syntax upon it for a human audience. I don't know about the syntax of questions in sign language, so I do not know what to look for in what little footage is available for chimps like Washoe, but seeing as how most claims about the length of statements demonstrated by chimps seems to top out at 3-4 symbols, it seems unlikely that they are producing any statement long enough to be a definitive question anyway. There is a world of difference between the statements made by chimps (read Herbert Terrace's 1979 paper, "Can an ape create a sentence?" for lots of examples of the syntax-less statements made by apes) and the sentences generated by children, which follow a sophisticated set of rules for different types of sentences. Chimps place the subject of the sentence indiscriminately before or after the verb, and the symbols that follow or precede the subject and verb also seem to obey no coherent rule. Once human children begin to talk in statements long enough to be considered sentences, they quickly begin to apply the syntactical rules of their culture to those statements. Chimps are extremely intelligent, and they have sophisticated cultures and notions of self, but to date there has never been a chimpanzee capable of uttering a question as coherent and meaningful as the titular interrogative posed by the fictional Tracy Jordan movie, "Who Dat Ninja?"