Chimpanzees can learn a new sound for a common object, suggesting that the ability to acquire the building blocks of language may be more ancient than humans, according to a new study.
When a band of captive chimpanzees was transferred from the Netherlands to Scotland's Edinburgh Zoo in 2010, they used different grunts to refer to apples. Three years later, after the two groups had bonded, they had converged toward a common sound for the fruit, according to the study published online Thursday in the journal Current Biology.
That acoustic convergence suggests that an important element of language cognition may have been present 7 million to 13 million years ago, in the last common ancestor of chimpanzees and humans.
Research has shown that primates can produce unique calls for things or events in their environment, such as the arrival of a predator or discovery of high-quality food. Experiments have shown that fellow members can extract valuable information from those calls, and act appropriately.
"We don't call them semantic signals like in human language – we call them functionally referential calls," said study co-author Simon Townsend, a comparative biologist at the University of Zurich.
"In animals there's always been a feeling that you can't really de-couple these referential calls from the underlying emotional state that the animal is experiencing," Townsend said. "In human language, there's no link between the way that we're feeling and the word and what it sounds like."
For that reason and others, most researchers had assumed that referential calls were innate and rigid among nonhuman primates, and not flexible and socially learned as they are among Homo sapiens.
"What we've demonstrated is that there is no difference, at least in chimpanzees," Townsend said.
Investigating whether chimps might be able to adopt vocalization from other bands has been difficult. Chimp groups don't suddenly merge in the wild. In fact, they're far more likely to go to war with each other, research shows.
So when seven chimpanzees from the Beekse Bergen Safari Park in the Netherlands were slated to be transferred to the Edinburgh Zoo, which already had six chimps, researchers at the University of York, St. Andrews and Zurich took advantage.
University of York evolutionary psychologist Katie Slocombe, the lead investigator of the study, had shown in 2006 that chimpanzees varied their calls with the degree of preference for food types. She noticed the Dutch chimps clearly preferred apples, and their calls had a higher pitch that made them distinct from their new Edinburgh zoo mates.
The researchers recorded each group's food grunts over the course of three years. For the first two years, though, little changed. But the process of getting to know one another was slow too. Researchers had suspected that any merger of calls would be related to socialization.
By 2013, it became clear that the acoustic signature of the grunts that the Dutch chimps associated with apples had come down in frequency - it was closer to that of the Edinburgh chimps, according to the study. And by then, the chimps no longer were hanging out in cliques. They had fully integrated, according to the study.
The Netherlands chimps also did not appear to have changed the intensity of their food preference -- they still were more inclined than their Scottish counterparts to choose apples over other foods, according to the study.
Other research has shown that primates and non-primates modulate social calls with changing circumstances, but this would be the first time anyone has documented that vocal learning from fellow species members altered the structure of these calls.
What drove the convergence remains unknown.
"There must be some sort of driving factor why they would need to do this when two chimp groups come together, and that could be that they want to be better understood," Townsend said. "But it could also be more likely that it's some aspect of a social bonding function – if you sound more like the one you're with, then you just have a better chance of making friends and having a place in the social group."
The researchers lean toward the latter explanation - the chimpanzees were driven by the need to get along.
Simon Kirby, who studies language evolution at the University of Edinburgh and was not involved in the study, said it was "the first hint that at least the sounds of referentially meaningful calls may be socially learned in a primate species other than human." That early skill in a common ancestor to humans could have been one of several that contributed to the eventual development of semantic language.
But University of Wisconsin psychologist Charles Snowdon, who specializes in primate and human cognition and was not involved in the study, said he suspects the results fall short of showing chimpanzees learned a novel vocalization.
"The slow pace of social integration reported suggests that these calls are really about affiliation and not about semantics," Snowdon said.
Besides, he noted, the calls were too similar in the first place: "If I learn that 'gato' means cat in Spanish then I've truly learned a new word to express a semantic concept, but if a Spanish speaker and I start saying "gat" with each other, we are simply converging on a term."