Bonobos communicate in squeals like human infants

Infants have a limited range of communication options -- but they make it work. Moms and dads know that a certain squeal can mean both "I'm happy" and "Change me." Long before they start speaking, children as young as three months old are able to use sounds to express both positive and negative emotions.

For a long time scientists thought that "functional flexibility," the ability to produce vocalizations in a range of emotional states and situations before language develops, was unique to human infants. But it turns out that may not be the case after all.

New research shows that wild bonobos, our closest living relative in the primate world, can vocalize in a similar manner by using a squeaking sound, termed "the peep," that requires context to be understood. Peeps are high-pitched sounds which are short in duration and produced with a closed mouth.

Researchers at the University of Birmingham, UK and the University of Neuchatel, Switzerland conducted research on bonobos in their native setting in the Congo. They found that bonobos utilized "the peep" in a wide range of situations across positive, negative and neutral situations without variation in acoustic structure, which means that the listener must take into account the context into order to discern what the bonobo is communicating.

"I was struck by how frequent their peeps were, and how many different contexts they produce them in," lead researcher Dr. Zanna Clay from the University of Birmingham's School of Psychology said in a press release. "It became apparent that because we couldn't always differentiate between peeps, we needed to understand the context to get to the root of their communication."

The researchers found that peeps made during negative contexts were acoustically distinct, however, which they believe is a result of higher subglottal air pressure during call production. Negative situations are charged, tense and urgent and can produce physiological consequences that could alter vocal tones.

The findings challenge the belief that animal vocalizations are tied to specific contexts or emotional state, which suggests that "functional flexibility" has evolutionary roots that predate the evolution of human speech.

"We felt that it was premature to conclude that this ability is uniquely human, especially as no one had really looked for it in the great apes," Dr. Clay said. "It appears that the more we look, the more similarity we find between animals and humans."