July 12, 1999
Volume 51, No. 35
Chimpanzees can tell, just like humans: What's in a
How do you distinguish friend from foe, your mother from your aunt? Humans depend mostly on visual cues, while other species use scent or sound.
Like humans, chimpanzees 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, Lisa Parr, a graduate student in the psychology department, and Candler Professor of Psychology Frans de Waal described 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. In a study funded by the National Center for Research Resources at the National Institutes of Health and conducted under the auspices of Yerkes' Living Links Center, Parr and de Waal demonstrated that chimpanzees using a computerized joystick program are able to accurately match portraits of unfamiliar females with their sons. The chimpanzees, however, failed to recognize the relationship between ape mothers and daughters.
"Perhaps facial similarities concerning sons, or males in general, are more important for chimpanzees," said Parr. This may relate to the male-centered social system of chimpanzees. De Waal's studies on the political strategies of chimpanzees during power struggles have long found that males take great risks on behalf of one another. Alliances often occur between brothers or other relatives, and kinship issues are also important for producing healthy offspring. Females are the migratory sex. At puberty they move into neighboring groups and avoid mating with male relatives, who remain in the natal group. Avoiding inbreeding is crucial to preventing genetic defects, and the ability to recognize facial similarities may help a female avoid migrating into groups in which the males resemble her mother-males that may be related to her.
Parr presented five adult chimpanzees with computer portraits of females and their offspring, along with unrelated chimps. Many of the photographs in Living Links' large and unique collection represent individuals in distant colonies, which ensured that the study subjects did not work with portraits of animals they had known previously.
The subjects were first presented with a sample photograph of an adult female, representing the stimulus to be matched. The sample photograph was taken away and test chimpanzees saw portraits of two additional chimps: one was the offspring of the adult female in the sample, 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.
Members of a family recognizing and helping one another in an altruistic fashion helps improve the survival of the genetic line. This "kin selection" is extremely important for the evolution of complex societies. "Complex societies are built around interactions between related animals," Parr said. "Family members share food, defend one another and refrain from having sex with one another. Relationships and alliances are then made between different families that make up the social fabric of a primate community."
Parr and de Waal's data suggest that chimpanzees are as good at recognizing faces-and seeing the similarities and differences between them-as humans are. That chimpanzees can do so may help their survival in ways that are not yet fully understood. But these characteristics 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.