Researchers at the Yerkes National Primate Research Center, Emory University, have found bonobos and chimpanzees use manual gestures of their hands, feet and limbs more flexibly than they do facial expressions and vocalizations, further supporting the evolution of human language began with gestures as the gestural origin hypothesis of language suggests. This study appears in the current issue of the Proceedings of the National Academy of Sciences.
Working with two groups of bonobos (13 animals) and two groups of chimpanzees (34 animals), Yerkes researchers Amy Pollick, PhD, and Frans de Waal, PhD, distinguished 31 manual gestures and 18 facial/vocal signals. They found both species used facial/vocal signals similarly, but the same did not hold true for the manual gestures. Rather, the researchers found both within and between species the manual gestures were less closely tied to a particular emotion and, thereby, serve a more adaptable function. For example, a single gesture may communicate an entirely different message depending upon the social context in which it is used.
"A chimpanzee may stretch out an open hand to another as a signal for support, whereas the same gesture toward a possessor of food signals a desire to share," said Pollick. "A scream, however, is a typical response for victims of intimidation, threat or attack. This is so for both bonobos and chimpanzees, and suggests the vocalization is relatively invariant," Pollick continued.
By studying similar types of communication in closely related species, researchers are able to determine shared ancestry. We know gestures are evolutionarily younger than facial expressions and vocalizations, as shown by their presence in apes and humans but not in monkeys. "A gesture that occurs in bonobos and chimpanzees as well as humans likely was present in the last common ancestor," said Pollick. "A good example of a shared gesture is the open-hand begging gesture, used by both apes and humans. This gesture can be used for food, if there is food around, but it also can be used to beg for help, for support, for money and so on. It's meaning is context-dependent," added de Waal.
Looking for further distinctions between species, the researchers found bonobos use gestures more flexibly than do chimpanzees. "Different groups of bonobos used gestures in specific contexts less consistently than did different groups of chimpanzees," said Pollick. The researcher's findings also suggest bonobos and chimpanzees engage in multi-modal communication, combining their gestures with facial expressions and vocalizations to communicate a message. "While chimpanzees produce more of these combinations, bonobos respond to them more often. This finding suggests the bonobo is a better model of symbolic communication in our early ancestors," concluded Pollick.
For more than seven decades, the Yerkes National Primate Research Center, Emory University, has been dedicated to advancing science and to improving human health and well-being. Today, the center, as one of only eight National Institutes of Health-funded national primate research centers, provides specialized scientific resources, expertise and training opportunities. Recognized as a multidisciplinary research institute, the Yerkes Research Center is making landmark discoveries in the fields of microbiology and immunology, neuroscience, psychobiology and sensory-motor systems. Research programs are seeking ways to: develop vaccines for infectious and noninfectious diseases, such as AIDS and Alzheimer's disease; treat cocaine addiction; interpret brain activity through imaging; increase understanding of progressive illnesses such as Parkinson's and Alzheimer's; unlock the secrets of memory; determine behavioral effects of hormone replacement therapy; address vision disorders; and advance knowledge about the evolutionary links between biology and behavior.
Journal
Proceedings of the National Academy of Sciences