The researchers used X-rays to take images of rhesus monkeys making various sounds as they ate, yawned and yelled at each other. The results show that macaques can easily make many different sounds, including the five most basic vowels in the English alphabet. The researchers speculate that rhesus monkeys cannot speak thousands of words and complete sentences because their brains are different from those of humans.
 Using a computer to simulate macaques to speak complete sentences, each word is relatively clear and not difficult to understand best supports the above researchers' conjecture.