June 21, 1999 Chimpanzees can tell: What's in a face?
How do you distinguish friend from foe, or your mother from your aunt? Humans depend mostly on visual cues, while other species use olfactory or auditory methods. Chimpanzees, like humans, also depend on visual discrimination. According to a report in the June 17 issue of Nature, chimpanzees can easily recognize faces of their brethren presented in digitized photographs. This ability to discriminate one group-mate from another has helped chimpanzees evolve to form the most complex of all mammal societies, characterized by individualized relationships, cooperative networks, and stable hierarchies of power.
In Nature, Living Links Center researchers at the Yerkes Primate Center, Lisa Parr and Frans de Waal, describe the first evidence in primates for an extension of these face recognition abilities to kin recognition that is purely visual and independent of previous experience with the individuals in question. Parr and de Waal demonstrate that chimpanzees, using a computerized joystick program, are able to accurately match portraits of unfamiliar females with their sons. The chimpanzees, however, fail to recognize the relationship between ape mothers and daughters.
Like Mother Like Son
"Perhaps facial similarities concerning sons, or males in general, are more important for chimpanzees," says Parr. This may relate to the male-centered social system of chimpanzees. It has long been known from Dr. de Waal's studies on the political strategies of chimpanzees during power struggles that males take great risks on behalf of one another. Alliances often occur between brothers or other relatives. Kinship issues are also important for producing healthy offspring. Females are the migratory sex; at puberty they move into neighboring groups, thus avoiding mating with their own male relatives, who remain in the natal group. Avoiding inbreeding is crucial to prevent genetic defects. The ability to recognize facial similarities may help a female avoid migrating into groups in which the males resemble her mother -- males which may well be related to herself.
Parr took advantage of the chimpanzee's proven face recognition abilities and adept computer skills when she presented five adult individuals with digitized portraits of females with their offspring, versus unrelated individuals. In doing so, she made use of the large and unique Living Links photo collection. Many of these photographs represent individuals in distant colonies, which ensured that the subjects in the study did not work with portraits of individuals they had known previously. They had to work purely on the basis of visual similarities.
The subjects were first presented with a sample photograph of an adult female, representing the stimulus to be matched. The sample then disappeared and test chimpanzees saw portraits of two additional individuals: one was the offspring of the individual in the sample, and the other an unrelated individual (matched for age and sex). The subjects made a correct response by moving the joystick-controlled cursor to select the related individual.
Why is this important? Members of a family recognizing and helping one another in an altruistic fashion helps improve the survival of the family's genetic line. This so-called "kin selection" is extremely important for the evolution of complex societies. "Complex societies are built around interactions between related animals,"explains Parr. "Family members share food, defend one another, and refrain from having sex with one another. Relationships and alliances are then made between different families which make up the social fabric of a primate community."
Parr and de Waal's data suggest that chimpanzees are as good at recognizing faces, seeing similarities and differences between them, as we are. That chimpanzees can do so may help their survival in ways that are not yet fully understood, but that may have played a role in our own evolution as well. In ever more extended family networks, where one doesn't always grow up with close kin, these skills may have served the same purpose of building kin-based alliances and avoiding incest.
The study was funded by the National Center for Research Resources of the National Institutes of Health.
Yerkes Primate Research Center of Emory University is the oldest scientific institution dedicated to primate research. Its programs cover a wide range of biomedical and behavioral sciences.
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The above story is based on materials provided by Emory University Health Sciences Center.
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