Along with their work with Viki, Keith Hayes also worked with other chimpanzees at the Yerkes facility. One question they explored: to what extent can chimpanzees voluntarily vocalize? Voluntary vocalization is essential in language — we often speak, not out of reflex, but simply because we want to. One project at Yerkes used operant conditioning to reward chimpanzees for producing any kind of sound at all. Here we see Betty, still a relatively young chimpanzee, has great difficulty producing vocalizations vouluntarily. She learned to excite herself, by jumping up and down, to produce a small sound.