It's no secret that primates are intelligent. In fact, the American Museum of Natural History explains that many people view primates as our cousins who we share 96%-99% of our DNA with.

Primates, like apes, have been taught to communicate in sign language with humans, but there's something interesting that's been observed over the years: Apes don't ask questions.

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According to the Koko Gorilla Foundation, some primates have developed a sign language vocabulary that exceeded 1,100 signs, possibly up to 6,000. Researchers have worked with primates to communicate about complex topics like emotions and past experiences, but it seems apes are still missing the ability to ask questions.

David Premack and Ann James Premack began studying apes and their ability to communicate with humans, and they even designed an entire study dedicated to having apes ask questions. In 1972, they released a paper titled, "Teaching Language to an Ape," that detailed their methodology in working with primates and language.

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In the paper, the authors wrote:

"In principle interrogation can be taught either by removing an element from a familiar situation in the animal's world or by removing the element from a language that maps the animal's world. It is probable that one can induce questions by purposefully removing key elements from a familiar situation. Suppose a chimpanzee received its daily ration of food at a specific time and place, and then one day the food was not there. A chimpanzee trained in the interrogative might inquire 'Where is my food?' or, in Sarah's case, 'My food is?' Sarah was never put in a situation that might induce such interrogation because for our purposes it was easier to teach Sarah to answer questions."

Despite the promising nature of the research, the Premacks later revealed that they were never successful in getting an ape to ask a question. They hypothesized that an ape named Sarah could understand questions being asked, but it was easier to ask her questions than to prompt her to ask questions.

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In a book titled "The Mind of an Ape," the Premacks explained:

"Though she [Sarah] understood the question, she did not herself ask any questions—unlike the child who asks interminable questions, such as 'What that? Who making noise? When Daddy come home? Me go Granny's house? Where puppy? Toy?' Sarah never delayed the departure of her trainer after her lessons by asking where the trainer was going, when she was returning, or anything else."

Further research could theoretically determine that apes, in fact, can ask questions but extensive research has yet to show that. Maybe it's just one of those things that sets humans apart from the rest.