It’s called “vocal functional flexibility.” We know that human infants exhibit this — not just uttering cries meaning negative emotional states and laughter meaning positive states — but also babbling away with sounds that could mean many things. In a 2013 paper, D. Kimbrough Oller and fellow researchers noted that if it could be shown that the latter type of vocalization occurs in non-human primates as well, it would “suggest deep roots for functional flexibility of vocalization in our primate heritage.” Chimpanzees and bonobos are our closest primate relatives. As it happens, no one has actually looked at young chimpanzees or bonobos to see if they show vocal functional flexibility, too.
A paper by Derry Taylor and other researchers at universities in the U.K., Switzerland, and France that was published in the journal iScience in October, describes the first systematic attempt to find out, looking at chimpanzees from birth up to the age of ten.
Taylor explained that while there are decades of research literature devoted to higher level features of human language like semantics and syntax, our language abilities don’t start at that level of complexity.
“Within a human lifetime, language is something that emerges from a kind of really complex developmental process,” Dr. Taylor told Salon in a video interview. “We didn’t really know anything about how the chimpanzee vocal repertoire develops. And how does that process kind of measure up to the process that we see in humans and it then raises another question, which is, ‘Okay, well what do we look for?’”
Taylor and his team considered the way in most animals, cries or sounds made by the infants tend to have a single function — a cry of alarm, for example. That’s not the case with all the sounds of human infants as they develop towards language, in which ultimately you need functionally-flexible sounds, to which the child learns to attach specific meanings.
“You can do many things with the same expression, and it’s something that you see in the pre-speech sounds of babies," Taylor said. "They [do] have laughs and cries that are relatively fixed in terms of how they’re used and what kind of function they have. But pre-speech sounds are different in that sense. They’re produced in really flexible ways and they’re kind of correspondingly flexible in the functions that they can serve in social interactions. And what we were looking for is basically whether there’s anything like that in chimpanzee vocal development.”
Many of the noises Taylor’s young chimpanzee subjects made were stereotyped sounds, just like other animals show, with each unique sound having a specific function. Until they started listening to grunting chimps, from newborns through to ten-year-old juveniles.
“With the grunts,” said Taylor, “it’s kind of not what you say, but more how you say it. They use sounds to express very different kinds of meanings, I guess you could say.”
So if this doesn’t mean that chimpanzees are going to grow up into conversationalists, what does it tell us? Well, if humans have the developmental foundations of language, and chimpanzees have something like it too, this suggests that our shared ancestors — way back in primate evolutionary history — already possessed them.
“The kind of key thing, the first thing we get from the comparative approach is that we can kind of put things on a timeline,” said Taylor. “So the logic is, if we find good evidence for something in humans and good evidence for something in chimpanzees, then we’re kind of justified in making the inference that this was also a trait that was held by the last common ancestor. So we can say with a reasonable degree of confidence that whatever those vocal communication systems were like, they probably at least had this type of flexibility.”
From this, Dr. Taylor says we can begin to articulate realistic scenarios for types of social interactions that would have been possible in such an ancestor.
“There was this very prominent view that [primate] vocalizations, in contrast to primate gestures, are really quite rigid in their form and their function. And this kind of shows that there’s more flexibility in the vocalizations than we previously thought.”
Just as Dr. Oller and his team suggested back in 2013, this suggests that this aspect of language may have evolved in ancestors common to humans and chimpanzees. But not everyone is ready to draw such conclusions — including Dr. Oller, who nevertheless welcomed the new research as the best study that exists on this topic.
"They wish to interpret the data as indicating that the capability of the human and inclination of the human infant to produce this phenomenon that we've called functional flexibility in their vocalizations is also present in the chimpanzee babies ... And in my view, there are a lot of reasons to hesitate to draw that conclusion,” Dr. Kimborough Oller told Salon. In particular, Oller points out that the number of sounds produced by young chimpanzees is vastly smaller than the number produced by young humans. He also believes they are qualitatively different.
“We are enormously more complicated in terms of what we can do socially than any other animal,” Oller argues. “And to try to minimize that by suggesting that what apes have in the way of vocal communicative capabilities is only different quantitatively from the human case is to me untenable. We are qualitatively different in terms of what we can do vocally from the others, and it starts in human infancy."
Taylor, who asserted that the purpose of his research is to look for both similarities and differences between humans and our primate relations, believes more research is in order, including looking in other primates. Oller’s work with human infants inspired the new chimpanzee study. Perhaps further research will clarify what exactly it means.
