Researchers have found two important building blocks of human speech in wild chimpanzees, one of our closest relatives.
A pair of studies finds that chimp communication includes both rhythmic structures and call combinations, two key elements of spoken language.
Taken together, the studies add to an emerging "early footprint" indicating how human language may have evolved, says Catherine Crockford, an author of one of the studies and a research director at the French National Centre for Scientific Research in Lyon.
The chimp findings also bolster research on communication in other primates, including orangutans and bonobos.
However, studies like these are increasingly hard to carry out, researchers say, as wild chimpanzee populations are depleted by hunting, the pet trade, and habitat destruction.
Drum beats with meaning
The study of rhythmic structures, which appears in the journal Current Biology, analyzed the drumming patterns of chimps from the rainforests in East and West Africa.
The trees in those rainforests often have large buttress roots extending from the trunk that make "fantastic resonant drumming surfaces," says Cat Hobaiter, an author of the study and a professor at the University of St. Andrews in Scotland.
Adult chimpanzees often drum with their feet, Hobaiter says, "so they're using their hands to hold on to those roots and then they're kind of dancing."
Chimps use drumming to communicate over long distances. The message may indicate what direction a particular chimp is traveling, or just be a social check-in.
"They have their own individual signatures," Hobaiter says, "so I know if I hear a drum, that's Frank or that's Fred."
Hobaiter was part of a team that analyzed hundreds of recorded drumming episodes from several different chimp communities. She says the analysis confirmed what field scientists had long suspected.
"Not only do chimpanzees have rhythmic structure in their drumming, but different populations of chimpanzees use different structures," Hobaiter says, a bit like a regional accent or dialect.
That suggests the rhythms are learned and under the control of the drummer — both important attributes of the rhythms that are integral to human language.
The results also suggest that rhythmic structures were already around when the first humans appeared more than 1 million years ago.
And Hobaiter says rhythm isn't just for music and dance.
"It's present in the back-and-forth of a conversation, in the timing of a slow country drawl or a fast-talking city accent," she says. "So rhythm is something that's really present in lots of aspects of our social behavior."
Combining calls to create new meanings
Spoken language also combines a finite number of vocal sounds in ways that allow for an unlimited number of meanings.
So Crockford and a team of researchers studied the call combinations used by 53 wild chimps in Côte d'Ivoire.
The team would begin observing the chimps each day at dawn and stay until evening, recording "everything they do, every activity change, every social interaction, every vocalization," Crockford says.
This allowed the team to know the context for the more-than-4,000 utterances they analyzed.
The utterances included a dozen distinct calls. Sometimes the calls were used on their own and sometimes in combinations. The analysis focused on two-call combinations, known as bigrams.
"What we found then is that there is some shift in meaning when these single calls are embedded into these bigrams — and that the meaning can change in several ways," Crockford says.
For example, a "hoo" call on its own often means a chimp is resting, while a "pant" call on its own usually means a chimp is playing.
But when the two calls are combined, it is likely to mean that a chimp is building a nest.
Previous research has found this use of call combinations to alter meaning, but only when it involved sounding an alarm about something dangerous, like a snake.
Some linguists and other scientists have been quick to point out that chimpanzee communication is far less flexible and complicated than human language.
Crockford agrees, saying that the evidence so far does not justify equating chimp calls with human words and sentences.
Nonetheless, humans and chimps may share one common motivation for communicating: gossip.
A chimp often can't see first-hand what's going on with other members of the troop. So Crockford says they may need to ask around to find out: Have they had a shift in status? Have they fallen out with somebody?
Crockford says it makes sense that chimps also have many other uses in daily life for their ability to combine calls.
"It probably didn't evolve just because once in a while we saw a predator," she says. "It probably evolved because we need to somehow navigate our social world."
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