In a group of chimpanzees now living in Tanzania’s Mihale Mountains National Park, grooming partners sometimes both raise their right (or left) arms above their heads and grasp each others’ wrists as they take turns cleaning one another. In a nearby Mihale chimp community studied about 20 years ago, grooming duos preferred to raise arms and clasp hands, palm-to-palm, as they tidied up one another.
This is the first evidence that chimps employ a social custom in which different communities arbitrarily modify a common behavior to identify fellow group members and foster social solidarity, proposes a team led by anthropologist William C. McGrew of Miami University in Oxford, Ohio.
This investigation, described in the February Current Anthropology, expands on earlier reports of separate cultural traditions in wild chimp groups (SN: 6/19/99, p. 388). It also adds to observations of social traditions in other animals, such as the adoption of vocal dialects by different groups of killer whales (SN: 10/28/00, p. 284).
Vocal dialects and other social customs proliferate in human societies. “The military salute appears to be an appropriate analogy [to chimps’ grooming hand-clasps],” McGrew says. For instance, a British soldier completes a salute with the palm facing forward, whereas his U.S. counterpart holds the palm down. Although the two gestures both signal respect to a higher-ranking officer, subtle differences clearly signify a saluter’s nationality.
McGrew first noted grooming chimps clasping hands in 1975 in a now-disbanded Mihale chimp group. He and his coworkers examined photographs of hand clasping taken during that expedition. They also inspected photos and videotapes of grooming in a nearby, still-intact Mihale chimp community observed in 1982 and 1996 and in a chimp group at Tanzania’s Gombe National Park studied for 40 years by Jane Goodall and others.
The now-defunct Mihale chimp group usually employed palm-to-palm clasping while grooming, McGrew’s group reports. In contrast, groomers in the other Mihale community performed hand-clasps in which one wrist crossed over the other.
During hand-clasps, a grooming partner of lower social rank typically supported some of the higher-ranking partner’s weight. Hand-clasps of either style occurred only when one partner took the lead and prompted reciprocation by the other.
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At Gombe, chimps were never reported to perform any type of hand-clasp during mutual grooming. However, grooming partners sometimes lifted the nongrooming arms and draped their hands over low-hanging tree branches, the researchers say.
McGrew plans to examine grooming hand-clasps in other wild chimp groups and in a captive chimp colony now being studied by Frans B.M. de Waal of Emory University in Atlanta. In 1992, an adult female in that community first initiated grooming hand-clasps with other adults. Many of her comrades have since taken up the practice.
“Grooming hand-clasps could provide a source of social identity for chimps,” de Waal says. “It may be a bit like secret handshakes in human societies.”